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

[Noralf Trønnes]

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 <da...@lechnology.com>
---
  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.

Noralf.