Re: [PATCH v2 1/4] drm/vc4: Use DRM Execution Contexts

[Christian König <christian.koenig@xxxxxxx>]

Am 17.12.24 um 12:09 schrieb Maíra Canal:
> Hi Christian,
>
> On 17/12/24 07:30, Christian König wrote:
> > Am 16.12.24 um 20:08 schrieb Melissa Wen:
> > > On 12/12, Maíra Canal wrote:
> > > > VC4 has internal copies of `drm_gem_lock_reservations()` and
> > > > `drm_gem_unlock_reservations()` inside the driver and ideally, we 
> > > > should
> > > > move those hard-coded functions to the generic functions provided 
> > > > by DRM.
> > > > But, instead of using the DRM GEM functions to (un)lock 
> > > > reservations, move
> > > > the new DRM Execution Contexts API.
> > > >
> > > > Signed-off-by: Maíra Canal <mcanal@xxxxxxxxxx>
> > > > ---
> > > >   drivers/gpu/drm/vc4/Kconfig   |  1 +
> > > >   drivers/gpu/drm/vc4/vc4_gem.c | 99 
> > > > ++++++++---------------------------
> > > >   2 files changed, 22 insertions(+), 78 deletions(-)
>
> [...]
>
> > > > -
> > > > -    ww_acquire_done(acquire_ctx);
> > > > +    int ret;
> > > >       /* Reserve space for our shared (read-only) fence references,
> > > >        * before we commit the CL to the hardware.
> > > >        */
> > > > -    for (i = 0; i < exec->bo_count; i++) {
> > > > -        bo = exec->bo[i];
> > > > +    drm_exec_init(exec_ctx, DRM_EXEC_INTERRUPTIBLE_WAIT, exec- 
> > > > >bo_count);
> > > > +    drm_exec_until_all_locked(exec_ctx) {
> > > > +        ret = drm_exec_prepare_array(exec_ctx, exec->bo,
> > > > +                         exec->bo_count, 1);
> > > Hi Maíra,
> > >
> > > So, I'm not familiar too drm_exec, but the original implementation of
> > > vc4_lock_bo_reservations() has a retry of locks from the contention and
> > > I don't see it inside the drm_exec_prepare_array(), why don't use the
> > > loop drm_exec_prepare_obj() plus drm_exec_retry_on_contention() 
> > > (similar
> > > to the typical usage documented for drm_exec)?
> >
> > The way how drm_exec and drm_exec_prepare_array is used seems to be 
> > correct here.
> >
> > drm_exec_prepare_array() basically just loops over all the GEM BOs in 
> > the array and calls drm_exec_prepare_obj() on them, so no need to 
> > open code that.
> >
> > drm_exec_retry_on_contention() is only needed if you have multiple 
> > calls to drm_exec_prepare_array() or drm_exec_prepare_obj(), so that 
> > the loop is restarted in between the calls.
>
> But doesn't `drm_exec_prepare_array()` have multiple calls to
> `drm_exec_prepare_obj()`? The fact that the multiple calls are wrapped
> in the function makes a difference?

Yeah. I know, it's not so easy to understand :)

What drm_exec_until_all_locked() and drm_exec_retry_on_contention() are 
basically doing is nicely wrapped error handling.

In other words drm_exec_retry_on_contention() does a "goto 
*__drm_exec_retry_ptr" if it detects that we have a contention. But you 
can't goto from a label in a function back into the caller.

So what drm_exec_prepare_array() does is to abort as soon as it sees the 
first error:

                ret = drm_exec_prepare_obj(exec, objects[i], num_fences);
                if (unlikely(ret))
                        return ret;


And in the caller we have:

drm_exec_until_all_locked(exec_ctx) {
    ret = drm_exec_prepare_array(exec_ctx, exec->bo, exec->bo_count, 1);
}

So the loop restarts after drm_exec_prepare_array() anyway and because 
of this using drm_exec_retry_on_contention() is not explicitly necessary.

Regards,
Christian.

