linux-kernel - Re: [PATCH] media: mediatek: vcodec: mark vdec_vp9_slice_map_counts_eob

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Message-ID: <20241119110226.dbd54clp46klvjl5@basti-XPS-13-9310>
Date: Tue, 19 Nov 2024 12:02:26 +0100
From: Sebastian Fricke <sebastian.fricke@...labora.com>
To: Nathan Chancellor <nathan@...nel.org>
Cc: Arnd Bergmann <arnd@...nel.org>, Tiffany Lin <tiffany.lin@...iatek.com>,
	Andrew-CT Chen <andrew-ct.chen@...iatek.com>,
	Yunfei Dong <yunfei.dong@...iatek.com>,
	Mauro Carvalho Chehab <mchehab@...nel.org>,
	Matthias Brugger <matthias.bgg@...il.com>,
	AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>,
	Arnd Bergmann <arnd@...db.de>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	Hans Verkuil <hverkuil@...all.nl>,
	Benjamin Gaignard <benjamin.gaignard@...labora.com>,
	linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	linux-mediatek@...ts.infradead.org, llvm@...ts.linux.dev
Subject: Re: [PATCH] media: mediatek: vcodec: mark
 vdec_vp9_slice_map_counts_eob_coef noinline

Hey Nathan,

On 18.11.2024 13:06, Nathan Chancellor wrote:
>On Fri, Oct 18, 2024 at 03:14:42PM +0000, Arnd Bergmann wrote:
>> From: Arnd Bergmann <arnd@...db.de>
>>
>> With KASAN enabled, clang fails to optimize the inline version of
>> vdec_vp9_slice_map_counts_eob_coef() properly, leading to kilobytes
>> of temporary values spilled to the stack:
>>
>> drivers/media/platform/mediatek/vcodec/decoder/vdec/vdec_vp9_req_lat_if.c:1526:12: error: stack frame size (2160) exceeds limit (2048) in 'vdec_vp9_slice_update_prob' [-Werror,-Wframe-larger-than]
>>
>> This seems to affect all versions of clang including the latest (clang-20),
>> but the degree of stack overhead is different per release.
>>
>> Marking the function as noinline_for_stack is harmless here and avoids
>> the problem completely.
>>
>> Signed-off-by: Arnd Bergmann <arnd@...db.de>
>
>Unfortunately, I have seen no moment on my upstream report and this
>warning is breaking allmodconfig builds because of -Werror. Can this be
>applied as a workaround for now (preferrably with a Cc: stable on it)?
>
>Reviewed-by: Nathan Chancellor <nathan@...nel.org>

I'll handle it asap, it will be part of 6.13

Regards,
Sebastian

>
>> ---
>> I have not come to a conclusion on how exactly clang fails to do this
>> right, but can provide the .config and/or preprocessed source files
>> and command line if we think this should be fixed in clang.
>> ---
>>  .../mediatek/vcodec/decoder/vdec/vdec_vp9_req_lat_if.c         | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/media/platform/mediatek/vcodec/decoder/vdec/vdec_vp9_req_lat_if.c b/drivers/media/platform/mediatek/vcodec/decoder/vdec/vdec_vp9_req_lat_if.c
>> index eea709d93820..47c302745c1d 100644
>> --- a/drivers/media/platform/mediatek/vcodec/decoder/vdec/vdec_vp9_req_lat_if.c
>> +++ b/drivers/media/platform/mediatek/vcodec/decoder/vdec/vdec_vp9_req_lat_if.c
>> @@ -1188,7 +1188,8 @@ static int vdec_vp9_slice_setup_lat(struct vdec_vp9_slice_instance *instance,
>>  	return ret;
>>  }
>>
>> -static
>> +/* clang stack usage explodes if this is inlined */
>> +static noinline_for_stack
>>  void vdec_vp9_slice_map_counts_eob_coef(unsigned int i, unsigned int j, unsigned int k,
>>  					struct vdec_vp9_slice_frame_counts *counts,
>>  					struct v4l2_vp9_frame_symbol_counts *counts_helper)
>> --
>> 2.39.5
>>
>