linux-kernel - Re: [PATCH v7 4/4] drm/vc4: Allocate binner bo when starting to use the V3D

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Date:   Thu, 25 Apr 2019 10:42:52 -0700
From:   Eric Anholt <eric@...olt.net>
To:     Paul Kocialkowski <paul.kocialkowski@...tlin.com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Cc:     David Airlie <airlied@...ux.ie>, Daniel Vetter <daniel@...ll.ch>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Eben Upton <eben@...pberrypi.org>,
        Paul Kocialkowski <paul.kocialkowski@...tlin.com>
Subject: Re: [PATCH v7 4/4] drm/vc4: Allocate binner bo when starting to use the V3D

Paul Kocialkowski <paul.kocialkowski@...tlin.com> writes:

> The binner BO is not required until the V3D is in use, so avoid
> allocating it at probe and do it on the first non-dumb BO allocation.
>
> Keep track of which clients are using the V3D and liberate the buffer
> when there is none left, using a kref. Protect the logic with a
> mutex to avoid race conditions.
>
> The binner BO is created at the time of the first render ioctl and is
> destroyed when there is no client and no exec job using it left.
>
> The Out-Of-Memory (OOM) interrupt also gets some tweaking, to avoid
> enabling it before having allocated a binner bo.
>
> We also want to keep the BO alive during runtime suspend/resume to avoid
> failing to allocate it at resume. This happens when the CMA pool is
> full at that point and results in a hard crash.
>
> Signed-off-by: Paul Kocialkowski <paul.kocialkowski@...tlin.com>
> ---
>  drivers/gpu/drm/vc4/vc4_bo.c  | 33 +++++++++++++++++++-
>  drivers/gpu/drm/vc4/vc4_drv.c |  6 ++++
>  drivers/gpu/drm/vc4/vc4_drv.h | 14 +++++++++
>  drivers/gpu/drm/vc4/vc4_gem.c | 13 ++++++++
>  drivers/gpu/drm/vc4/vc4_irq.c | 21 +++++++++----
>  drivers/gpu/drm/vc4/vc4_v3d.c | 58 +++++++++++++++++++++++++++--------
>  6 files changed, 125 insertions(+), 20 deletions(-)
>
> diff --git a/drivers/gpu/drm/vc4/vc4_bo.c b/drivers/gpu/drm/vc4/vc4_bo.c
> index 88ebd681d7eb..2b3ec5926fe2 100644
> --- a/drivers/gpu/drm/vc4/vc4_bo.c
> +++ b/drivers/gpu/drm/vc4/vc4_bo.c
> @@ -799,13 +799,38 @@ vc4_prime_import_sg_table(struct drm_device *dev,
>  	return obj;
>  }
>  
> +static int vc4_grab_bin_bo(struct vc4_dev *vc4, struct vc4_file *vc4file)
> +{
> +	int ret;
> +
> +	if (!vc4->v3d)
> +		return -ENODEV;
> +
> +	if (vc4file->bin_bo_used)
> +		return 0;
> +
> +	ret = vc4_v3d_bin_bo_get(vc4);
> +	if (ret)
> +		return ret;
> +


> +	vc4file->bin_bo_used = true;

I think I found one last race.  Multiple threads could be in an ioctl
trying to grab the bin BO at the same time (while this is only during
app startup, since the fd only needs to get the ref once, it's
particularly plausible given that allocating the bin BO is slow).  I
think if you replace this line with:

	mutex_lock(&vc4->bin_bo_lock);
        if (vc4file->bin_bo_used) {
        	mutex_unlock(&vc4->bin_bo_lock);
                vc4_v3d_bin_bo_put(vc4);
        } else {
        	vc4file->bin_bo_used = true;
        	mutex_unlock(&vc4->bin_bo_lock);
        }

that will be the last change we need.  If you agree with this, feel free
to squash it in and apply the series with:

Reviewed-by: Eric Anholt <eric@...olt.net>

> +
> +	return 0;
> +}
> +
>  int vc4_create_bo_ioctl(struct drm_device *dev, void *data,
>  			struct drm_file *file_priv)
>  {
>  	struct drm_vc4_create_bo *args = data;
> +	struct vc4_file *vc4file = file_priv->driver_priv;
> +	struct vc4_dev *vc4 = to_vc4_dev(dev);
>  	struct vc4_bo *bo = NULL;
>  	int ret;
>  
> +	ret = vc4_grab_bin_bo(vc4, vc4file);
> +	if (ret)
> +		return ret;
> +
>  	/*
>  	 * We can't allocate from the BO cache, because the BOs don't
>  	 * get zeroed, and that might leak data between users.
> @@ -846,6 +871,8 @@ vc4_create_shader_bo_ioctl(struct drm_device *dev, void *data,
>  			   struct drm_file *file_priv)
>  {
>  	struct drm_vc4_create_shader_bo *args = data;
> +	struct vc4_file *vc4file = file_priv->driver_priv;
> +	struct vc4_dev *vc4 = to_vc4_dev(dev);
>  	struct vc4_bo *bo = NULL;
>  	int ret;
>  
> @@ -865,6 +892,10 @@ vc4_create_shader_bo_ioctl(struct drm_device *dev, void *data,
>  		return -EINVAL;
>  	}
>  
> +	ret = vc4_grab_bin_bo(vc4, vc4file);
> +	if (ret)
> +		return ret;
> +
>  	bo = vc4_bo_create(dev, args->size, true, VC4_BO_TYPE_V3D_SHADER);
>  	if (IS_ERR(bo))
>  		return PTR_ERR(bo);
> @@ -894,7 +925,7 @@ vc4_create_shader_bo_ioctl(struct drm_device *dev, void *data,
>  	 */
>  	ret = drm_gem_handle_create(file_priv, &bo->base.base, &args->handle);
>  
> - fail:
> +fail:
>  	drm_gem_object_put_unlocked(&bo->base.base);
>  
>  	return ret;

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