linux-kernel - Re: [PATCH RFT 10/14] drm/msm/a6xx: Stop tracking macrotile

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Message-ID: <c4921bf8-5eb5-458e-8afa-eeb86d5b5f34@oss.qualcomm.com>
Date: Fri, 9 May 2025 14:45:48 +0200
From: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
To: Connor Abbott <cwabbott0@...il.com>,
        Konrad Dybcio <konradybcio@...nel.org>
Cc: Bjorn Andersson <andersson@...nel.org>, Rob Clark <robdclark@...il.com>,
        Abhinav Kumar <quic_abhinavk@...cinc.com>,
        Dmitry Baryshkov <lumag@...nel.org>,
        Akhil P Oommen <quic_akhilpo@...cinc.com>, Sean Paul <sean@...rly.run>,
        David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
        Marijn Suijten <marijn.suijten@...ainline.org>,
        linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, freedreno@...ts.freedesktop.org,
        Konrad Dybcio <konrad.dybcio@....qualcomm.com>
Subject: Re: [PATCH RFT 10/14] drm/msm/a6xx: Stop tracking macrotile_mode
 (again)

On 5/8/25 8:33 PM, Connor Abbott wrote:
> On Thu, May 8, 2025 at 2:14 PM Konrad Dybcio <konradybcio@...nel.org> wrote:
>>
>> From: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
>>
>> SC8180X (A680) and SA8775P (A663) require a write to that register,
>> while other SKUs are fine with the default value. Don't overwrite it
>> needlessly, requiring the developer to read the value back from
>> hardware just to put it in the driver again, introducing much more room
>> for error.
> 
> I'm not sure I understand that last sentence. The original reason I
> always wrote it was that for host image copy we need to know the value
> of macrotile_mode, so again the value exposed to userspace must match
> what's set in the HW. We can't read the value from the HW and send it
> to userspace, because userspace queries this when creating the
> physical device during device enumeration and we really don't want to
> spuriously turn on the device then. That means the safest thing is to
> always program it, guaranteeing that it always matches. Otherwise we
> just have to hope that the default value matches what we expect it to
> be.
> 
> I know you're copying this from kgsl, but kgsl doesn't expose the
> macrotile_mode to userspace. I expect that HIC was added afterwards
> and only works via hacks there (if it's even supported at all on the
> relevant SoCs).

Alright, I think I'll include it in the common UBWC config (even though
it only concerns the GPU), as IIUC it may differ between platforms
implementing the same GPU SKU

Konrad