> Best Regards,
> - Maíra
>
> > Regards,
> > Christian.
> >
> > > Also, probably you already considered that: we can extend this 
> > > update to
> > > v3d, right?
> > >
> > > Melissa
> > >
> > > > +    }
> > > > -        ret = dma_resv_reserve_fences(bo->resv, 1);
> > > > -        if (ret) {
> > > > -            vc4_unlock_bo_reservations(dev, exec, acquire_ctx);
> > > > -            return ret;
> > > > -        }
> > > > +    if (ret) {
> > > > +        drm_exec_fini(exec_ctx);
> > > > +        return ret;
> > > >       }
> > > >       return 0;
> > > > @@ -679,7 +620,7 @@ vc4_lock_bo_reservations(struct drm_device *dev,
> > > >    */
> > > >   static int
> > > >   vc4_queue_submit(struct drm_device *dev, struct vc4_exec_info *exec,
> > > > -         struct ww_acquire_ctx *acquire_ctx,
> > > > +         struct drm_exec *exec_ctx,
> > > >            struct drm_syncobj *out_sync)
> > > >   {
> > > >       struct vc4_dev *vc4 = to_vc4_dev(dev);
> > > > @@ -708,7 +649,7 @@ vc4_queue_submit(struct drm_device *dev, struct 
> > > > vc4_exec_info *exec,
> > > >       vc4_update_bo_seqnos(exec, seqno);
> > > > -    vc4_unlock_bo_reservations(dev, exec, acquire_ctx);
> > > > +    drm_exec_fini(exec_ctx);
> > > >       list_add_tail(&exec->head, &vc4->bin_job_list);
> > > > @@ -1123,7 +1064,7 @@ vc4_submit_cl_ioctl(struct drm_device *dev, 
> > > > void *data,
> > > >       struct drm_vc4_submit_cl *args = data;
> > > >       struct drm_syncobj *out_sync = NULL;
> > > >       struct vc4_exec_info *exec;
> > > > -    struct ww_acquire_ctx acquire_ctx;
> > > > +    struct drm_exec exec_ctx;
> > > >       struct dma_fence *in_fence;
> > > >       int ret = 0;
> > > > @@ -1216,7 +1157,7 @@ vc4_submit_cl_ioctl(struct drm_device *dev, 
> > > > void *data,
> > > >       if (ret)
> > > >           goto fail;
> > > > -    ret = vc4_lock_bo_reservations(dev, exec, &acquire_ctx);
> > > > +    ret = vc4_lock_bo_reservations(exec, &exec_ctx);
> > > >       if (ret)
> > > >           goto fail;
> > > > @@ -1224,7 +1165,7 @@ vc4_submit_cl_ioctl(struct drm_device *dev, 
> > > > void *data,
> > > >           out_sync = drm_syncobj_find(file_priv, args->out_sync);
> > > >           if (!out_sync) {
> > > >               ret = -EINVAL;
> > > > -            goto fail;
> > > > +            goto fail_unreserve;
> > > >           }
> > > >           /* We replace the fence in out_sync in vc4_queue_submit 
> > > > since
> > > > @@ -1239,7 +1180,7 @@ vc4_submit_cl_ioctl(struct drm_device *dev, 
> > > > void *data,
> > > >        */
> > > >       exec->args = NULL;
> > > > -    ret = vc4_queue_submit(dev, exec, &acquire_ctx, out_sync);
> > > > +    ret = vc4_queue_submit(dev, exec, &exec_ctx, out_sync);
> > > >       /* The syncobj isn't part of the exec data and we need to 
> > > > free our
> > > >        * reference even if job submission failed.
> > > > @@ -1248,13 +1189,15 @@ vc4_submit_cl_ioctl(struct drm_device *dev, 
> > > > void *data,
> > > >           drm_syncobj_put(out_sync);
> > > >       if (ret)
> > > > -        goto fail;
> > > > +        goto fail_unreserve;
> > > >       /* Return the seqno for our job. */
> > > >       args->seqno = vc4->emit_seqno;
> > > >       return 0;
> > > > +fail_unreserve:
> > > > +    drm_exec_fini(&exec_ctx);
> > > >   fail:
> > > >       vc4_complete_exec(&vc4->base, exec);
> > > > --
> > > > 2.47.1