Re: [PATCH] drm/tinydrm: mipi-dbi: Fix unbalanced DMA access

[David Lechner]

On 08/04/2017 01:49 AM, Noralf TrÃnnes wrote:
> Den 04.08.2017 00.41, skrev David Lechner:
> > On 08/01/2017 03:14 PM, David Lechner wrote:
> > > If we return here and import_attach is true, then 
> > > dma_buf_end_cpu_access()
> > > will not be called balance dma_buf_begin_cpu_access().
> > >
> > > Fix by setting ret instead of returning.
> > >
> > > Signed-off-by: David Lechner <david@xxxxxxxxxxxxxx>
> > > ---
> > >   drivers/gpu/drm/tinydrm/mipi-dbi.c | 3 ++-
> > >   1 file changed, 2 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/gpu/drm/tinydrm/mipi-dbi.c 
> > > b/drivers/gpu/drm/tinydrm/mipi-dbi.c
> > > index c83eeb7..e10fa4b 100644
> > > --- a/drivers/gpu/drm/tinydrm/mipi-dbi.c
> > > +++ b/drivers/gpu/drm/tinydrm/mipi-dbi.c
> > > @@ -183,7 +183,8 @@ static int mipi_dbi_buf_copy(void *dst, struct 
> > > drm_framebuffer *fb,
> > >           dev_err_once(fb->dev->dev, "Format is not supported: %s\n",
> > >                    drm_get_format_name(fb->format->format,
> > >                            &format_name));
> > > -        return -EINVAL;
> > > +        ret = -EINVAL;
> > > +        break;
> > >       }
> > >         if (import_attach)
> >
> >
> > I just realized that the next line here can mask ret.
> >
> >
> >     if (import_attach)
> >         ret = dma_buf_end_cpu_access(import_attach->dmabuf,
> >                          DMA_FROM_DEVICE);
> >
> > So, we should either ignore the return value from 
> > dma_buf_end_cpu_access() always or add some logic to ignore it if ret 
> > is already an error.
> >
> > In some of the other patches I have been sending, we have the same 
> > situation. I those, I have opted to just ignore the return value from 
> > dma_buf_end_cpu_access(). e.g...
> >
> >
> >     if (import_attach)
> >         dma_buf_end_cpu_access(import_attach->dmabuf, DMA_FROM_DEVICE);
> >
> > Is this a reasonable thing to do?
>
> mipi_dbi_buf_copy() can be used by other rgb565 controllers, hence
> the format check I think. So when that happens, it can be moved to
> tinydrm-helpers.
>
> Currently it's not possible to get an illegal format, since mipi-dbi
> only supports those two formats.
> Userspace can't set an illegal format, beacuse it's checked:
> drm_atomic_commit -> drm_atomic_check_only -> drm_atomic_plane_check ->
> drm_plane_check_pixel_format
>
> So I think we can just leave this alone, or put the check before
> import_attach if you really want to put this straight.

I guess we can just leave it alone for now.