linux-kernel - Re: [PATCH v2] drm/stm: drv: Improve data transfers

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Date:   Tue, 30 Jan 2018 16:58:07 +0200
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Philippe Cornu <philippe.cornu@...com>
Cc:     Yannick Fertre <yannick.fertre@...com>,
        Benjamin Gaignard <benjamin.gaignard@...aro.org>,
        Vincent Abriou <vincent.abriou@...com>,
        David Airlie <airlied@...ux.ie>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Archit Taneja <architt@...eaurora.org>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Fabien Dessenne <fabien.dessenne@...com>,
        Mickael Reulier <mickael.reulier@...com>,
        Ludovic Barre <ludovic.barre@...com>,
        Alexandre Torgue <alexandre.torgue@...com>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>
Subject: Re: [PATCH v2] drm/stm: drv: Improve data transfers

Hi Philippe,

Thank you for the patch.

On Tuesday, 30 January 2018 12:42:00 EET Philippe Cornu wrote:
> To optimize data transfers, align pitch on 128 bytes & height
> on 4 bytes. This optimization is not applicable on hw without MMU.
> 
> Signed-off-by: Yannick Fertre <yannick.fertre@...com>
> Signed-off-by: Vincent Abriou <vincent.abriou@...com>
> Signed-off-by: Philippe Cornu <philippe.cornu@...com>
> ---
> Changes in v2: Rename stm_dumb_create() to stm_gem_cma_dumb_create() and
>   move CONFIG_MMU inside the function following comments from Benjamin
>   Gaignard.
> 
>  drivers/gpu/drm/stm/drv.c | 20 +++++++++++++++++++-
>  1 file changed, 19 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/stm/drv.c b/drivers/gpu/drm/stm/drv.c
> index 8fe954c27fba..8bc7e8418b8d 100644
> --- a/drivers/gpu/drm/stm/drv.c
> +++ b/drivers/gpu/drm/stm/drv.c
> @@ -31,6 +31,24 @@ static const struct drm_mode_config_funcs
> drv_mode_config_funcs = { .atomic_commit = drm_atomic_helper_commit,
>  };
> 
> +static int stm_gem_cma_dumb_create(struct drm_file *file,
> +				   struct drm_device *dev,
> +				   struct drm_mode_create_dumb *args)
> +{
> +#ifdef CONFIG_MMU
> +	unsigned int min_pitch = DIV_ROUND_UP(args->width * args->bpp, 8);
> +
> +	/*
> +	 * in order to optimize data transfer, pitch is aligned on
> +	 * 128 bytes, height is aligned on 4 bytes
> +	 */
> +	args->pitch = roundup(min_pitch, 128);
> +	args->height = roundup(args->height, 4);
> +#endif
> +
> +	return drm_gem_cma_dumb_create_internal(file, dev, args);

In the !CONFIG_MMU case you now bypass the pitch and size calculations 
performed by drm_gem_cma_dumb_create(), allowing userspace to allocate 
arbitrarily large buffers. Is that intentional ?

> +}
> +
>  DEFINE_DRM_GEM_CMA_FOPS(drv_driver_fops);
> 
>  static struct drm_driver drv_driver = {
> @@ -44,7 +62,7 @@ static struct drm_driver drv_driver = {
>  	.minor = 0,
>  	.patchlevel = 0,
>  	.fops = &drv_driver_fops,
> -	.dumb_create = drm_gem_cma_dumb_create,
> +	.dumb_create = stm_gem_cma_dumb_create,
>  	.prime_handle_to_fd = drm_gem_prime_handle_to_fd,
>  	.prime_fd_to_handle = drm_gem_prime_fd_to_handle,
>  	.gem_free_object_unlocked = drm_gem_cma_free_object,

-- 
Regards,

Laurent Pinchart