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

[Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>]

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@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)

Konrad