[PATCH] drm/msm/a6xx: Fix excessive stack usage

[Dmitry Baryshkov]

On Mon, Oct 28, 2024 at 11:39:16AM +0100, Konrad Dybcio wrote:
> On 28.10.2024 10:52 AM, Akhil P Oommen wrote:
> > On 10/28/2024 12:13 AM, Arnd Bergmann wrote:
> >> On Sun, Oct 27, 2024, at 18:05, Akhil P Oommen wrote:
> >>> Clang-19 and above sometimes end up with multiple copies of the large
> >>> a6xx_hfi_msg_bw_table structure on the stack. The problem is that
> >>> a6xx_hfi_send_bw_table() calls a number of device specific functions to
> >>> fill the structure, but these create another copy of the structure on
> >>> the stack which gets copied to the first.
> >>>
> >>> If the functions get inlined, that busts the warning limit:
> >>>
> >>> drivers/gpu/drm/msm/adreno/a6xx_hfi.c:631:12: error: stack frame size 
> >>> (1032) exceeds limit (1024) in 'a6xx_hfi_send_bw_table' 
> >>> [-Werror,-Wframe-larger-than]
> >>>
> >>> Fix this by kmalloc-ating struct a6xx_hfi_msg_bw_table instead of using
> >>> the stack. Also, use this opportunity to skip re-initializing this table
> >>> to optimize gpu wake up latency.
> >>>
> >>> Cc: Arnd Bergmann <arnd at kernel.org>
> >>
> >> Please change this to "Reported-by:"
> > 
> > Sure.
> > 
> >>
> >> The patch looks correct to me, just one idea for improvement.
> >>
> >>> b/drivers/gpu/drm/msm/adreno/a6xx_gmu.h
> >>> index 94b6c5cab6f4..b4a79f88ccf4 100644
> >>> --- a/drivers/gpu/drm/msm/adreno/a6xx_gmu.h
> >>> +++ b/drivers/gpu/drm/msm/adreno/a6xx_gmu.h
> >>> @@ -99,6 +99,7 @@ struct a6xx_gmu {
> >>>  	struct completion pd_gate;
> >>>
> >>>  	struct qmp *qmp;
> >>> +	struct a6xx_hfi_msg_bw_table *bw_table;
> >>>  };
> >>
> >> I think the bw_table is better just embedded
> >> in here rather than referenced as a pointer:
> >>
> > There are some low tier chipsets with relatively lower RAM size that
> > doesn't require this table. So, dynamically allocating this here helps
> > to save 640 bytes (minus the overhead of tracking).
> 
> I'd second this, said chipsets often ship with 1-2 GiB of RAM (which
> is still a lot in comparison, but you know.. every little bit counts)

Okay from my side. Yeah, poor Gnome runnning on top of 1 GiB device is
very sad.

-- 
With best wishes
Dmitry