When IIO goes through the available scan masks in order to select the
best suiting one, it will just accept the first listed subset of channels
which meets the user's requirements. If driver lists a mask which is a
subset of some of the masks previously in the array of
avaliable_scan_masks, then the latter one will never be selected.

Add a warning if driver registers masks which can't be used due to the
available_scan_masks-array ordering.

Suggested-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>

---
The change was suggested by Jonathan here:
https://lore.kernel.org/lkml/20230924170726.41443502@jic23-huawei/
---
 drivers/iio/industrialio-core.c | 57 +++++++++++++++++++++++++++++++++
 1 file changed, 57 insertions(+)

diff --git a/drivers/iio/industrialio-core.c b/drivers/iio/industrialio-core.c
index c77745b594bd..d4f37f4eeec0 100644
--- a/drivers/iio/industrialio-core.c
+++ b/drivers/iio/industrialio-core.c
@@ -1896,6 +1896,53 @@ static int iio_check_extended_name(const struct iio_dev *indio_dev)
 
 static const struct iio_buffer_setup_ops noop_ring_setup_ops;
 
+static void iio_sanity_check_avail_scan_masks(struct iio_dev *indio_dev)
+{
+	unsigned int num_masks, masklength, longs_per_mask;
+	const unsigned long *av_masks;
+	int i;
+
+	av_masks = indio_dev->available_scan_masks;
+	masklength = indio_dev->masklength;
+	longs_per_mask = BITS_TO_LONGS(masklength);
+
+	if (bitmap_empty(av_masks, masklength))
+		dev_warn(indio_dev->dev.parent, "empty scan mask\n");
+
+	for (num_masks = 0; *av_masks; num_masks++)
+		av_masks += longs_per_mask;
+
+	if (num_masks < 2)
+		return;
+
+	av_masks = indio_dev->available_scan_masks;
+
+	/*
+	 * Go through all the masks from first to one before the last, and see
+	 * that no mask found later from the available_scan_masks array is a
+	 * subset of mask found earlier. If this happens, then the mask found
+	 * later will never get used because scanning the array is stopped when
+	 * the first suitable mask is found. Drivers should order the array of
+	 * available masks in the order of preference (presumably the least
+	 * costy to access masks first).
+	 */
+	for (i = 0; i < num_masks - 1; i++) {
+		const unsigned long *mask1;
+		int j;
+
+		mask1 = av_masks + i * longs_per_mask;
+		for (j = i + 1; j < num_masks; j++) {
+			const unsigned long *mask2;
+
+			mask2 = av_masks + j * longs_per_mask;
+			if (bitmap_subset(mask2, mask1, masklength))
+				dev_warn(indio_dev->dev.parent,
+					 "available_scan_mask %d subset of %d. Never used\n",
+					 j, i);
+		}
+	}
+}
+
 int __iio_device_register(struct iio_dev *indio_dev, struct module *this_mod)
 {
 	struct iio_dev_opaque *iio_dev_opaque = to_iio_dev_opaque(indio_dev);
@@ -1934,6 +1981,16 @@ int __iio_device_register(struct iio_dev *indio_dev, struct module *this_mod)
 		goto error_unreg_debugfs;
 	}
 
+	/*
+	 * In order to not wreck utter havoc we just warn for now. Might want
+	 * to convert this to a failure after people have had time to act upon
+	 * the warning. It'd be nice to check this earlier, but we need the
+	 * iio_buffers_alloc_sysfs_and_mask() to have the masklength set.
+	 */
+	if (indio_dev->available_scan_masks)
+		iio_sanity_check_avail_scan_masks(indio_dev);
+
+
 	ret = iio_device_register_sysfs(indio_dev);
 	if (ret) {
 		dev_err(indio_dev->dev.parent,

base-commit: 5e99f692d4e32e3250ab18d511894ca797407aec
-- 
2.41.0


-- 
Matti Vaittinen, Linux device drivers
ROHM Semiconductors, Finland SWDC
Kiviharjunlenkki 1E
90220 OULU
FINLAND

~~~ "I don't think so," said Rene Descartes. Just then he vanished ~~~
Simon says - in Latin please.
~~~ "non cogito me" dixit Rene Descarte, deinde evanescavit ~~~
Thanks to Simon Glass for the translation =] 

On Tue, 3 Oct 2023 12:49:45 +0300
Matti Vaittinen <mazziesaccount@gmail.com> wrote:

> When IIO goes through the available scan masks in order to select the
> best suiting one, it will just accept the first listed subset of channels
> which meets the user's requirements. If driver lists a mask which is a
> subset of some of the masks previously in the array of
> avaliable_scan_masks, then the latter one will never be selected.
> 
> Add a warning if driver registers masks which can't be used due to the
> available_scan_masks-array ordering.
> 
> Suggested-by: Jonathan Cameron <jic23@kernel.org>
> Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
Hi Matti

Thanks for doing this.  A few comments inline + maybe we need to think
about a unit test for the matching code. I feel we aren't pushing the
corners of that in any drivers so far so it might bite us later.

