linux-kernel - Re: [PATCH 1/2] irqchip/qcom-pdc: don't read version register if it is not available

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Date:   Tue, 29 Aug 2023 09:55:38 +0530
From:   "Maulik Shah (mkshah)" <quic_mkshah@...cinc.com>
To:     Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Marc Zyngier <maz@...nel.org>
CC:     Andy Gross <agross@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        <linux-arm-msm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        Neil Armstrong <neil.armstrong@...aro.org>
Subject: Re: [PATCH 1/2] irqchip/qcom-pdc: don't read version register if it
 is not available

Hi,

On 8/28/2023 3:48 PM, Dmitry Baryshkov wrote:
> On Mon, 28 Aug 2023 at 13:04, Marc Zyngier <maz@...nel.org> wrote:
>> On Mon, 28 Aug 2023 10:46:10 +0100,
>> Dmitry Baryshkov <dmitry.baryshkov@...aro.org> wrote:
>>> On Mon, 28 Aug 2023 at 12:36, Maulik Shah (mkshah)
>>> <quic_mkshah@...cinc.com> wrote:
>>>> Hi Dmitry,
>>>>
>>>> This patch may be useful if there was a case where some PDCs don't have
>>>> version register populated/available,
>>>> In all PDC versions, version register is always available but due to reg
>>>> size not good enough in device tree for SM8150 it failed to read.
>>>>
>>>> reg size in device node must be expanded if its too small to access all
>>>> registers and i think
>>>> additional check in driver to check if size is good enough would not be
>>>> of much use.
>>> Unfortunately, it doesn't work this way. DT files are ABI. Even if we
>>> change the DT, the kernel should continue working with the older
>>> version.
>>> Thus, we have to add such bandaid code, which will keep the kernel
>>> from crashing if old DT was used.
>> You're missing the point: all existing PDC HW have version register.
>> The fact that the DT is crap doesn't invalidate this simple fact. It
>> is thus perfectly possible for the driver to *ignore* the crap and do
>> the right thing by expanding the size of the mapping, rather than
>> falling back to the non-versioned code.
> Ah. Interesting idea. If that's the overall consensus I can send v2
> doing this. Not sure what is better though.
if expanding register size and reading version register looks too hacky 
the other way is to have "qcom,pdc-v3.2" compatible for newer targets 
post which don't need to read version register to figure out as the 
compatible string is enough to tell if v3.2 HW behavior needs to apply.

I am ok with either approach but biased towards using version register 
rather than compatibles.

Thanks,
Maulik
>> There is definitely precedents for this sort of behaviour, such as the
>> ARM GICv2 probe code.