Re: [PATCH 02/10] dt-bindings: PCI: qcom: Do not require 'msi-map-mask'

From: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
To: Johan Hovold <johan@kernel.org>
Cc: "Krzysztof Kozlowski" <krzysztof.kozlowski@linaro.org>,
	"Johan Hovold" <johan+linaro@kernel.org>,
	"Bjorn Andersson" <andersson@kernel.org>,
	"Bjorn Helgaas" <bhelgaas@google.com>,
	"Konrad Dybcio" <konrad.dybcio@linaro.org>,
	"Lorenzo Pieralisi" <lpieralisi@kernel.org>,
	"Krzysztof Wilczyński" <kw@linux.com>,
	"Rob Herring" <robh@kernel.org>,
	"Krzysztof Kozlowski" <krzysztof.kozlowski+dt@linaro.org>,
	"Conor Dooley" <conor+dt@kernel.org>,
	linux-arm-msm@vger.kernel.org, linux-pci@vger.kernel.org,
	devicetree@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 02/10] dt-bindings: PCI: qcom: Do not require 'msi-map-mask'
Date: Wed, 21 Feb 2024 10:56:07 +0530	[thread overview]
Message-ID: <20240221052607.GB11693@thinkpad> (raw)
In-Reply-To: <ZdRXpQnbDbojlMkV@hovoldconsulting.com>

On Tue, Feb 20, 2024 at 08:41:25AM +0100, Johan Hovold wrote:
> On Fri, Feb 16, 2024 at 10:24:06PM +0530, Manivannan Sadhasivam wrote:
> > On Wed, Feb 14, 2024 at 02:38:57PM +0100, Krzysztof Kozlowski wrote:
> > > On 14/02/2024 13:54, Johan Hovold wrote:
> > > > On Wed, Feb 14, 2024 at 01:01:20PM +0100, Krzysztof Kozlowski wrote:
> > > >> On 12/02/2024 17:50, Johan Hovold wrote:
> > > >>> Whether the 'msi-map-mask' property is needed or not depends on how the
> > > >>> MSI interrupts are mapped and it should therefore not be described as
> > > >>> required.
> > > >>
> > > >> I could imagine that on all devices the interrupts are mapped in a way
> > > >> you need to provide msi-map-mask. IOW, can there be a Qualcomm platform
> > > >> without msi-map-mask?
> > > > 
> > > > I don't have access to the documentation so I'll leave that for you guys
> > > > to determine. I do note that the downstream DT does not use it and that
> > > > we have a new devicetree in linux-next which also does not have it:
> > > > 
> > > > 	https://lore.kernel.org/r/20240125-topic-sm8650-upstream-pcie-its-v1-1-cb506deeb43e@linaro.org
> > > > 
> > > > But at least the latter looks like an omission that should be fixed.
> > > 
> > > Hm, either that or the mask for sm8450 was not needed as well. Anyway,
> > > thanks for explanation, appreciated!
> > 
> > msi-map-mask is definitely needed as it would allow all the devices under the
> > same bus to reuse the MSI identifier. Currently, excluding this property will
> > not cause any issue since there is a single device under each bus. But we cannot
> > assume that is going to be the case on all boards.
> 
> Are you saying that there is never a use case for an identity mapping?
> Just on Qualcomm hardware or in general?
> 
> It looks like we have a fairly large number of mainline devicetrees that
> do use an identity mapping here (i.e. do not specify 'msi-map-mask') and
> the binding document also has an explicit example of this.
> 
> 	Documentation/devicetree/bindings/pci/pci-msi.txt

I don't know how other platforms supposed to work without this property for more
than one devices. Maybe they were not tested enough?

But for sure, Qcom SoCs require either per device MSI identifier or
msi-map-mask.

- Mani

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