Still that's a job for another day.

Jonathan

> 
> ---
> The change was suggested by Jonathan here:
> https://lore.kernel.org/lkml/20230924170726.41443502@jic23-huawei/
> ---
>  drivers/iio/industrialio-core.c | 57 +++++++++++++++++++++++++++++++++
>  1 file changed, 57 insertions(+)
> 
> diff --git a/drivers/iio/industrialio-core.c b/drivers/iio/industrialio-core.c
> index c77745b594bd..d4f37f4eeec0 100644
> --- a/drivers/iio/industrialio-core.c
> +++ b/drivers/iio/industrialio-core.c
> @@ -1896,6 +1896,53 @@ static int iio_check_extended_name(const struct iio_dev *indio_dev)
>  
>  static const struct iio_buffer_setup_ops noop_ring_setup_ops;
>  
> +static void iio_sanity_check_avail_scan_masks(struct iio_dev *indio_dev)
> +{
> +	unsigned int num_masks, masklength, longs_per_mask;
> +	const unsigned long *av_masks;
> +	int i;
> +
> +	av_masks = indio_dev->available_scan_masks;
> +	masklength = indio_dev->masklength;
> +	longs_per_mask = BITS_TO_LONGS(masklength);
> +
> +	if (bitmap_empty(av_masks, masklength))
> +		dev_warn(indio_dev->dev.parent, "empty scan mask\n");

They'd definitely notice this one as you'd never be able to enable the
buffer - if someone hasn't tested that, then meh.  Still this function
is called sanity_check so might as well check for insanity.


> +
> +	for (num_masks = 0; *av_masks; num_masks++)

I think we can't just check *av_masks - need bitmap_empty() as first
long might be 0 but could be bits set in the next one.

> +		av_masks += longs_per_mask;
hmm. Makes me wonder if the available scan mask stuff actually works
for large numbers of channels (so more than one long).  I don't think
we have any drivers that both have large channel counts and use
available_scan_masks.   The code is there to support matching in this
case but probably wants a selftest at somepoint to make sure it will work
if such a device comes along...


> +
> +	if (num_masks < 2)
> +		return;

Not sure it's worth bothering with this early exit route.  The loops
will be trivial anyway if num_masks == 1.


> +
> +	av_masks = indio_dev->available_scan_masks;
> +
> +	/*
> +	 * Go through all the masks from first to one before the last, and see
> +	 * that no mask found later from the available_scan_masks array is a
> +	 * subset of mask found earlier. If this happens, then the mask found
> +	 * later will never get used because scanning the array is stopped when
> +	 * the first suitable mask is found. Drivers should order the array of
> +	 * available masks in the order of preference (presumably the least
> +	 * costy to access masks first).
> +	 */
> +	for (i = 0; i < num_masks - 1; i++) {
> +		const unsigned long *mask1;
> +		int j;
> +
> +		mask1 = av_masks + i * longs_per_mask;
> +		for (j = i + 1; j < num_masks; j++) {
> +			const unsigned long *mask2;
> +
> +			mask2 = av_masks + j * longs_per_mask;
> +			if (bitmap_subset(mask2, mask1, masklength))
> +				dev_warn(indio_dev->dev.parent,
> +					 "available_scan_mask %d subset of %d. Never used\n",
> +					 j, i);
> +		}
> +	}
> +}
> +
>  int __iio_device_register(struct iio_dev *indio_dev, struct module *this_mod)
>  {
>  	struct iio_dev_opaque *iio_dev_opaque = to_iio_dev_opaque(indio_dev);
> @@ -1934,6 +1981,16 @@ int __iio_device_register(struct iio_dev *indio_dev, struct module *this_mod)
>  		goto error_unreg_debugfs;
>  	}
>  
> +	/*
> +	 * In order to not wreck utter havoc we just warn for now. Might want
> +	 * to convert this to a failure after people have had time to act upon
> +	 * the warning. It'd be nice to check this earlier, but we need the
> +	 * iio_buffers_alloc_sysfs_and_mask() to have the masklength set.

