There are mailbox clients that can discover the mailbox channel ID at
run-time. For such cases passing the channel identifier via DT is
redundant. Add support for referencing controllers solely by node.

Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>
---
 Documentation/devicetree/bindings/mailbox/mailbox.txt | 19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

diff --git a/Documentation/devicetree/bindings/mailbox/mailbox.txt b/Documentation/devicetree/bindings/mailbox/mailbox.txt
index af8ecee2ac68..0c4295a62f61 100644
--- a/Documentation/devicetree/bindings/mailbox/mailbox.txt
+++ b/Documentation/devicetree/bindings/mailbox/mailbox.txt
@@ -5,9 +5,10 @@ assign appropriate mailbox channel to client drivers.
 
 * Mailbox Controller
 
-Required property:
-- #mbox-cells: Must be at least 1. Number of cells in a mailbox
-		specifier.
+Optional property:
+- #mbox-cells: Must be at least 1. Number of cells in a mailbox specifier.
+               The property becomes mandatory for the cases where the clients
+               reference the controller via the mboxes property.
 
 Example:
 	mailbox: mailbox {
@@ -19,7 +20,11 @@ Example:
 * Mailbox Client
 
 Required property:
+Clients must reference the mailbox controller either via the mboxes or mbox
+properties.
 - mboxes: List of phandle and mailbox channel specifiers.
+- mbox: phandle pointing to the controller. Used by clients that can discover
+        the channel identifiers at runtime.
 
 Optional property:
 - mbox-names: List of identifier strings for each mailbox channel.
@@ -29,7 +34,13 @@ Optional property:
 	  communication between the mailbox client and the remote.
 
 
-Example:
+Example using mbox:
+	power-management {
+		...
+		mbox = <&mailbox>;
+	};
+
+Example using mboxes:
 	pwr_cntrl: power {
 		...
 		mbox-names = "pwr-ctrl", "rpc";

-- 
2.47.1.613.gc27f4b7a9f-goog

On Thu, Dec 19, 2024 at 01:07:46PM +0000, Tudor Ambarus wrote:
> There are mailbox clients that can discover the mailbox channel ID at
> run-time. For such cases passing the channel identifier via DT is
> redundant. Add support for referencing controllers solely by node.

I don't really get your implementation, why not just allow #mbox-cells = 0?
That's what's done for things like fixed frequency clocks that only have
a single output.

Cheers,
Conor.

> 
> Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>
> ---
>  Documentation/devicetree/bindings/mailbox/mailbox.txt | 19 +++++++++++++++----
>  1 file changed, 15 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/devicetree/bindings/mailbox/mailbox.txt b/Documentation/devicetree/bindings/mailbox/mailbox.txt
> index af8ecee2ac68..0c4295a62f61 100644
> --- a/Documentation/devicetree/bindings/mailbox/mailbox.txt
> +++ b/Documentation/devicetree/bindings/mailbox/mailbox.txt
> @@ -5,9 +5,10 @@ assign appropriate mailbox channel to client drivers.
>  
>  * Mailbox Controller
>  
> -Required property:
> -- #mbox-cells: Must be at least 1. Number of cells in a mailbox
> -		specifier.
> +Optional property:
> +- #mbox-cells: Must be at least 1. Number of cells in a mailbox specifier.
> +               The property becomes mandatory for the cases where the clients
> +               reference the controller via the mboxes property.
>  
>  Example:
>  	mailbox: mailbox {
> @@ -19,7 +20,11 @@ Example:
>  * Mailbox Client
>  
>  Required property:
> +Clients must reference the mailbox controller either via the mboxes or mbox
> +properties.
>  - mboxes: List of phandle and mailbox channel specifiers.
> +- mbox: phandle pointing to the controller. Used by clients that can discover
> +        the channel identifiers at runtime.
>  
>  Optional property:
>  - mbox-names: List of identifier strings for each mailbox channel.
> @@ -29,7 +34,13 @@ Optional property:
>  	  communication between the mailbox client and the remote.
>  
>  
> -Example:
> +Example using mbox:
> +	power-management {
> +		...
> +		mbox = <&mailbox>;
> +	};
> +
> +Example using mboxes:
>  	pwr_cntrl: power {
>  		...
>  		mbox-names = "pwr-ctrl", "rpc";
> 
> -- 
> 2.47.1.613.gc27f4b7a9f-goog
> 

Hi, Conor,

On 12/19/24 2:11 PM, Conor Dooley wrote:
>> There are mailbox clients that can discover the mailbox channel ID at
>> run-time. For such cases passing the channel identifier via DT is
>> redundant. Add support for referencing controllers solely by node.
> I don't really get your implementation, why not just allow #mbox-cells = 0?
> That's what's done for things like fixed frequency clocks that only have
> a single output.

Ah, indeed!

instead of:
	of_parse_phandle(dev->of_node, "mbox", 0);
I can do a:
	of_parse_phandle_with_args(dev->of_node, "mboxes",
                                   "#mbox-cells", 0, &of_args)
where #mbox-cells = 0;

Or ... can I pass NULL for cells_name and make the #mbox-cells property
optional and still keeping its requirement of being at least 1?

