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

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Message-ID: <bf38f469923e1c2a16252c6d1d2bcb2b@agner.ch>
Date:   Tue, 18 Sep 2018 11:14:22 -0700
From:   Stefan Agner <stefan@...er.ch>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>
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

On 14.09.2018 02:23, Laurent Pinchart wrote:
> 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.
> 

There is history to that, and I'd really rather prefer to keep that similar across the kernel. There is already precedence in the kernel, the display timings (which is a consumer) defines it from the driving perspective too (see DISPLAY_FLAGS_PIXDATA_POSEDGE).

That is why I introduced the bus flags using the same perspective.

If we _really_ want it from sampling side, we should create new defines which are explicit about that, e.g.: DRM_BUS_FLAG_PIXDATA_SAMPLE_NEGEDGE.

But again, since even the display flags use the driving perspective, I'd rather prefer to have it that way also for bridges too.

--
Stefan


>> 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:
>>  	 *