It's not going to break anyone if they get this wrong, they will just waste time
and possibly power reading too many channels!  So warn is appropriate I think.

I'm not sure the comment adds much in general so I'd slim it down or drop it
from v2.

> +	 */
> +	if (indio_dev->available_scan_masks)
> +		iio_sanity_check_avail_scan_masks(indio_dev);
> +
One blank line is enough ;)

> +
>  	ret = iio_device_register_sysfs(indio_dev);
>  	if (ret) {
>  		dev_err(indio_dev->dev.parent,
> 
> base-commit: 5e99f692d4e32e3250ab18d511894ca797407aec

Hi Again Jonathan.

On 10/5/23 18:30, Jonathan Cameron wrote:
> On Tue, 3 Oct 2023 12:49:45 +0300
> Matti Vaittinen <mazziesaccount@gmail.com> wrote:
> 
>> When IIO goes through the available scan masks in order to select the
>> best suiting one, it will just accept the first listed subset of channels
>> which meets the user's requirements. If driver lists a mask which is a
>> subset of some of the masks previously in the array of
>> avaliable_scan_masks, then the latter one will never be selected.
>>
>> Add a warning if driver registers masks which can't be used due to the
>> available_scan_masks-array ordering.
>>
>> Suggested-by: Jonathan Cameron <jic23@kernel.org>
>> Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
> Hi Matti
> 
> Thanks for doing this.  A few comments inline + maybe we need to think
> about a unit test for the matching code. I feel we aren't pushing the
> corners of that in any drivers so far so it might bite us later.
> 
> Still that's a job for another day.
> 
> Jonathan
> 
>>
>> ---
>> The change was suggested by Jonathan here:
>> https://lore.kernel.org/lkml/20230924170726.41443502@jic23-huawei/
>> ---
>>   drivers/iio/industrialio-core.c | 57 +++++++++++++++++++++++++++++++++
>>   1 file changed, 57 insertions(+)
>>
>> diff --git a/drivers/iio/industrialio-core.c b/drivers/iio/industrialio-core.c
>> index c77745b594bd..d4f37f4eeec0 100644
>> --- a/drivers/iio/industrialio-core.c
>> +++ b/drivers/iio/industrialio-core.c
>> @@ -1896,6 +1896,53 @@ static int iio_check_extended_name(const struct iio_dev *indio_dev)

...

>> +
>> +	for (num_masks = 0; *av_masks; num_masks++)
> 
> I think we can't just check *av_masks - need bitmap_empty() as first
> long might be 0 but could be bits set in the next one.
> 
>> +		av_masks += longs_per_mask;

I did switch this to:
+       for (num_masks = 0; !bitmap_empty(av_masks, masklength); 
num_masks++)
+               av_masks += longs_per_mask;

but this kind of freaks me out.

I think in kernel we see two ways of constructing and passing arrays to 
frameworks. One is creating a NULL terminated array, the other being an 
array which size is given. The available_scan_masks is using the first 
approach.

The array represents bitmasks, which are thought to be of arbitrary 
length. The type of array items is longs though. When building an arry 
like this, it is easy to just do:

unsigned long masks[] = {
	mask1_hi,
	mask1_lo,
	mask2_hi,
	mask2_lo,
	...
	maskN_lo,
	/* sentinel */
	0
}

(By the way, I've always hated that 'sentinel' comment as it - in my 
opinion - is not worth adding. I think the meaning of 0 should be 
obvious, but here I just added it to alleviate the problem).

Here, if I'm not mistaken, the check I implemented would go reading out 
of the array bounds.

Knowing how easy it would be slip the above array past my reviewing eyes 
- I find this scary. And ugly part of this is that we can't detect this 
in the iio-core side, because we have no way of knowing how big the 
array and sentinel are. What makes this worse is that the core does:

for (i = 0; i < indio_dev->num_channels; i++)
                         ml = max(ml, channels[i].scan_index + 1);
                 indio_dev->masklength = ml;

so, masklength may not be what was set in driver.

I did quick and dirty grep for "_scan_mask\[" in iio directory and 
didn't spot any bigger than a few channels masks. Still, this makes me 
worried.

