linux-kernel - Re: [PATCH v2 1/8] drm/bridge: use bus flags in bridge timings

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Date:   Fri, 14 Sep 2018 12:23:52 +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 v2 1/8] drm/bridge: use bus flags in bridge timings

Hi Stefan,

Thankk you for the patch.

On Wednesday, 12 September 2018 21:32:15 EEST Stefan Agner wrote:
> The DRM bus flags conveys additional information on pixel data on
> the bus. All currently available bus flags might be of interest for
> a bridge. In the case at hand a dumb VGA bridge needs a specific
> data enable polarity (DRM_BUS_FLAG_DE_LOW).
> 
> Replace the sampling_edge field with input_bus_flags and allow all
> currently documented bus flags.
> 
> This changes the perspective from sampling side to the driving
> side for the currently supported flags. We assume that the sampling
> edge is always the opposite of the driving edge (hence we need to
> invert the DRM_BUS_FLAG_PIXDATA_[POS|NEG]EDGE flags). This is an
> assumption we already make for displays. For all we know it is a
> safe assumption for bridges too.

Please don't, that will get confusing very quickly. Flag names such as 
DRM_BUS_FLAG_PIXDATA_NEGEDGE don't mention sampling or driving. There's only a 
small comment next to their definition, which will easily be overlooked. I'd 
rather update the definition to cover both sampling and driving, and document 
the input_bus_flags field to explain that all flags refer to sampling.

> 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..d5aa0f931ef2
> 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,
> +	.input_bus_flags = DRM_BUS_FLAG_PIXDATA_NEGEDGE,
>  	.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,
> +	.input_bus_flags = DRM_BUS_FLAG_PIXDATA_NEGEDGE,
>  	/* 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,
> +	.input_bus_flags = DRM_BUS_FLAG_PIXDATA_NEGEDGE,
>  	/* 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..45e90f4b46c3 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:
> +	 * @input_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).
> +	 * Additional settings this bridge requires for the pixel data on
> +	 * the input bus (e.g. pixel signal polarity). See also
> +	 * &drm_display_info->bus_flags.
>  	 */
> -	u32 sampling_edge;
> +	u32 input_bus_flags;
>  	/**
>  	 * @setup_time_ps:
>  	 *

-- 
Regards,

Laurent Pinchart