linux-kernel - Re: [PATCH] drm: mxsfb: check framebuffer pitch

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Date:   Mon, 7 Sep 2020 19:17:12 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Stefan Agner <stefan@...er.ch>
Cc:     marex@...x.de, airlied@...ux.ie, daniel@...ll.ch,
        shawnguo@...nel.org, s.hauer@...gutronix.de, kernel@...gutronix.de,
        festevam@...il.com, linux-imx@....com,
        dri-devel@...ts.freedesktop.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm: mxsfb: check framebuffer pitch

Hi Stefan,

Thank you for the patch.

On Mon, Sep 07, 2020 at 06:03:43PM +0200, Stefan Agner wrote:
> The lcdif IP does not support a framebuffer pitch (stride) other than
> the CRTC width. Check for equality and reject the state otherwise.
> 
> This prevents a distorted picture when using 640x800 and running the
> Mesa graphics stack. Mesa tires to use a cache aligned stride, which

s/tires/tries/

> leads at that particular resolution to width != stride. Currently
> Mesa has no fallback behavior, but rejecting this configuration allows
> userspace to handle the issue correctly.

I'm increasingly impressed by how featureful this IP core is :-)

> Signed-off-by: Stefan Agner <stefan@...er.ch>
> ---
>  drivers/gpu/drm/mxsfb/mxsfb_kms.c | 22 ++++++++++++++++++----
>  1 file changed, 18 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/mxsfb/mxsfb_kms.c b/drivers/gpu/drm/mxsfb/mxsfb_kms.c
> index b721b8b262ce..79aa14027f91 100644
> --- a/drivers/gpu/drm/mxsfb/mxsfb_kms.c
> +++ b/drivers/gpu/drm/mxsfb/mxsfb_kms.c
> @@ -403,14 +403,28 @@ static int mxsfb_plane_atomic_check(struct drm_plane *plane,
>  {
>  	struct mxsfb_drm_private *mxsfb = to_mxsfb_drm_private(plane->dev);
>  	struct drm_crtc_state *crtc_state;
> +	unsigned int pitch;
> +	int ret;
>  
>  	crtc_state = drm_atomic_get_new_crtc_state(plane_state->state,
>  						   &mxsfb->crtc);
>  
> -	return drm_atomic_helper_check_plane_state(plane_state, crtc_state,
> -						   DRM_PLANE_HELPER_NO_SCALING,
> -						   DRM_PLANE_HELPER_NO_SCALING,
> -						   false, true);
> +	ret = drm_atomic_helper_check_plane_state(plane_state, crtc_state,
> +						  DRM_PLANE_HELPER_NO_SCALING,
> +						  DRM_PLANE_HELPER_NO_SCALING,
> +						  false, true);
> +	if (ret || !plane_state->visible)

Would it be more explict to check for !plane_state->fb ? Otherwise I'll
have to verify that !fb always implies !visible :-)

> +		return ret;
> +
> +	pitch = crtc_state->mode.hdisplay *
> +		plane_state->fb->format->cpp[0];

This holds on a single line.

> +	if (plane_state->fb->pitches[0] != pitch) {
> +		dev_err(plane->dev->dev,
> +			"Invalid pitch: fb and crtc widths must be the same");

I'd turn this into a dev_dbg(), printing error messages to the kernel
log in response to user-triggered conditions is a bit too verbose and
could flood the log.

Wouldn't it be best to catch this issue when creating the framebuffer ?

> +		return -EINVAL;
> +	}
> +
> +	return 0;
>  }
>  
>  static void mxsfb_plane_primary_atomic_update(struct drm_plane *plane,

-- 
Regards,

Laurent Pinchart