BTW: I did also:

Author: Matti Vaittinen <mazziesaccount@gmail.com>
Date:   Fri Oct 6 13:53:11 2023 +0300

     iio: buffer: use bitmap_empty() to find last mask

     When IIO buffer code is scanning the array of available masks for
     matching the user's enable request to channel configuration 
supported by
     driver, the code uses a 'check for long 0' as indication of last mask.
     This does not work right for channel masks greater than BITS_PER_LONG.

     Use bitmap_empty() to find the last element in available_scan_masks
     array.

     Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
     ---
     NOTE: This is potentially hazardous change. Please, don't pick without
     thorough check and understanding.

diff --git a/drivers/iio/industrialio-buffer.c 
b/drivers/iio/industrialio-buffer.c
index 176d31d9f9d8..1e59afddcf9a 100644
--- a/drivers/iio/industrialio-buffer.c
+++ b/drivers/iio/industrialio-buffer.c
@@ -413,7 +413,7 @@ static const unsigned long 
*iio_scan_mask_match(const unsigned long *av_masks,
  {
         if (bitmap_empty(mask, masklength))
                 return NULL;
-       while (*av_masks) {
+       while (!bitmap_empty(av_masks, masklength)) {
                 if (strict) {
                         if (bitmap_equal(mask, av_masks, masklength))
                                 return av_masks;

but this is just as fragile - for obvious reasons.

One way around this would be to have the first bit in the long always 
set for a valid mask - and take this into account when going through the 
masks. It's probably somewhat more confusing than current code though - 
but it would allow using just a single long (with all - or  at least 
first - bits zero to indicate end of masks).

Other option I see is to just error out if available_scan_masks array is 
given with larger than one 'long' wide masks and worry things when this 
breaks.

Anyways, I don't like using bitmap_empty() for array of bitmaps which 
may be longer than BITS_PER_LONG unless we can sanity check the size of 
the array...

How do you feel about this?

Yours,
	-- Matti

-- 
Matti Vaittinen
Linux kernel developer at ROHM Semiconductors
Oulu Finland

~~ When things go utterly wrong vim users can always type :help! ~~

On Fri, 6 Oct 2023 14:10:16 +0300
Matti Vaittinen <mazziesaccount@gmail.com> wrote:

> Hi Again Jonathan.
> 
> On 10/5/23 18:30, Jonathan Cameron wrote:
> > On Tue, 3 Oct 2023 12:49:45 +0300
> > Matti Vaittinen <mazziesaccount@gmail.com> wrote:
> >   
> >> When IIO goes through the available scan masks in order to select the
> >> best suiting one, it will just accept the first listed subset of channels
> >> which meets the user's requirements. If driver lists a mask which is a
> >> subset of some of the masks previously in the array of
> >> avaliable_scan_masks, then the latter one will never be selected.
> >>
> >> Add a warning if driver registers masks which can't be used due to the
> >> available_scan_masks-array ordering.
> >>
> >> Suggested-by: Jonathan Cameron <jic23@kernel.org>
> >> Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>  
> > Hi Matti
> > 
> > Thanks for doing this.  A few comments inline + maybe we need to think
> > about a unit test for the matching code. I feel we aren't pushing the
> > corners of that in any drivers so far so it might bite us later.
> > 
> > Still that's a job for another day.
> > 
> > Jonathan
> >   
> >>
> >> ---
> >> The change was suggested by Jonathan here:
> >> https://lore.kernel.org/lkml/20230924170726.41443502@jic23-huawei/
> >> ---
> >>   drivers/iio/industrialio-core.c | 57 +++++++++++++++++++++++++++++++++
> >>   1 file changed, 57 insertions(+)
> >>
> >> diff --git a/drivers/iio/industrialio-core.c b/drivers/iio/industrialio-core.c
> >> index c77745b594bd..d4f37f4eeec0 100644
> >> --- a/drivers/iio/industrialio-core.c
> >> +++ b/drivers/iio/industrialio-core.c
> >> @@ -1896,6 +1896,53 @@ static int iio_check_extended_name(const struct iio_dev *indio_dev)  
> 
> ...
> 
> >> +
> >> +	for (num_masks = 0; *av_masks; num_masks++)  
> > 
> > I think we can't just check *av_masks - need bitmap_empty() as first
> > long might be 0 but could be bits set in the next one.
> >   
> >> +		av_masks += longs_per_mask;  
> 
> I did switch this to:
> +       for (num_masks = 0; !bitmap_empty(av_masks, masklength); 
> num_masks++)
> +               av_masks += longs_per_mask;
> 
> but this kind of freaks me out.

