RE: [RFC][PATCH] Revert "drm/panel-simple: drop use of data-mapping property"

[Christoph Niedermaier <cniedermaier@dh-electronics.com>]

From: Laurent Pinchart [mailto:laurent.pinchart@ideasonboard.com]
Sent: Thursday, February 3, 2022 12:46 AM
> 
> Hi Christoph,
> 

Hi Laurent,

> On Tue, Feb 01, 2022 at 12:07:17PM +0100, Christoph Niedermaier wrote:
>> Without the data-mapping devicetree property my display won't
>> work properly. It is flickering, because the bus flags won't
>> be assigned without a defined bus format by the imx parallel
>> display driver. There was a discussion about the removal [1]
>> and an agreement that a better solution is needed, but it is
>> missing so far. So what would be the better approach?
>>
>> [1] https://patchwork.freedesktop.org/patch/357659/?series=74705&rev=1
>>
>> This reverts commit d021d751c14752a0266865700f6f212fab40a18c.
>>
>> Signed-off-by: Christoph Niedermaier <cniedermaier@dh-electronics.com>
>> Cc: Marek Vasut <marex@denx.de>
>> Cc: Sam Ravnborg <sam@ravnborg.org>
>> Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
>> Cc: Maxime Ripard <mripard@kernel.org>
>> Cc: Philipp Zabel <p.zabel@pengutronix.de>
>> Cc: David Airlie <airlied@linux.ie>
>> Cc: Daniel Vetter <daniel@ffwll.ch>
>> Cc: Shawn Guo <shawnguo@kernel.org>
>> Cc: Sascha Hauer <s.hauer@pengutronix.de>
>> Cc: Pengutronix Kernel Team <kernel@pengutronix.de>
>> Cc: Fabio Estevam <festevam@gmail.com>
>> Cc: NXP Linux Team <linux-imx@nxp.com>
>> Cc: linux-arm-kernel@lists.infradead.org
>> To: dri-devel@lists.freedesktop.org
>> ---
>>  drivers/gpu/drm/panel/panel-simple.c | 11 +++++++++++
>>  1 file changed, 11 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/panel/panel-simple.c b/drivers/gpu/drm/panel/panel-simple.c
>> index 3c08f9827acf..2c683d94a3f3 100644
>> --- a/drivers/gpu/drm/panel/panel-simple.c
>> +++ b/drivers/gpu/drm/panel/panel-simple.c
>> @@ -453,6 +453,7 @@ static int panel_dpi_probe(struct device *dev,
>>       struct panel_desc *desc;
>>       unsigned int bus_flags;
>>       struct videomode vm;
>> +     const char *mapping;
>>       int ret;
>>
>>       np = dev->of_node;
>> @@ -477,6 +478,16 @@ static int panel_dpi_probe(struct device *dev,
>>       of_property_read_u32(np, "width-mm", &desc->size.width);
>>       of_property_read_u32(np, "height-mm", &desc->size.height);
>>
>> +     of_property_read_string(np, "data-mapping", &mapping);
>> +     if (!strcmp(mapping, "rgb24"))
>> +             desc->bus_format = MEDIA_BUS_FMT_RGB888_1X24;
>> +     else if (!strcmp(mapping, "rgb565"))
>> +             desc->bus_format = MEDIA_BUS_FMT_RGB565_1X16;
>> +     else if (!strcmp(mapping, "bgr666"))
>> +             desc->bus_format = MEDIA_BUS_FMT_RGB666_1X18;
>> +     else if (!strcmp(mapping, "lvds666"))
>> +             desc->bus_format = MEDIA_BUS_FMT_RGB666_1X24_CPADHI;
> 
> You're right that there's an issue, but a revert isn't the right option.
> The commit you're reverting never made it in a stable release, because
> it was deemed to not be a good enough option.
> 
> First of all, any attempt to fix this should include an update to the DT
> binding. Second, as this is about DPI panels, the LVDS option should be
> dropped. Finally, I've shared some initial thoughts in [1], maybe you
> can reply to that e-mail to continue the discussion there ?

According to your thoughts in [1] you mean that the bus format should be
build out of the devicetree properties bus-width and data-shift. It would
be possible for evenly structured busses like RGB888_1X24 and RGB666_1X18,
but what about a bus like RGB565_1X16, where each color has different
bus width. Also the order of the colors should be defined to differ
between busses like RGB888_1X24 and GBR888_1X24.
Are there any ideas how can this be covered?

> 
> [1] https://lore.kernel.org/all/20200303185531.GJ11333@pendragon.ideasonboard.com/
> 
>> +
>>       /* Extract bus_flags from display_timing */
>>       bus_flags = 0;
>>       vm.flags = timing->flags;
> 


Regards
Christoph