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

[Guenter Roeck]

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@xxxxxxxxxx>
> Signed-off-by: Marco Pagani <marpagan@xxxxxxxxxx>

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 ? 

Thanks,
Guenter