Good because I'm fairly sure you need to reduce the masklength int hat
bitmap_empty as well.  It is getting a bit complex.


> 
> I think in kernel we see two ways of constructing and passing arrays to 
> frameworks. One is creating a NULL terminated array, the other being an 
> array which size is given. The available_scan_masks is using the first 
> approach.
> 
> The array represents bitmasks, which are thought to be of arbitrary 
> length. The type of array items is longs though. When building an arry 
> like this, it is easy to just do:
> 
> unsigned long masks[] = {
> 	mask1_hi,
> 	mask1_lo,
> 	mask2_hi,
> 	mask2_lo,
> 	...
> 	maskN_lo,
> 	/* sentinel */
> 	0
> }
> 
> (By the way, I've always hated that 'sentinel' comment as it - in my 
> opinion - is not worth adding. I think the meaning of 0 should be 
> obvious, but here I just added it to alleviate the problem).
> 
> Here, if I'm not mistaken, the check I implemented would go reading out 
> of the array bounds.

It does indeed.


> 
> Knowing how easy it would be slip the above array past my reviewing eyes 
> - I find this scary. And ugly part of this is that we can't detect this 
> in the iio-core side, because we have no way of knowing how big the 
> array and sentinel are. What makes this worse is that the core does:
> 
> for (i = 0; i < indio_dev->num_channels; i++)
>                          ml = max(ml, channels[i].scan_index + 1);
>                  indio_dev->masklength = ml;
> 
> so, masklength may not be what was set in driver.

IIRC this is there to allow for sparse scan_index values.  Those are
very rare, but I think there are a few drivers doing that because
it allowed for slightly simpler code a long time back.  May not even
matter today.  Key is that mask_length is big enough to allow the
bits at all present scan_index values to be set.

So it should always match with the drivers where this is used to make
sure available_scan_masks has the right number of longs per entry,
but the drivers may need to be a little clever if they are both
doing large numbers of channels and sparse scan_index values.
AFAIK there are none doing that. Going further I don't recall any
drivers that use the available_scan_masks stuff going beyond 32
channels (so needing more than one unsigned long per element).

> 
> I did quick and dirty grep for "_scan_mask\[" in iio directory and 
> didn't spot any bigger than a few channels masks. Still, this makes me 
> worried.
> 
> BTW: I did also:
> 
> Author: Matti Vaittinen <mazziesaccount@gmail.com>
> Date:   Fri Oct 6 13:53:11 2023 +0300
> 
>      iio: buffer: use bitmap_empty() to find last mask
> 
>      When IIO buffer code is scanning the array of available masks for
>      matching the user's enable request to channel configuration 
> supported by
>      driver, the code uses a 'check for long 0' as indication of last mask.
>      This does not work right for channel masks greater than BITS_PER_LONG.
> 
>      Use bitmap_empty() to find the last element in available_scan_masks
>      array.
> 
>      Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
>      ---
>      NOTE: This is potentially hazardous change. Please, don't pick without
>      thorough check and understanding.
> 
> diff --git a/drivers/iio/industrialio-buffer.c 
> b/drivers/iio/industrialio-buffer.c
> index 176d31d9f9d8..1e59afddcf9a 100644
> --- a/drivers/iio/industrialio-buffer.c
> +++ b/drivers/iio/industrialio-buffer.c
> @@ -413,7 +413,7 @@ static const unsigned long 
> *iio_scan_mask_match(const unsigned long *av_masks,
>   {
>          if (bitmap_empty(mask, masklength))
>                  return NULL;
> -       while (*av_masks) {
> +       while (!bitmap_empty(av_masks, masklength)) {
>                  if (strict) {
>                          if (bitmap_equal(mask, av_masks, masklength))
>                                  return av_masks;
> 
> but this is just as fragile - for obvious reasons.
Ah. yes, that is indeed a bug.  I'm not sure your fix is particularly
fragile though.  This comes back to us having no drivers that actually use
big bitmaps yet.

Key is that we need the available_scan_masks null terminator to be the
same length as any other entry - so if multiple unsigned longs needed
then multiple 0's should be there.  We should definitely document that
and ideally add a test case.   We can bulk out the dummy driver
to trigger these and provide an example of how available_scan_masks
should be set.

