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

[Stefan Agner]

On 05.09.2018 00:44, Laurent Pinchart wrote:
> Hi Stefan,
> 
> On Wednesday, 5 September 2018 10:06:28 EEST Laurent Pinchart wrote:
>> 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@xxxxxxxx>
>> > ---
>> >
>> >  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.

Good point! I actually really don't like that we use the same flags here
but from a different perspective. Especially since the flags defines
document things differently:

/* drive data on pos. edge */
#define DRM_BUS_FLAG_PIXDATA_POSEDGE	(1<<2)
/* drive data on neg. edge */
#define DRM_BUS_FLAG_PIXDATA_NEGEDGE	(1<<3)

Using the opposite perspective would also need translation in crtc
drivers... So far no driver uses sampling_edge.

I would prefer if we always use the meaning as documented by the flags.

I guess we would need to convert DRM_BUS_FLAG_PIXDATA_POSEDGE ->
DRM_BUS_FLAG_PIXDATA_NEGEDGE.

Linus Walleij, you added sampling edge, any thoughts?


>>
>> 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.

Naming the field input_bus_flags seems very sensible. How about:

struct drm_bridge_timings {
	/**
	 * @input_bus_flags:
	 *
	 * Specifies the settings for the pixel data on the input
	 * bus of this bridge (like pixel signal polarity). Note the
	 * flags are typically controller centric! See also
	 * &drm_display_info->bus_flags.
	 */
	u32 input_bus_flags;

> 
> And I should of course make sure to wake up before reviewing patches. 
> Obviously only one driver is currently affected by the rename. More will use 
> the flags later though, so the argument could still hold.

It is only one bridge driver making use of bridge timings. As far as I
can see currently no driver actually makes use of the bridge timings
sampling_edge currently...

But yes, agreed, we should make a sensible choice now to avoid churn
down the line.

--
Stefan

> 
>> >  	/**
>> >
>> >  	 * @setup_time_ps:
>> >  	 *