[PATCH v5 6/8] drm/panfrost: Support synchronization jobs

[Steven Price]

On 30/09/2021 20:09, Boris Brezillon wrote:
> Sometimes, all the user wants to do is add a synchronization point.
> Userspace can already do that by submitting a NULL job, but this implies
> submitting something to the GPU when we could simply skip the job and
> signal the done fence directly.
> 
> v5:
> * New patch
> 
> Signed-off-by: Boris Brezillon <boris.brezillon at collabora.com>

I had thought we would be fine without kbase's "dependency only atom"
because we don't have the fan-{in,out} problems that kbase's atoms
produce. But if we're ending up with real hardware NULL jobs then this
is clearly better.

A couple of minor points below, but as far as I can tell this is
functionally correct.

Reviewed-by: Steven Price <steven.price at arm.com>

> ---
>  drivers/gpu/drm/panfrost/panfrost_drv.c | 9 +++++++--
>  drivers/gpu/drm/panfrost/panfrost_job.c | 6 ++++++
>  include/uapi/drm/panfrost_drm.h         | 7 +++++++
>  3 files changed, 20 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panfrost/panfrost_drv.c b/drivers/gpu/drm/panfrost/panfrost_drv.c
> index 30dc158d56e6..89a0c484310c 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_drv.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_drv.c
> @@ -542,7 +542,9 @@ static const struct panfrost_submit_ioctl_version_info submit_versions[] = {
>  	[1] = { 48, 8, 16 },
>  };
>  
> -#define PANFROST_JD_ALLOWED_REQS PANFROST_JD_REQ_FS
> +#define PANFROST_JD_ALLOWED_REQS \
> +	(PANFROST_JD_REQ_FS | \
> +	 PANFROST_JD_REQ_DEP_ONLY)
>  
>  static int
>  panfrost_submit_job(struct drm_device *dev, struct drm_file *file_priv,
> @@ -559,7 +561,10 @@ panfrost_submit_job(struct drm_device *dev, struct drm_file *file_priv,
>  	if (args->requirements & ~PANFROST_JD_ALLOWED_REQS)
>  		return -EINVAL;
>  
> -	if (!args->head)
> +	/* If this is a dependency-only job, the job chain head should be NULL,
> +	 * otherwise it should be non-NULL.
> +	 */
> +	if ((args->head != 0) != !(args->requirements & PANFROST_JD_REQ_DEP_ONLY))

NIT: There's confusion over NULL vs 0 here - the code is correct
(args->head is a u64 and not a pointer for a kernel) but the comment
makes it seem like it should be a pointer.

We could side-step the mismatch by rewriting as below:

  !args->head == !(args->requirements & PANFROST_JD_REQ_DEP_ONLY)

Although I'm not convinced whether that's more readable or not!

>  		return -EINVAL;
>  
>  	bo_stride = submit_versions[version].bo_ref_stride;
> diff --git a/drivers/gpu/drm/panfrost/panfrost_job.c b/drivers/gpu/drm/panfrost/panfrost_job.c
> index 0367cee8f6df..6d8706d4a096 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_job.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_job.c
> @@ -192,6 +192,12 @@ static void panfrost_job_hw_submit(struct panfrost_job *job, int js)
>  	u64 jc_head = job->jc;
>  	int ret;
>  
> +	if (job->requirements & PANFROST_JD_REQ_DEP_ONLY) {
> +		/* Nothing to execute, signal the fence directly. */
> +		dma_fence_signal_locked(job->done_fence);
> +		return;
> +	}
> +

It took me a while to convince myself that the reference counting for
the PM reference is correct. Before panfrost_job_hw_submit() always
returned with an extra reference, but now we have a case which doesn't.
AFAICT this is probably fine because we dereference on the path where
the hardware has completed the job (which obviously won't happen here).
But I'm still a bit uneasy whether the reference counts are always correct.

Steve

>  	panfrost_devfreq_record_busy(&pfdev->pfdevfreq);
>  
>  	ret = pm_runtime_get_sync(pfdev->dev);
> diff --git a/include/uapi/drm/panfrost_drm.h b/include/uapi/drm/panfrost_drm.h
> index 5e3f8a344f41..b9df066970f6 100644
> --- a/include/uapi/drm/panfrost_drm.h
> +++ b/include/uapi/drm/panfrost_drm.h
> @@ -46,6 +46,13 @@ extern "C" {
>  #define DRM_IOCTL_PANFROST_PERFCNT_DUMP		DRM_IOW(DRM_COMMAND_BASE + DRM_PANFROST_PERFCNT_DUMP, struct drm_panfrost_perfcnt_dump)
>  
>  #define PANFROST_JD_REQ_FS (1 << 0)
> +
> +/*
> + * Dependency only job. The job chain head should be set to 0 when this flag
> + * is set.
> + */
> +#define PANFROST_JD_REQ_DEP_ONLY (1 << 1)
> +
>  /**
>   * struct drm_panfrost_submit - ioctl argument for submitting commands to the 3D
>   * engine.
>