> 
> One way around this would be to have the first bit in the long always 
> set for a valid mask - and take this into account when going through the 
> masks. It's probably somewhat more confusing than current code though - 
> but it would allow using just a single long (with all - or  at least 
> first - bits zero to indicate end of masks).

Too complex.

> 
> Other option I see is to just error out if available_scan_masks array is 
> given with larger than one 'long' wide masks and worry things when this 
> breaks.

That would kick the problem into the long grass.

> 
> Anyways, I don't like using bitmap_empty() for array of bitmaps which 
> may be longer than BITS_PER_LONG unless we can sanity check the size of 
> the array...
> 
> How do you feel about this?

Agreed it's problematic as that null terminator isn't clearly forced to
be big enough.  Hmm. Can we cheat for any drivers that actually need large
masks (when they come along) and use an appropriate 2D array.

unsigned long available_masks[][2] = {
	{mask0_ll, mask0_hl},
	{mask0_ll, mask0_hl},
	{}
};

	iio_dev->available_scan_masks = (unsigned long *)available_masks;

If we put such an example into the dummy / example driver then that might
act to avoid us getting bugs in future + test the fix you have above and
related.

Jonathan

> 
> Yours,
> 	-- Matti
>

On 10/5/23 18:30, Jonathan Cameron wrote:
> On Tue, 3 Oct 2023 12:49:45 +0300
> Matti Vaittinen <mazziesaccount@gmail.com> wrote:
> 
>> When IIO goes through the available scan masks in order to select the
>> best suiting one, it will just accept the first listed subset of channels
>> which meets the user's requirements. If driver lists a mask which is a
>> subset of some of the masks previously in the array of
>> avaliable_scan_masks, then the latter one will never be selected.
>>
>> Add a warning if driver registers masks which can't be used due to the
>> available_scan_masks-array ordering.
>>
>> Suggested-by: Jonathan Cameron <jic23@kernel.org>
>> Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
> Hi Matti
> 
> Thanks for doing this.  A few comments inline + maybe we need to think
> about a unit test for the matching code. I feel we aren't pushing the
> corners of that in any drivers so far so it might bite us later.

I am extremely conservative what comes to adding unit tests. I have seen 
some projects where the amount of existing unit test code made code 
changes very much very slow - stopping people doing any improvements. 
Basically, no one wanted to touch the existing code unless it was 
absolutely must because even a minor code change caused several tests to 
break. OTOH, that unit test setup did not only test that end result of a 
function was expected - it did also check the calls done from the 
function to be tested - checking for example that the certain prints 
appeared with certain inputs and so on. That project stopped being fun 
very quickly...

But yes. After spending a while reading IIO code, I agree that _some_ 
parts of it could benefit from a few carefully designed unit tests. (And 
sorry, I haven't checked what tests are existing already - so may be 
there already is relevant tests) :) Channel data demuxing and the mask 
handling are indeed the first to come to my mind ;) I wouldn't dare to 
touch that part without some testing.

> Still that's a job for another day.

Hey, we need to have something for tomorrow, right? :)

> 
>>
>> ---
>> The change was suggested by Jonathan here:
>> https://lore.kernel.org/lkml/20230924170726.41443502@jic23-huawei/
>> ---
>>   drivers/iio/industrialio-core.c | 57 +++++++++++++++++++++++++++++++++
>>   1 file changed, 57 insertions(+)
>>
>> diff --git a/drivers/iio/industrialio-core.c b/drivers/iio/industrialio-core.c
>> index c77745b594bd..d4f37f4eeec0 100644
>> --- a/drivers/iio/industrialio-core.c
>> +++ b/drivers/iio/industrialio-core.c
>> @@ -1896,6 +1896,53 @@ static int iio_check_extended_name(const struct iio_dev *indio_dev)
>>   
>>   static const struct iio_buffer_setup_ops noop_ring_setup_ops;
>>   
>> +static void iio_sanity_check_avail_scan_masks(struct iio_dev *indio_dev)
>> +{
>> +	unsigned int num_masks, masklength, longs_per_mask;
>> +	const unsigned long *av_masks;
>> +	int i;
>> +
>> +	av_masks = indio_dev->available_scan_masks;
>> +	masklength = indio_dev->masklength;
>> +	longs_per_mask = BITS_TO_LONGS(masklength);
>> +
>> +	if (bitmap_empty(av_masks, masklength))
>> +		dev_warn(indio_dev->dev.parent, "empty scan mask\n");
> 
> They'd definitely notice this one as you'd never be able to enable the
> buffer - if someone hasn't tested that, then meh.  Still this function
> is called sanity_check so might as well check for insanity.
> 
> 
>> +
>> +	for (num_masks = 0; *av_masks; num_masks++)
> 
> I think we can't just check *av_masks - need bitmap_empty() as first
> long might be 0 but could be bits set in the next one.

