Re: [PATCH] drm: exynos: Fix fb_videomode <-> drm_mode_modeinfo conversion

On Thu, Mar 08, 2012 at 04:31:51PM +0100, Laurent Pinchart wrote:

Hi Joonyoung,

On Thursday 08 March 2012 22:05:50 Joonyoung Shim wrote:

quoted

On 03/08/2012 08:34 PM, Laurent Pinchart wrote:

quoted

The fb_videomode structure stores the front porch and back porch in the
right_margin and left_margin fields respectively. right_margin should
thus be computed with hsync_start - hdisplay, and left_margin with
htotal - hsync_end. The same holds for the vertical direction.

        Active               Front           Sync            Back
        Region               Porch                           Porch

<-------------------><----------------><-------------><---------------->

   //////////////////|
  
  ////////////////// |

//////////////////  |..................               ..................

                                        _______________

<------ xres -------><- right_margin -><- hsync_len -><- left_margin -->

<---- hdisplay ----->
<------------ hsync_start ------------>
<--------------------- hsync_end -------------------->
<--------------------------------- htotal ----------------------------->

Fix the fb_videomode<->  drm_mode_modeinfo conversion functions
accordingly.

Signed-off-by: Laurent Pinchart<laurent.pinchart@ideasonboard.com>
---

  drivers/gpu/drm/exynos/exynos_drm_connector.c |   16 ++++++++--------
  1 files changed, 8 insertions(+), 8 deletions(-)

While trying to understand how the fb_videomode and drm_mode_modeinfo
fields map to each other, I found what might be a bug in the Exynos DRM
driver. Could you please check and confirm that my understanding is
correct ?

Good catch. You can refer Documentation/fb/framebuffer.txt to know in
detail.

Acked-by: Joonyoung Shim <redacted>

Thank you. There's a similar issue in exynos_mipi_dsi_set_display_mode() where 
front and back porch are inverted. I'll submit a patch for that as well.

Damned. This seems to be a common misunderstanding. I never really
thought about it and always assumed that 'front porch' and 'back porch'
refers to the picture and not to the sync signal. I was so sure that
I didn't even care to google it before I saw this patch.

Sascha

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