[PATCH 1/2] drm/sched: add drm_sched_prealloc_dependency_slots

[Philipp Stanner]

On Tue, 2025-03-18 at 13:03 +0100, Christian König wrote:
> The problem is that drivers sometimes need to add dependencies
> without
> allocating any memory.
> 
> Add a function which preallocates slots by inserting signaled stub
> fences
> into the dependency array.

I think I get what you're doing here, but it would certainly be helpful
to provide some more justification in the commit message a la: "Some
drivers have to [...]. With drm_sched_job_add_dependency() that's not
possible because [...]"

> 
> Signed-off-by: Christian König <christian.koenig at amd.com>
> ---
>  drivers/gpu/drm/scheduler/sched_main.c | 41
> ++++++++++++++++++++++++--
>  include/drm/gpu_scheduler.h            |  2 ++
>  2 files changed, 40 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/scheduler/sched_main.c
> b/drivers/gpu/drm/scheduler/sched_main.c
> index 4d4219fbe49d..2085eea89513 100644
> --- a/drivers/gpu/drm/scheduler/sched_main.c
> +++ b/drivers/gpu/drm/scheduler/sched_main.c
> @@ -852,6 +852,38 @@ void drm_sched_job_arm(struct drm_sched_job
> *job)
>  }
>  EXPORT_SYMBOL(drm_sched_job_arm);
>  
> +/**
> + * drm_sched_job_prealloc_dependency_slots - avoid ENOMEM on adding
> dependencies
> + * @job: scheduler job where dependencies will be added
> + * @num_deps: number of dependencies to preallocate slots for
> + *
> + * Preallocate memory so that no ENOMEM can come later while adding
> + * dependencies.

Similarly, should have a sentence that clarifies for when this is
needed.

> + *
> + * Return:
> + * 0 on success, or an error on failing to expand the array.
> + */
> +int drm_sched_job_prealloc_dependency_slots(struct drm_sched_job
> *job,
> +					    unsigned int num_deps)
> +{
> +	struct dma_fence *fence;
> +	u32 id = 0;
> +	int ret;
> +
> +	while (num_deps--) {
> +		fence = dma_fence_get_stub();
> +		ret = xa_alloc(&job->dependencies, &id, fence,
> xa_limit_32b,
> +			       GFP_KERNEL);

So this would fill the xarr with already signaled fences which then
later will be replaced with unsignaled fences?

Help me out here: would it also work to add NULL instead of that stub-
fence?


> +		if (ret != 0) {

btw. style should be consistent with the while() above where you ommit
'> 0'.

I personally prefer having the comparison operators, but implicitly
checking for 0 is well established in the kernel, so I guess both is
OK.

> +			dma_fence_put(fence);
> +			return ret;
> +		}
> +	}
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL(drm_sched_job_prealloc_dependency_slots);
> +
>  /**
>   * drm_sched_job_add_dependency - adds the fence as a job dependency
>   * @job: scheduler job to add the dependencies to
> @@ -878,10 +910,12 @@ int drm_sched_job_add_dependency(struct
> drm_sched_job *job,
>  	 * engines involved, rather than the number of BOs.
>  	 */
>  	xa_for_each(&job->dependencies, index, entry) {
> -		if (entry->context != fence->context)
> +		bool signaled = dma_fence_is_signaled(entry);
> +
> +		if (!signaled && entry->context != fence->context)
>  			continue;
>  
> -		if (dma_fence_is_later(fence, entry)) {
> +		if (signaled || dma_fence_is_later(fence, entry)) {
>  			dma_fence_put(entry);
>  			xa_store(&job->dependencies, index, fence,
> GFP_KERNEL);
>  		} else {
> @@ -890,7 +924,8 @@ int drm_sched_job_add_dependency(struct
> drm_sched_job *job,
>  		return 0;
>  	}
>  
> -	ret = xa_alloc(&job->dependencies, &id, fence, xa_limit_32b,
> GFP_KERNEL);
> +	ret = xa_alloc(&job->dependencies, &id, fence, xa_limit_32b,
> +		       GFP_KERNEL);

Would leave that unchanged for git-blame.


Thx
P.

>  	if (ret != 0)
>  		dma_fence_put(fence);
>  
> diff --git a/include/drm/gpu_scheduler.h
> b/include/drm/gpu_scheduler.h
> index 1a7e377d4cbb..916e820b27ff 100644
> --- a/include/drm/gpu_scheduler.h
> +++ b/include/drm/gpu_scheduler.h
> @@ -632,6 +632,8 @@ 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_slots(struct drm_sched_job
> *job,
> +					    unsigned int num_deps);
>  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,