Ah. In case where we have bitmap consisting of many longs. Indeed. By 
the way, I think I stole this check from the actual matching code - we 
should probably fix it as well.

>> +		av_masks += longs_per_mask;
> hmm. Makes me wonder if the available scan mask stuff actually works
> for large numbers of channels (so more than one long).

After you pointed out the problem in for-condition - it probably does 
not work for all cases.

>  I don't think
> we have any drivers that both have large channel counts and use
> available_scan_masks.   The code is there to support matching in this
> case but probably wants a selftest at somepoint to make sure it will work
> if such a device comes along...
> 
> 
>> +
>> +	if (num_masks < 2)
>> +		return;
> 
> Not sure it's worth bothering with this early exit route.  The loops
> will be trivial anyway if num_masks == 1.

I probably thought about the num_masks == 0 when adding this check. 
Decided we might just early exit while checking.

>> +
>> +	av_masks = indio_dev->available_scan_masks;
>> +
>> +	/*
>> +	 * Go through all the masks from first to one before the last, and see
>> +	 * that no mask found later from the available_scan_masks array is a
>> +	 * subset of mask found earlier. If this happens, then the mask found
>> +	 * later will never get used because scanning the array is stopped when
>> +	 * the first suitable mask is found. Drivers should order the array of
>> +	 * available masks in the order of preference (presumably the least
>> +	 * costy to access masks first).
>> +	 */
>> +	for (i = 0; i < num_masks - 1; i++) {
>> +		const unsigned long *mask1;
>> +		int j;
>> +
>> +		mask1 = av_masks + i * longs_per_mask;
>> +		for (j = i + 1; j < num_masks; j++) {
>> +			const unsigned long *mask2;
>> +
>> +			mask2 = av_masks + j * longs_per_mask;
>> +			if (bitmap_subset(mask2, mask1, masklength))
>> +				dev_warn(indio_dev->dev.parent,
>> +					 "available_scan_mask %d subset of %d. Never used\n",
>> +					 j, i);
>> +		}
>> +	}
>> +}
>> +
>>   int __iio_device_register(struct iio_dev *indio_dev, struct module *this_mod)
>>   {
>>   	struct iio_dev_opaque *iio_dev_opaque = to_iio_dev_opaque(indio_dev);
>> @@ -1934,6 +1981,16 @@ int __iio_device_register(struct iio_dev *indio_dev, struct module *this_mod)
>>   		goto error_unreg_debugfs;
>>   	}
>>   
>> +	/*
>> +	 * In order to not wreck utter havoc we just warn for now. Might want
>> +	 * to convert this to a failure after people have had time to act upon
>> +	 * the warning. It'd be nice to check this earlier, but we need the
>> +	 * iio_buffers_alloc_sysfs_and_mask() to have the masklength set.
> 
> It's not going to break anyone if they get this wrong, they will just waste time
> and possibly power reading too many channels!  So warn is appropriate I think.
> 
> I'm not sure the comment adds much in general so I'd slim it down or drop it
> from v2.

I'm fine with dropping the comment. My mindset is easily leaning too 
much on developing new drivers when I think of checks like this one. 
It'd be nice to get a noticeable kick immediately when developing a 
driver - but yes, one should be kicked just by the warning alone.

> 
>> +	 */
>> +	if (indio_dev->available_scan_masks)
>> +		iio_sanity_check_avail_scan_masks(indio_dev);
>> +
> One blank line is enough ;)

Again... Thanks!

>> +
>>   	ret = iio_device_register_sysfs(indio_dev);
>>   	if (ret) {
>>   		dev_err(indio_dev->dev.parent,
>>
>> base-commit: 5e99f692d4e32e3250ab18d511894ca797407aec

Yours,
	-- Matti

> 

-- 
Matti Vaittinen
Linux kernel developer at ROHM Semiconductors
Oulu Finland

~~ When things go utterly wrong vim users can always type :help! ~~