Re: [PATCH 1/2] dt-bindings: mailbox : arm,mhuv2: Allow for more RX interrupts

[Cristian Marussi <cristian.marussi@arm.com>]

On Wed, Mar 29, 2023 at 06:44:31PM +0100, Sudeep Holla wrote:
> On Wed, Mar 29, 2023 at 04:39:35PM +0100, Cristian Marussi wrote:
> > The ARM MHUv2 Receiver block can indeed support more interrupts, up to the
> > maximum number of available channels, but anyway no more than the maximum
> > number of supported interrupt for an AMBA device.
> > 
> > Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
> > ---
> > Cc: Rob Herring <robh+dt@kernel.org>
> > Cc: Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>
> > Cc: devicetree@vger.kernel.org
> > 
> >  .../devicetree/bindings/mailbox/arm,mhuv2.yaml      | 13 +++++++++----
> >  1 file changed, 9 insertions(+), 4 deletions(-)
> > 
> > diff --git a/Documentation/devicetree/bindings/mailbox/arm,mhuv2.yaml b/Documentation/devicetree/bindings/mailbox/arm,mhuv2.yaml
> > index a4f1fe63659a..5a57f4e2a623 100644
> > --- a/Documentation/devicetree/bindings/mailbox/arm,mhuv2.yaml
> > +++ b/Documentation/devicetree/bindings/mailbox/arm,mhuv2.yaml
> > @@ -69,10 +69,15 @@ properties:
> >  
> >    interrupts:
> >      description: |
> > -      The MHUv2 controller always implements an interrupt in the "receiver"
> > -      mode, while the interrupt in the "sender" mode was not available in the
> > -      version MHUv2.0, but the later versions do have it.
> > -    maxItems: 1
> > +      The MHUv2 controller always implements at least an interrupt in the
> > +      "receiver" mode, while the interrupt in the "sender" mode was not
> > +      available in the version MHUv2.0, but the later versions do have it.
> > +      In "receiver" mode, beside a single combined interrupt, there could be
> > +      multiple interrupts, up to the number of implemented channels but anyway
> > +      no more than the maximum number of interrupts potentially supported by
> > +      AMBA.
> > +    minItems: 1
> > +    maxItems: 9
> 

Hi,

> I am not sure 9 is the correct value here. IIUC it is just what Linux defines
> as AMBA_NR_IRQS. Looking at the history it was bumped from 2 to 9 for use
> by PL330 DMA driver. I couldn't find anything to relate this 9 in any
> AMBA or other related specification.
> 

Yes, I could not find either where the 9 comes from, but it is what
currently each amba device is limited to, at the software level, in terms of
interrupts that can be detected.

> Ideally I would say we don't know what the max here. We just have a platform
> implementing 2 interrupts now. Do we for with 2 for now and change it if some
> new users require more in the future ?
> 

By the spec seems to me that the maximum number of interrupts are equal to
the maximum possible channels (124), or one combined interrupt.

But these in turn, as said, are capped by the AMBA_NR_IRQS and I have
only seen one system using 2. (for which I need this series to work)

> I will leave that to the DT maintainers but 9 is simply random based on Linux
> code so I would rather choose some other random number with a better reasoning
> than 9 as AMBA code in the kernel is limiting it to 9.
> 

Agreed. Aiming to describe any possible hw in the DT, I would say 124 at
this point. (even though implausible not to use the combined interrupt
at that point...)


Thanks,
Cristian