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: <865y4zfppf.wl-maz@kernel.org>
Date:   Mon, 28 Aug 2023 11:04:12 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
Cc:     "Maulik Shah (mkshah)" <quic_mkshah@...cinc.com>,
        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

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.

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

	M.

-- 
Without deviation from the norm, progress is not possible.