[PATCH] drm/imx: parallel-display: Adjust bus_flags and bus_format handling

[Philipp Zabel]

Hi Marek,

On Thu, 2019-11-14 at 14:17 +0100, Marek Vasut wrote:
> The bus_flags and bus_format handling logic does not seem to cover
> all potential usecases. Specifically, this seems to fail with an
> "edt,etm0700g0edh6" display attached to an 24bit display interface,
> with interface-pix-fmt = "rgb24" set in DT.

interface-pix-fmt is a legacy property that was never intended to be
used as an override for the panel bus format. The bus flags were
supposed to be set from the display-timings node, back when there was no
of-graph connected panel at all.

That being said, there isn't really a proper alternative that allows to
override the bus format requested by the panel driver in the device tree
to account for weird wiring. We could reuse the bus-width endpoint
property documented in [1], but that wouldn't completely specify how the
RGB components are to be mapped onto the parallel bus.

[1] Documentation/devicetree/bindings/media/video-interfaces.txt

I do wonder whether for your case it would be better to implement a
MEDIA_BUS_FMT_RGB666_1X24_CPADLO though, to leave the LSBs untouched
instead of risking to dump them into accidental PCB antennae.

> In this specific setup, the panel-simple.c driver entry for the display
> sets .bus_flags to non-zero value. However, as imxpd->bus_format is set
> from the DT property "interface-pix-fmt", imx_pd_encoder_atomic_check()
> will set imx_crtc_state->bus_flags = imxpd->bus_flags even though the
> imxpd->bus_flags is zero, while the di->bus_flags is correctly set by
> the panel-simple.c and non-zero.
>
> The result is incorrect flags being
> used for the display configuration and thus an image corruption.
> (Specifically, DRM_BUS_FLAG_PIXDATA_POSEDGE is not propagated and thus
> the ipuv3 clocks pixels on the wrong edge).
> 
> This patch fixes the problem by overriding the imx_crtc_state->bus_format
> from the imxpd->bus_format only if the DT property "interface-pix-fmt" is
> present or if the DI provides no formats. Similarly for bus_flags, which
> are set from imxpd->bus_flags only if the DI provides no formats.
> 
> Signed-off-by: Marek Vasut <marex at denx.de>
> Cc: Daniel Vetter <daniel at ffwll.ch>
> Cc: David Airlie <airlied at linux.ie>
> Cc: Fabio Estevam <festevam at gmail.com>
> Cc: NXP Linux Team <linux-imx at nxp.com>
> Cc: Philipp Zabel <p.zabel at pengutronix.de>
> Cc: Sascha Hauer <s.hauer at pengutronix.de>
> Cc: Shawn Guo <shawnguo at kernel.org>
> Cc: linux-arm-kernel at lists.infradead.org
> To: dri-devel at lists.freedesktop.org
> ---
>  drivers/gpu/drm/imx/parallel-display.c | 13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/imx/parallel-display.c b/drivers/gpu/drm/imx/parallel-display.c
> index 35518e5de356..92f00b12c068 100644
> --- a/drivers/gpu/drm/imx/parallel-display.c
> +++ b/drivers/gpu/drm/imx/parallel-display.c
> @@ -113,13 +113,16 @@ static int imx_pd_encoder_atomic_check(struct drm_encoder *encoder,
>  	struct drm_display_info *di = &conn_state->connector->display_info;
>  	struct imx_parallel_display *imxpd = enc_to_imxpd(encoder);
>  
> -	if (!imxpd->bus_format && di->num_bus_formats) {
> -		imx_crtc_state->bus_flags = di->bus_flags;
> +	if (imxpd->bus_format || !di->num_bus_formats)

I see no reason to invert the logic here. Let's keep the common case
first.

> +		imx_crtc_state->bus_format = imxpd->bus_format;
> +	else
>  		imx_crtc_state->bus_format = di->bus_formats[0];
> -	} else {
> +
> +	if (di->num_bus_formats)
> +		imx_crtc_state->bus_flags = di->bus_flags;
> +	else
>  		imx_crtc_state->bus_flags = imxpd->bus_flags;
> -		imx_crtc_state->bus_format = imxpd->bus_format;
> -	}
> +
>  	imx_crtc_state->di_hsync_pin = 2;
>  	imx_crtc_state->di_vsync_pin = 3;

So while I don't like this being used at all, I think the patch improves
consistency, as imx_pd_connector_get_modes doesn't allow to override the
panel's modes with DT display-timings either.

regards
Philipp