[PATCH 24/33] drm/panel-simple: Fix dotclock for Ortustech COM37H3M

[H. Nikolaus Schaller]

Hi Ville,

> Am 02.03.2020 um 21:34 schrieb Ville Syrjala <ville.syrjala at linux.intel.com>:
> 
> From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> 
> The currently listed dotclock disagrees with the currently
> listed vrefresh rate. Change the dotclock to match the vrefresh.
> 
> Someone tell me which (if either) of the dotclock or vreresh is
> correct?

Data sheet of COM37H3M99DTC says:

			MIN	TYP	MAX
CLK frequency 	fCLK	18	19.8	27	MHz
VSYNC Frequency	fVSYNC	54	60	66	Hz
VSYNC cycle time tv	646	650	700	H
HSYNC frequency	fHSYNC	--	39.0	50.0	Khz
HSYNC cycle time th	504	508	630	CLK

But data sheet of COM37H3M05DTC says

			MIN	TYP	MAX
CLK frequency 	fCLK	--	22.4	26.3	MHz (in VGA mode - there is also an QVGA mode)
VSYNC Frequency	fVSYNC	54	60	66	Hz
VSYNC cycle time tv	--	650	--	H
HSYNC frequency	fHSYNC	--	39.3	--	Khz
HSYNC cycle time th	--	570	--	CLK

So there are two different subvariants of the same panel.

If I remember correctly, the 05 is older (April 2010)
and the 99DTC newer (Dec 2011).

So 22 MHz isn't outside of either spec but may be higher
than needed for the 99DTC. I.e. the panel is probably
running at higher frame rate than 60 fps.

Hm. I think we should define some compromise. I.e.

.clock = 22230
.vrefresh = 60
.vtotal = 650
.htotal = 570

Probably we originally tried to do this with the parameter
set but got something wrong.

If you agree with this approach, I can try to figure out
the other parameters so that they should fit both panel
variants. I can only test with COM37H3M99DTC since I
do no longer have a device with COM37H3M05DTC.

In general it seems that the structure drm_display_mode
is overdetermined.

Either .clock could be calculated from .vrefresh (and
the other .vtotal and .htotal) or vice versa like I
did for the proposal above.

I haven't looked into the driver code, but would it be
possible to specify .clock = 0 (or leave it out) to
calculate it bottom up? This would avoid such inconsistencies.

On the other hand it is not easily visible any more
from the code if the clock is in range of the data
sheet limits.

BR and thanks,
Nikolaus

> 
> Cc: H. Nikolaus Schaller <hns at goldelico.com>
> Cc: Sam Ravnborg <sam at ravnborg.org>
> Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
> ---
> drivers/gpu/drm/panel/panel-simple.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/panel/panel-simple.c b/drivers/gpu/drm/panel/panel-simple.c
> index ca72b73408e9..f9b4f84bffb3 100644
> --- a/drivers/gpu/drm/panel/panel-simple.c
> +++ b/drivers/gpu/drm/panel/panel-simple.c
> @@ -2617,7 +2617,7 @@ static const struct panel_desc ontat_yx700wv03 = {
> };
> 
> static const struct drm_display_mode ortustech_com37h3m_mode  = {
> -	.clock = 22153,
> +	.clock = 19842,
> 	.hdisplay = 480,
> 	.hsync_start = 480 + 8,
> 	.hsync_end = 480 + 8 + 10,
> -- 
> 2.24.1
>