Re: [PATCH] drm: mxsfb: check framebuffer pitch

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Tue, Sep 08, 2020 at 03:33:04PM +0300, Laurent Pinchart wrote:
> On Tue, Sep 08, 2020 at 02:29:02PM +0200, Daniel Vetter wrote:
> > On Tue, Sep 8, 2020 at 2:07 PM Stefan Agner <stefan@agner.ch> wrote:
> > > On 2020-09-08 10:48, Daniel Vetter wrote:
> > >> On Tue, Sep 08, 2020 at 11:18:25AM +0300, Tomi Valkeinen wrote:
> > >>> On 08/09/2020 10:55, Stefan Agner wrote:
> > >>>> On 2020-09-07 20:18, Daniel Vetter wrote:
> > >>>>> 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@agner.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
> > >>>>>
> > >>>>
> > >>>> Sounds sensible. From what I can tell fb_create is the proper callback
> > >>>> to implement this at addfb time. Will give this a try.
> > >>>>
> > >>>> FWIW, I got the idea from drivers/gpu/drm/tilcdc/tilcdc_plane.c. Maybe
> > >>>> should be moved to addfb there too?
> > >>>
> > >>> But you don't know the crtc width when creating the framebuffer.
> > >>
> > >> Hm right this is a different check. What we could check in fb_create for
> > >> both is that the logical fb size matches exactly the pitch. That's not
> > >> sufficient criteria, but it will at least catch some of them already.
> > >>
> > >> But yeah we'd need both here.
> > >
> > > After validating width of framebuffer against pitch, the only thing we
> > > need to check here is that the width matches. From what I can tell,
> > > least for mxsfb, this should be covered by
> > > drm_atomic_helper_check_plane_state's can_position parameter set to
> > > false.
> > 
> > This only checks against the src rectangle of the crtc state, there's
> > nothing forcing that the size of the fb matches the src rectangle
> > exactly. I guess we could maybe add that as another parameter for hw
> > like yours or tilcdc. Naming is a bit tricky, maybe
> > require_matching_fb or src_must_match_fb or something like that.
> 
> Can we turn those parameters into flags ? false, true, false is hard to
> read.

Even the flags approach doesn't really scale past some point. Is there
a particularly convincing reason for stuffing yet another check into
this function as opposed to just introducing a separate function?
I prefer clear single purpose functions over swiss army knives.

> 
> > > So I think in my case I can get away by only checking the framebuffer.
> > 
> > You still need both I think.
> 
> -- 
> Regards,
> 
> Laurent Pinchart
> _______________________________________________
> dri-devel mailing list
> dri-devel@lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/dri-devel

-- 
Ville Syrjälä
Intel

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