[RESEND,V2] drm: fsl-dcu: Fix no fb check bug

[Stefan Agner]

On 2016-01-26 13:18, Emil Velikov wrote:
> On 14 January 2016 at 08:23, Meng Yi <meng.yi at nxp.com> wrote:
>>> >>         switch (fb->pixel_format) {
>>> >>         case DRM_FORMAT_RGB565:
>>> >>         case DRM_FORMAT_RGB888:
>>> >> @@ -85,9 +88,6 @@ static void fsl_dcu_drm_plane_atomic_update(struct
>>> drm_plane *plane,
>>> >>         unsigned int alpha, bpp;
>>> >>         int index, ret;
>>> >>
>>> >> -       if (!fb)
>>> >> -               return;
>>> >> -
>>> > ... which no longer has the !fb check, and we'll crash with null deref
>>> > a few lines below ?
>>>
>>>
>>> If there is a legitimate situation where fb is null which also ultimately leads to a
>>> atomic_commit, I guess we should keep the return here...
>>
>> I think I made a mistake here, fb check should not be removed . As Stefan mentioned, if fb check in fsl_dcu_drm_plane_atomic_check return 0, fsl_dcu_drm_plane_atomic_update will ultimately called, and we'll crash since plane->state->fb is NULL.
>>
> I believe you meant "Emil" in the above ;-) But seriously... afaict a
> fair few drivers do a similar !fb (even !state->crtc) check(s)...
> which makes me wonder if core DRM isn't the better place for it ? Or
> perhaps that's intentional as core provides the flexibility for each
> driver to mangle with the fb between .check and .disable ?
> 

There seem to be a consensus to check crtc and fb on atomic_check.

However, in atomic_update, some drives do a NULL check on crtc only, and
some on both, crtc and fb.

The comment in drm_atomic_plane_disabling says CRTC and FB should always
be NULL together... So I guess it does not really matter all that much,
unless there is anyway a bug.

Furthermore, it seems that in the null case, atomic_disable is called
anyway (which this driver implements). Not sure if there is another case
in which either of this two could be NULL and atomic_update could be
called.

Since this patch is mainly addressing the null check in atomic_check, I
will apply it without the change in atomic_update for now.

--
Stefan