linux-kernel - Re: [PATCH v5] drm/test: add a test suite for GEM objects backed by shmem

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Date: Thu, 22 Feb 2024 07:52:26 -0800
From: Guenter Roeck <linux@...ck-us.net>
To: Marco Pagani <marpagan@...hat.com>
Cc: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 David Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>,
 Sumit Semwal <sumit.semwal@...aro.org>,
 Christian Koenig <christian.koenig@....com>,
 Javier Martinez Canillas <javierm@...hat.com>, linux-kernel@...r.kernel.org,
 dri-devel@...ts.freedesktop.org, linux-media@...r.kernel.org,
 linaro-mm-sig@...ts.linaro.org
Subject: Re: [PATCH v5] drm/test: add a test suite for GEM objects backed by
 shmem

Hi Marco,

On 2/22/24 07:32, Marco Pagani wrote:
> 
> 
> On 2024-02-18 16:49, Guenter Roeck wrote:
>> Hi,
>>
>> On Thu, Nov 30, 2023 at 06:14:16PM +0100, Marco Pagani wrote:
>>> This patch introduces an initial KUnit test suite for GEM objects
>>> backed by shmem buffers.
>>>
>>> Suggested-by: Javier Martinez Canillas <javierm@...hat.com>
>>> Signed-off-by: Marco Pagani <marpagan@...hat.com>
>>
>> When running this in qemu, I get lots of warnings backtraces in the drm
>> core.
>>
>> WARNING: CPU: 0 PID: 1341 at drivers/gpu/drm/drm_gem_shmem_helper.c:327
>> WARNING: CPU: 0 PID: 1341 at drivers/gpu/drm/drm_gem_shmem_helper.c:173
>> WARNING: CPU: 0 PID: 1341 at drivers/gpu/drm/drm_gem_shmem_helper.c:385
>> WARNING: CPU: 0 PID: 1341 at drivers/gpu/drm/drm_gem_shmem_helper.c:211
>> WARNING: CPU: 0 PID: 1345 at kernel/dma/mapping.c:194
>> WARNING: CPU: 0 PID: 1347 at drivers/gpu/drm/drm_gem_shmem_helper.c:429
>> WARNING: CPU: 0 PID: 1349 at drivers/gpu/drm/drm_gem_shmem_helper.c:445
>>
>> It looks like dma_resv_assert_held() asserts each time it is executed.
>> The backtrace in kernel/dma/mapping.c is triggered by
>> 	if (WARN_ON_ONCE(!dev->dma_mask))
>> 		return 0;
>> in __dma_map_sg_attrs().
>>
>> Is this a possible problem in the test code, or can it be caused by
>> some limitations or bugs in the qemu emulation ? If so, do you have any
>> thoughts or ideas what those limitations / bugs might be ?
> 
> Hi Guenter,
> 
> Thanks for reporting this issue. As you correctly noted, the warnings appear to
> be caused by the dma_mask in the mock device being uninitialized. I'll send a
> patch to fix it soon.
> 

Thanks a lot for the update.

In this context, the TTM unit tests fail as well in qemu, with worse result:
It seems there is some bad cleanup after a failed test case, causing list
corruptions in the drm core and ultimately a crash. I don't know if this
is also caused by the missing dma_mask initialization.

Thanks,
Guenter