It's possible for the polling loop in busy_loop() to get scheduled away
for a long time.

  status = ipc_read_status(scu); // status = IPC_STATUS_BUSY
  <long time scheduled away>
  if (!(status & IPC_STATUS_BUSY))

If this happens, then the status bit could change while the task is
scheduled away and this function would never read the status again after
timing out. Instead, the function will return -ETIMEDOUT when it's
possible that scheduling didn't work out and the status bit was cleared.
Bit polling code should always check the bit being polled one more time
after the timeout in case this happens.

Fix this by reading the status once more after the while loop breaks.
The read_poll_timeout() macro implements all of this, and it is
shorter, so use that macro here to consolidate code and fix this.

There were some concerns with using read_poll_timeout() because it uses
timekeeping, and timekeeping isn't running early on or during the late
stages of system suspend or early stages of system resume, but an audit
of the code concluded that this code isn't called during those times so
it is safe to use the macro.

Cc: Prashant Malani <pmalani@chromium.org>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Fixes: e7b7ab3847c9 ("platform/x86: intel_scu_ipc: Sleeping is fine when polling")
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
---
 drivers/platform/x86/intel_scu_ipc.c | 19 ++++++++-----------
 1 file changed, 8 insertions(+), 11 deletions(-)

diff --git a/drivers/platform/x86/intel_scu_ipc.c b/drivers/platform/x86/intel_scu_ipc.c
index 6851d10d6582..5a37becc65aa 100644
--- a/drivers/platform/x86/intel_scu_ipc.c
+++ b/drivers/platform/x86/intel_scu_ipc.c
@@ -19,6 +19,7 @@
 #include <linux/init.h>
 #include <linux/interrupt.h>
 #include <linux/io.h>
+#include <linux/iopoll.h>
 #include <linux/module.h>
 #include <linux/slab.h>
 
@@ -231,19 +232,15 @@ static inline u32 ipc_data_readl(struct intel_scu_ipc_dev *scu, u32 offset)
 /* Wait till scu status is busy */
 static inline int busy_loop(struct intel_scu_ipc_dev *scu)
 {
-	unsigned long end = jiffies + IPC_TIMEOUT;
+	u8 status;
+	int err;
 
-	do {
-		u32 status;
+	err = read_poll_timeout(ipc_read_status, status, !(status & IPC_STATUS_BUSY),
+				100, jiffies_to_usecs(IPC_TIMEOUT), false, scu);
+	if (err)
+		return err;
 
-		status = ipc_read_status(scu);
-		if (!(status & IPC_STATUS_BUSY))
-			return (status & IPC_STATUS_ERR) ? -EIO : 0;
-
-		usleep_range(50, 100);
-	} while (time_before(jiffies, end));
-
-	return -ETIMEDOUT;
+	return (status & IPC_STATUS_ERR) ? -EIO : 0;
 }
 
 /* Wait till ipc ioc interrupt is received or timeout in 10 HZ */
-- 
https://chromeos.dev

Hi,

On 9/11/2023 12:39 PM, Stephen Boyd wrote:
> It's possible for the polling loop in busy_loop() to get scheduled away
> for a long time.
> 
>   status = ipc_read_status(scu); // status = IPC_STATUS_BUSY
>   <long time scheduled away>
>   if (!(status & IPC_STATUS_BUSY))
> 
> If this happens, then the status bit could change while the task is
> scheduled away and this function would never read the status again after
> timing out. Instead, the function will return -ETIMEDOUT when it's
> possible that scheduling didn't work out and the status bit was cleared.
> Bit polling code should always check the bit being polled one more time
> after the timeout in case this happens.
> 
> Fix this by reading the status once more after the while loop breaks.
> The read_poll_timeout() macro implements all of this, and it is
> shorter, so use that macro here to consolidate code and fix this.
> 
> There were some concerns with using read_poll_timeout() because it uses
> timekeeping, and timekeeping isn't running early on or during the late
> stages of system suspend or early stages of system resume, but an audit
> of the code concluded that this code isn't called during those times so
> it is safe to use the macro.
> 
> Cc: Prashant Malani <pmalani@chromium.org>
> Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> Fixes: e7b7ab3847c9 ("platform/x86: intel_scu_ipc: Sleeping is fine when polling")
> Signed-off-by: Stephen Boyd <swboyd@chromium.org>
> ---

Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>


-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer

On Mon, Sep 11, 2023 at 12:39:33PM -0700, Stephen Boyd wrote:
> It's possible for the polling loop in busy_loop() to get scheduled away
> for a long time.
> 
>   status = ipc_read_status(scu); // status = IPC_STATUS_BUSY
>   <long time scheduled away>
>   if (!(status & IPC_STATUS_BUSY))
> 
> If this happens, then the status bit could change while the task is
> scheduled away and this function would never read the status again after
> timing out. Instead, the function will return -ETIMEDOUT when it's
> possible that scheduling didn't work out and the status bit was cleared.
> Bit polling code should always check the bit being polled one more time
> after the timeout in case this happens.
> 
> Fix this by reading the status once more after the while loop breaks.
> The read_poll_timeout() macro implements all of this, and it is
> shorter, so use that macro here to consolidate code and fix this.
> 
> There were some concerns with using read_poll_timeout() because it uses
> timekeeping, and timekeeping isn't running early on or during the late
> stages of system suspend or early stages of system resume, but an audit
> of the code concluded that this code isn't called during those times so
> it is safe to use the macro.
> 
> Cc: Prashant Malani <pmalani@chromium.org>
> Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>

On Mon, Sep 11, 2023 at 12:39:33PM -0700, Stephen Boyd wrote:
> It's possible for the polling loop in busy_loop() to get scheduled away
> for a long time.
> 
>   status = ipc_read_status(scu); // status = IPC_STATUS_BUSY
>   <long time scheduled away>
>   if (!(status & IPC_STATUS_BUSY))
> 
> If this happens, then the status bit could change while the task is
> scheduled away and this function would never read the status again after
> timing out. Instead, the function will return -ETIMEDOUT when it's
> possible that scheduling didn't work out and the status bit was cleared.
> Bit polling code should always check the bit being polled one more time
> after the timeout in case this happens.
> 
> Fix this by reading the status once more after the while loop breaks.
> The read_poll_timeout() macro implements all of this, and it is
> shorter, so use that macro here to consolidate code and fix this.
> 
> There were some concerns with using read_poll_timeout() because it uses
> timekeeping, and timekeeping isn't running early on or during the late
> stages of system suspend or early stages of system resume, but an audit
> of the code concluded that this code isn't called during those times so
> it is safe to use the macro.

...

> +	err = read_poll_timeout(ipc_read_status, status, !(status & IPC_STATUS_BUSY),
> +				100, jiffies_to_usecs(IPC_TIMEOUT), false, scu);

Since "false" you probably can utilize readx_poll_timeout().

> +	if (err)
> +		return err;
>  
-- 
With Best Regards,
Andy Shevchenko

On Tue, Sep 12, 2023 at 12:17:22AM +0300, Andy Shevchenko wrote:
> On Mon, Sep 11, 2023 at 12:39:33PM -0700, Stephen Boyd wrote:

...

> > +	err = read_poll_timeout(ipc_read_status, status, !(status & IPC_STATUS_BUSY),
> > +				100, jiffies_to_usecs(IPC_TIMEOUT), false, scu);
> 
> Since "false" you probably can utilize readx_poll_timeout().

...and because only a single parameter taken.

> > +	if (err)
> > +		return err;

With that,
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

-- 
With Best Regards,
Andy Shevchenko

Quoting Andy Shevchenko (2023-09-11 14:17:22)
> On Mon, Sep 11, 2023 at 12:39:33PM -0700, Stephen Boyd wrote:
> > It's possible for the polling loop in busy_loop() to get scheduled away
> > for a long time.
> >
> >   status = ipc_read_status(scu); // status = IPC_STATUS_BUSY
> >   <long time scheduled away>
> >   if (!(status & IPC_STATUS_BUSY))
> >
> > If this happens, then the status bit could change while the task is
> > scheduled away and this function would never read the status again after
> > timing out. Instead, the function will return -ETIMEDOUT when it's
> > possible that scheduling didn't work out and the status bit was cleared.
> > Bit polling code should always check the bit being polled one more time
> > after the timeout in case this happens.
> >
> > Fix this by reading the status once more after the while loop breaks.
> > The read_poll_timeout() macro implements all of this, and it is
> > shorter, so use that macro here to consolidate code and fix this.
> >
> > There were some concerns with using read_poll_timeout() because it uses
> > timekeeping, and timekeeping isn't running early on or during the late
> > stages of system suspend or early stages of system resume, but an audit
> > of the code concluded that this code isn't called during those times so
> > it is safe to use the macro.
>
> ...
>
> > +     err = read_poll_timeout(ipc_read_status, status, !(status & IPC_STATUS_BUSY),
> > +                             100, jiffies_to_usecs(IPC_TIMEOUT), false, scu);
>
> Since "false" you probably can utilize readx_poll_timeout().
>

You mean 'addr' will be 'scu'? Ok.