[PATCH 1/3] drm/sched: add drm_sched_prealloc_dependency_slots v3

[Tvrtko Ursulin]

On 15/05/2025 16:00, Christian König wrote:
> Sometimes drivers need to be able to submit multiple jobs which depend on
> each other to different schedulers at the same time, but using
> drm_sched_job_add_dependency() can't fail any more after the first job is
> initialized.
> 
> This function preallocate memory for dependency slots so that no ENOMEM
> can come later while adding dependencies.
> 
> v2: rework implementation an documentation
> v3: rework from scratch, use separate function to add preallocated deps
> 
> Signed-off-by: Christian König <christian.koenig at amd.com>
> ---
>   drivers/gpu/drm/scheduler/sched_main.c | 45 ++++++++++++++++++++++++++
>   include/drm/gpu_scheduler.h            |  4 +++
>   2 files changed, 49 insertions(+)
> 
> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
> index f7118497e47a..b95e7089aa70 100644
> --- a/drivers/gpu/drm/scheduler/sched_main.c
> +++ b/drivers/gpu/drm/scheduler/sched_main.c
> @@ -858,6 +858,51 @@ void drm_sched_job_arm(struct drm_sched_job *job)
>   }
>   EXPORT_SYMBOL(drm_sched_job_arm);
>   
> +/**
> + * drm_sched_job_prealloc_dependency_slot - avoid ENOMEM on adding dependencies
> + * @job: scheduler job where dependencies will be added
> + * @id: id for the allocated slot
> +  *
> + * Sometimes drivers need to be able to submit multiple jobs which depend on
> + * each other to different schedulers at the same time, but using
> + * drm_sched_job_add_dependency() can't fail any more after the first job is
> + * initialized.
> + *
> + * This function preallocate memory for a dependency slot so that no ENOMEM can
> + * come later while adding dependencies. The index of the preallocated slot is
> + * returned in @id.
> + *
> + * Return:
> + * 0 on success, or an error on failing to expand the array.
> + */
> +int drm_sched_job_prealloc_dependency_slot(struct drm_sched_job *job,
> +					   u32 *id)
> +{
> +	return xa_alloc(&job->dependencies, id, NULL, xa_limit_32b, GFP_KERNEL);
> +}
> +EXPORT_SYMBOL(drm_sched_job_prealloc_dependency_slot);
> +
> +/**
> + * drm_sched_job_add_prealloc_dep - add dependency to preallocated slot
> + * @job: scheduler job where dependencies will be added
> + * @id: the preallocated slot index
> + * @fence: the dependency to add
> + *
> + * Consumes @fence and adds it to the preallocated slot dependency.
> + */
> +void drm_sched_job_add_prealloc_dep(struct drm_sched_job *job, u32 id,
> +				    struct dma_fence *fence)
> +{
> +	fence = xa_store(&job->dependencies, id, fence, GFP_ATOMIC);

Add assert that the passed id exists (was preallocated) and is NULL?

Also, if someone preallocates and does not consume the slot will that 
confuse the iteration in drm_sched_job_dependency()?

Regards,

Tvrtko

> +	/*
> +	 * Be defensive just in case driver messed it up and used preallocated
> +	 * slot twice.
> +	 */
> +	if (WARN_ON(fence))
> +		dma_fence_put(fence);
> +}
> +EXPORT_SYMBOL(drm_sched_job_add_prealloc_dep);
> +
>   /**
>    * drm_sched_job_add_dependency - adds the fence as a job dependency
>    * @job: scheduler job to add the dependencies to
> diff --git a/include/drm/gpu_scheduler.h b/include/drm/gpu_scheduler.h
> index d860db087ea5..0286e0934317 100644
> --- a/include/drm/gpu_scheduler.h
> +++ b/include/drm/gpu_scheduler.h
> @@ -632,6 +632,10 @@ int drm_sched_job_init(struct drm_sched_job *job,
>   		       u32 credits, void *owner);
>   void drm_sched_job_arm(struct drm_sched_job *job);
>   void drm_sched_entity_push_job(struct drm_sched_job *sched_job);
> +int drm_sched_job_prealloc_dependency_slot(struct drm_sched_job *job,
> +					   u32 *id);
> +void drm_sched_job_add_prealloc_dep(struct drm_sched_job *job, u32 id,
> +				    struct dma_fence *fence);
>   int drm_sched_job_add_dependency(struct drm_sched_job *job,
>   				 struct dma_fence *fence);
>   int drm_sched_job_add_syncobj_dependency(struct drm_sched_job *job,