linux-kernel - Re: [PATCH] dt-bindings: qcom: geni-se: Fix '#address-cells' & '#size-cells' related dt-binding error

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Date:   Wed, 18 Jan 2023 21:23:32 -0600
From:   Bjorn Andersson <andersson@...nel.org>
To:     Bhupesh Sharma <bhupesh.sharma@...aro.org>
Cc:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        linux-arm-msm@...r.kernel.org, devicetree@...r.kernel.org,
        agross@...nel.org, konrad.dybcio@...aro.org,
        linux-kernel@...r.kernel.org, bhupesh.linux@...il.com,
        robh+dt@...nel.org, krzysztof.kozlowski+dt@...aro.org
Subject: Re: [PATCH] dt-bindings: qcom: geni-se: Fix '#address-cells' &
 '#size-cells' related dt-binding error

On Mon, Jan 16, 2023 at 09:13:12PM +0530, Bhupesh Sharma wrote:
> On Mon, 16 Jan 2023 at 13:23, Krzysztof Kozlowski
> <krzysztof.kozlowski@...aro.org> wrote:
> >
> > On 15/01/2023 22:33, Bhupesh Sharma wrote:
> > > On Sun, 15 Jan 2023 at 20:57, Krzysztof Kozlowski
> > > <krzysztof.kozlowski@...aro.org> wrote:
> > >>
> > >> On 13/01/2023 21:10, Bhupesh Sharma wrote:
> > >>> Fix the following '#address-cells' & '#size-cells' related
> > >>> dt-binding error:
> > >>>
> > >>>    $ make dtbs_check
> > >>>
> > >>>    From schema: Documentation/devicetree/bindings/soc/qcom/qcom,geni-se.yaml
> > >>>         arch/arm64/boot/dts/qcom/sm4250-oneplus-billie2.dtb: geniqup@...0000:
> > >>>               #address-cells:0:0: 2 was expected
> > >>>       From schema: Documentation/devicetree/bindings/soc/qcom/qcom,geni-se.yaml
> > >>
> > >> Don't we want rather to unify the soc address range?
> > >
> > > Well, the assumption in the original dt-bindings was that every reg
> > > variable is 4 * u32 wide (as most new qcom SoCs set #address- and
> > > #size-cells to <2>). However, that is not the case for all of the
> > > SoCs.
> >
> > Hm, which device of that SoC cannot be used with address/size cells 2?
> 
> As noted in the git log already the geniqup on sm6115 / sm4250 cannot
> be used with address/size cells 2 (See:
> https://github.com/torvalds/linux/blob/master/arch/arm64/boot/dts/qcom/sm6115.dtsi#L795)
> 

I'm not able to find the reasoning you're referring to. We do have cells
of 2 for these nodes on all other platforms.  If there is a specific
problem, that can be documented and you can probably use ranges to
reduce keep the cells of 1 in the geni wrappers.


The reason why we have cells = 2 on most platforms is because the SMMU
reports that it's capable of more address bits than the buses will
handle. So without cells = 2, we can't describe dma-ranges appropriately
and you get page faults due to truncated addresses on the bus when the
iommu iova has been picking addresses for you.

Regards,
Bjorn

> > > So, ideally we shouldn't set the  "#address-cells" and  "#size-cells":
> > > as const: 2 in the bindings.
> > >
> > > See as an example:
> > > https://www.kernel.org/doc/Documentation/devicetree/bindings/usb/usb-device.yaml
> >
> >
> > How USB device - so entirely different device, not MMIO! - is related here?
> >
> > Best regards,
> > Krzysztof
> >