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

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Message-ID: <20200907181855.GE2352366@phenom.ffwll.local>
Date:   Mon, 7 Sep 2020 20:18:55 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:     Stefan Agner <stefan@...er.ch>, 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

On Mon, Sep 07, 2020 at 07:17:12PM +0300, Laurent Pinchart wrote:
> 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 ?

Yeah this should be verified at addfb time. We try to validate as early as
possible.
-Daniel

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

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch