linux-kernel - Re: [PATCH 1/6] drm/bridge: use bus flags in bridge timings

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Message-ID: <4035252.QuWadVx7pr@avalon>
Date:   Wed, 05 Sep 2018 10:06:28 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Stefan Agner <stefan@...er.ch>
Cc:     linus.walleij@...aro.org, airlied@...ux.ie, robh+dt@...nel.org,
        mark.rutland@....com, shawnguo@...nel.org, s.hauer@...gutronix.de,
        p.zabel@...gutronix.de, kernel@...gutronix.de,
        fabio.estevam@....com, linux-imx@....com, architt@...eaurora.org,
        a.hajda@...sung.com, gustavo@...ovan.org,
        maarten.lankhorst@...ux.intel.com, sean@...rly.run,
        marcel.ziswiler@...adex.com, max.krummenacher@...adex.com,
        dri-devel@...ts.freedesktop.org, devicetree@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/6] drm/bridge: use bus flags in bridge timings

Hi Stefan,

Thank you for the patch.

On Wednesday, 5 September 2018 08:21:08 EEST Stefan Agner wrote:
> The DRM bus flags convey additional information on pixel data on
> the bus. All current available bus flags might be of interest for
> a bridge. Remove the sampling_edge field and use bus_flags.
> 
> In the case at hand a dumb VGA bridge needs a specific data enable
> polarity (DRM_BUS_FLAG_DE_LOW).
> 
> Signed-off-by: Stefan Agner <stefan@...er.ch>
> ---
>  drivers/gpu/drm/bridge/dumb-vga-dac.c |  6 +++---
>  include/drm/drm_bridge.h              | 11 +++++------
>  2 files changed, 8 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/gpu/drm/bridge/dumb-vga-dac.c
> b/drivers/gpu/drm/bridge/dumb-vga-dac.c index 9b706789a341..7a5c24967115
> 100644
> --- a/drivers/gpu/drm/bridge/dumb-vga-dac.c
> +++ b/drivers/gpu/drm/bridge/dumb-vga-dac.c
> @@ -234,7 +234,7 @@ static int dumb_vga_remove(struct platform_device *pdev)
> */
>  static const struct drm_bridge_timings default_dac_timings = {
>  	/* Timing specifications, datasheet page 7 */
> -	.sampling_edge = DRM_BUS_FLAG_PIXDATA_POSEDGE,
> +	.bus_flags = DRM_BUS_FLAG_PIXDATA_POSEDGE,
>  	.setup_time_ps = 500,
>  	.hold_time_ps = 1500,
>  };
> @@ -245,7 +245,7 @@ static const struct drm_bridge_timings
> default_dac_timings = { */
>  static const struct drm_bridge_timings ti_ths8134_dac_timings = {
>  	/* From timing diagram, datasheet page 9 */
> -	.sampling_edge = DRM_BUS_FLAG_PIXDATA_POSEDGE,
> +	.bus_flags = DRM_BUS_FLAG_PIXDATA_POSEDGE,
>  	/* From datasheet, page 12 */
>  	.setup_time_ps = 3000,
>  	/* I guess this means latched input */
> @@ -258,7 +258,7 @@ static const struct drm_bridge_timings
> ti_ths8134_dac_timings = { */
>  static const struct drm_bridge_timings ti_ths8135_dac_timings = {
>  	/* From timing diagram, datasheet page 14 */
> -	.sampling_edge = DRM_BUS_FLAG_PIXDATA_POSEDGE,
> +	.bus_flags = DRM_BUS_FLAG_PIXDATA_POSEDGE,
>  	/* From datasheet, page 16 */
>  	.setup_time_ps = 2000,
>  	.hold_time_ps = 500,
> diff --git a/include/drm/drm_bridge.h b/include/drm/drm_bridge.h
> index bd850747ce54..85d4b51eae19 100644
> --- a/include/drm/drm_bridge.h
> +++ b/include/drm/drm_bridge.h
> @@ -244,14 +244,13 @@ struct drm_bridge_funcs {
>   */
>  struct drm_bridge_timings {
>  	/**
> -	 * @sampling_edge:
> +	 * @bus_flags:
>  	 *
> -	 * Tells whether the bridge samples the digital input signal
> -	 * from the display engine on the positive or negative edge of the
> -	 * clock, this should reuse the DRM_BUS_FLAG_PIXDATA_[POS|NEG]EDGE
> -	 * bitwise flags from the DRM connector (bit 2 and 3 valid).
> +	 * Tells what additional settings for the pixel data on the bus
> +	 * this bridge requires (like pixel signal polarity). See also
> +	 * &drm_display_info->bus_flags.
>  	 */
> -	u32 sampling_edge;
> +	u32 bus_flags;

While I'm not opposed to extending the bridge structure to allow specifying 
other flags, I think we're losing information here. The sampling_edge field 
makes it clear that the DRM_BUS_FLAG_PIXDATA_(NEG|POS)EDGE flags specified on 
which clock edge signals are sampled. bus_flags could be interpreted 
differently, for instance as specifying on which clock edge signals are output 
on the other side of the bridge.

We could easily fix this by specifying that the bus_flags field refers to the 
input side of the bridge. We could also rename the field to input_bus_flags. 
The rename could be delayed until needed, but that would result in yet another 
change to all bridge drivers, so we may want to do it now as your patch 
touches all the drivers already. Other options might also be possible.

>  	/**
>  	 * @setup_time_ps:
>  	 *

-- 
Regards,

Laurent Pinchart