Re: [PATCH v8 2/3] drm: Add drm_user_fence helper

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Low] Unconditional use of mmput_async() breaks compilation and linking on NOMMU configurations.
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commit 890c55fa04660dfbfe7364ac5c78b52162fca15b
Author: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>

drm: Add drm_user_fence helper

This commit introduces a common DRM user fence helper providing a
kref-managed, MM-borrowing dma-fence-callback-to-workqueue pattern.
It is used by drivers that must access userspace memory from a kthread
context when a GPU fence signals.

> diff --git a/drivers/gpu/drm/Makefile b/drivers/gpu/drm/Makefile
> index 8895ca0476d89..3557e6a8d31c9 100644
> --- a/drivers/gpu/drm/Makefile
> +++ b/drivers/gpu/drm/Makefile
> @@ -69,6 +69,7 @@ drm-y := \
>  	drm_syncobj.o \
>  	drm_sysfs.o \
>  	drm_trace_points.o \
> +	drm_user_fence.o \
>  	drm_vblank.o \

[ ... ]

> diff --git a/drivers/gpu/drm/drm_user_fence.c b/drivers/gpu/drm/drm_user_fence.c
> new file mode 100644
> index 0000000000000..f7882208a8b73
> --- /dev/null
> +++ b/drivers/gpu/drm/drm_user_fence.c

[ ... ]

> +static void drm_user_fence_do_work(struct drm_work_fence *wfence)
> +{
> +	struct drm_user_fence *ufence =
> +		container_of(wfence, struct drm_user_fence, base);
> +	struct mm_struct *mm = NULL;
> +
> +	if (mmget_not_zero(ufence->mm)) {
> +		mm = ufence->mm;
> +		kthread_use_mm(mm);
> +	}
> +
> +	ufence->ops->writeback(ufence, !!mm);
> +
> +	if (mm) {
> +		kthread_unuse_mm(mm);
> +		mmput_async(mm);	/* requires CONFIG_MMU — GPU requires MMU */

[Severity: Low]
Does this unconditional call to mmput_async() break the build on systems
without CONFIG_MMU?

While the comment notes that the GPU requires an MMU, the drm_user_fence.o
file is added unconditionally to the core DRM build in
drivers/gpu/drm/Makefile. If the kernel is built with CONFIG_DRM enabled
but CONFIG_MMU disabled, this will result in an implicit function
declaration and an undefined reference to mmput_async() during linking.

> +	}
> +}

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Sashiko AI review · https://sashiko.dev/#/patchset/20260908084627.352535-1-srinivasan.shanmugam@amd.com?part=2