linux-kernel - Re: [PATCH RFT 04/14] drm/msm/a6xx: Get a handle to the common UBWC config

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Message-ID: <CAF6AEGuDBdmyS+N4pR3gEYz0mSLkKqYZjCASk8am2atdGKq5UA@mail.gmail.com>
Date: Tue, 13 May 2025 15:06:18 -0700
From: Rob Clark <robdclark@...il.com>
To: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
Cc: Konrad Dybcio <konradybcio@...nel.org>, Bjorn Andersson <andersson@...nel.org>, 
	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
Subject: Re: [PATCH RFT 04/14] drm/msm/a6xx: Get a handle to the common UBWC config

On Fri, May 9, 2025 at 10:00 AM Konrad Dybcio
<konrad.dybcio@....qualcomm.com> wrote:
>
> On 5/9/25 3:52 PM, Rob Clark wrote:
> > On Fri, May 9, 2025 at 5:31 AM Konrad Dybcio
> > <konrad.dybcio@....qualcomm.com> wrote:
> >>
> >> On 5/8/25 8:41 PM, Rob Clark wrote:
> >>> On Thu, May 8, 2025 at 11:13 AM Konrad Dybcio <konradybcio@...nel.org> wrote:
> >>>>
> >>>> From: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
> >>>>
> >>>> Start the great despaghettification by getting a pointer to the common
> >>>> UBWC configuration, which houses e.g. UBWC versions that we need to
> >>>> make decisions.
> >>>>
> >>>> Signed-off-by: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
> >>>> ---
> >>>>  drivers/gpu/drm/msm/adreno/a6xx_gpu.c   | 16 ++++++++++++++--
> >>>>  drivers/gpu/drm/msm/adreno/adreno_gpu.c |  6 ++++++
> >>>>  drivers/gpu/drm/msm/adreno/adreno_gpu.h |  3 +++
> >>>>  3 files changed, 23 insertions(+), 2 deletions(-)
> >>>>
> >>>> diff --git a/drivers/gpu/drm/msm/adreno/a6xx_gpu.c b/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
> >>>> index b161b5cd991fc645dfcd69754b82be9691775ffe..89eb725f0950f3679d6214366cfbd22d5bcf4bc7 100644
> >>>> --- a/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
> >>>> +++ b/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
> >>>> @@ -585,8 +585,13 @@ static void a6xx_set_cp_protect(struct msm_gpu *gpu)
> >>>>         gpu_write(gpu, REG_A6XX_CP_PROTECT(protect->count_max - 1), protect->regs[i]);
> >>>>  }
> >>>>
> >>>> -static void a6xx_calc_ubwc_config(struct adreno_gpu *gpu)
> >>>> +static int a6xx_calc_ubwc_config(struct adreno_gpu *gpu)
> >>>>  {
> >>>> +       /* Inherit the common config and make some necessary fixups */
> >>>> +       gpu->common_ubwc_cfg = qcom_ubwc_config_get_data();
> >>>
> >>> This does look a bit funny given the devm_kzalloc() below.. I guess
> >>> just so that the ptr is never NULL?
> >>
> >> Yeah, would you prefer this is changed?
> >
> > I think having an all zeros ubwc cfg isn't really going to work
> > anyways, so probably drop the kzalloc().  Or if there is a case that
> > I'm not thinking of offhand where it makes sense to have an all 0's
> > cfg, then add a comment to avoid future head scratching, since
> > otherwise it looks like a bug to be fixed.
>
> So my own lack of comments bit me.
>
> Without the allocation this will fall apart badly..
> I added this hunk:
>
> ---------------------
> /* Inherit the common config and make some necessary fixups */
> common_cfg = if (IS_ERR(common_cfg))
>         return ERR_PTR(-EINVAL);
>
> *adreno_gpu->ubwc_config = *common_cfg;
> ---------------------
>
> to get the common data but take away the const qualifier.. because
> we still override some HBB values and we can't yet fully trust the
> common config, as the smem getter is not yet plumbed up.

So I get that common_ubwc_cfg is the const thing without fixups (and
agree that it should say const), and ubwc_config is the fixed up
thing.  But don't see how that necessitates the zeroalloc.  Couldn't
you just:


  if (!IS_ERR_OR_NULL(adreno_gpu->common_ubwc_cfg)
    adreno_gpu->ubwc_config = *adreno_gpu->common_ubwc_cfg;

> I can add a commit discarding all the HBB overrides (matching or not)
> or we can keep the zeroalloc around for some time (i'd rather keep
> the function returning const so that when things are ready nobody gets
> to poke at the source of *truth*)

We can keep the overrides to start (although the goal should be to
remove them).. but qcom_ubwc_config_get_data() not finding anything
seems like more or less a fatal condition.

BR,
-R