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

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Message-ID: <8876d95c-e7ea-478a-bb88-b97e5cf67f08@linaro.org>
Date:   Mon, 28 Aug 2023 12:20:09 +0200
From:   Konrad Dybcio <konrad.dybcio@...aro.org>
To:     Marc Zyngier <maz@...nel.org>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
Cc:     "Maulik Shah (mkshah)" <quic_mkshah@...cinc.com>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <andersson@...nel.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

On 28.08.2023 12:04, Marc Zyngier 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.
I wasn't sure if this would cross the "too hacky" boundary, but if
you're fine with it, I'll happily ack this approach.

Konrad