Thanks!
ta

On Thu, Dec 19, 2024 at 03:42:11PM +0000, Tudor Ambarus wrote:
> Hi, Conor,
> 
> On 12/19/24 2:11 PM, Conor Dooley wrote:
> >> There are mailbox clients that can discover the mailbox channel ID at
> >> run-time. For such cases passing the channel identifier via DT is
> >> redundant. Add support for referencing controllers solely by node.
> > I don't really get your implementation, why not just allow #mbox-cells = 0?
> > That's what's done for things like fixed frequency clocks that only have
> > a single output.
> 
> Ah, indeed!
> 
> instead of:
> 	of_parse_phandle(dev->of_node, "mbox", 0);
> I can do a:
> 	of_parse_phandle_with_args(dev->of_node, "mboxes",
>                                    "#mbox-cells", 0, &of_args)
> where #mbox-cells = 0;
> 
> Or ... can I pass NULL for cells_name and make the #mbox-cells property
> optional and still keeping its requirement of being at least 1?

I think the mbox-cells = 0 approach is preferred, that property is what
marks it as a mailbox controller after all. Perhaps Rob or Krzysztof can
comment?

On 12/19/24 6:58 PM, Conor Dooley wrote:
> On Thu, Dec 19, 2024 at 03:42:11PM +0000, Tudor Ambarus wrote:
>> Hi, Conor,
>>
>> On 12/19/24 2:11 PM, Conor Dooley wrote:
>>>> There are mailbox clients that can discover the mailbox channel ID at
>>>> run-time. For such cases passing the channel identifier via DT is
>>>> redundant. Add support for referencing controllers solely by node.
>>> I don't really get your implementation, why not just allow #mbox-cells = 0?
>>> That's what's done for things like fixed frequency clocks that only have
>>> a single output.
>>
>> Ah, indeed!
>>
>> instead of:
>> 	of_parse_phandle(dev->of_node, "mbox", 0);
>> I can do a:
>> 	of_parse_phandle_with_args(dev->of_node, "mboxes",
>>                                    "#mbox-cells", 0, &of_args)
>> where #mbox-cells = 0;
>>
>> Or ... can I pass NULL for cells_name and make the #mbox-cells property
>> optional and still keeping its requirement of being at least 1?
> 
> I think the mbox-cells = 0 approach is preferred, that property is what
> marks it as a mailbox controller after all. Perhaps Rob or Krzysztof can
> comment?

I think using mbox-cells = 0 is better indeed. In my proposal I
considered the list to always have a single phandle, thus a reference to
a single mailbox controller, whereas it may be possible that clients to
reference multiple mailbox controllers. If so, #mbox-cells needs to be
defined in all the controllers, for consistency reasons, similar to what
happens with fixed clocks, as you already mentioned.

Thus I'll change the method to:
struct mbox_chan *mbox_request_channel_by_args(struct mbox_client *cl,
                       int index, const struct mbox_xlate_args *spec);
and use of_parse_phandle_with_args() in it.

Thanks, Conor!
ta

On Fri, Dec 20, 2024 at 07:51:32AM +0000, Tudor Ambarus wrote:
> 
> 
> On 12/19/24 6:58 PM, Conor Dooley wrote:
> > On Thu, Dec 19, 2024 at 03:42:11PM +0000, Tudor Ambarus wrote:
> >> Hi, Conor,
> >>
> >> On 12/19/24 2:11 PM, Conor Dooley wrote:
> >>>> There are mailbox clients that can discover the mailbox channel ID at
> >>>> run-time. For such cases passing the channel identifier via DT is
> >>>> redundant. Add support for referencing controllers solely by node.
> >>> I don't really get your implementation, why not just allow #mbox-cells = 0?
> >>> That's what's done for things like fixed frequency clocks that only have
> >>> a single output.
> >>
> >> Ah, indeed!
> >>
> >> instead of:
> >> 	of_parse_phandle(dev->of_node, "mbox", 0);
> >> I can do a:
> >> 	of_parse_phandle_with_args(dev->of_node, "mboxes",
> >>                                    "#mbox-cells", 0, &of_args)
> >> where #mbox-cells = 0;
> >>
> >> Or ... can I pass NULL for cells_name and make the #mbox-cells property
> >> optional and still keeping its requirement of being at least 1?
> > 
> > I think the mbox-cells = 0 approach is preferred, that property is what
> > marks it as a mailbox controller after all. Perhaps Rob or Krzysztof can
> > comment?
> 
> I think using mbox-cells = 0 is better indeed. In my proposal I
> considered the list to always have a single phandle, thus a reference to
> a single mailbox controller, whereas it may be possible that clients to
> reference multiple mailbox controllers. If so, #mbox-cells needs to be
> defined in all the controllers, for consistency reasons, similar to what
> happens with fixed clocks, as you already mentioned.

Oh right, I had totally forgotten about that, that's a solid argument
for the mbox-cells = 0 approach for sure.