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

[Stefan Agner <stefan@agner.ch>]

On 06.09.2018 05:25, Laurent Pinchart wrote:
> Hi Linus,
> 
> On Thursday, 6 September 2018 14:07:41 EEST Linus Walleij wrote:
>> On Wed, Sep 5, 2018 at 8:32 PM Stefan Agner <stefan@agner.ch> wrote:
>> > On 05.09.2018 00:44, Laurent Pinchart wrote:
>> >
>> > 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)
>>
>> Maybe a stupid comment from my side, but can't we just change the
>> documentation to match the usecases?
>>
>> /* Trigger pixel data latch on positive edge */
>> #define DRM_BUS_FLAG_PIXDATA_POSEDGE    (1<<2)
> 
> The flags are used for the drm_connector bus_flags field, and really mean 
> driving on the positive/negative edges. We this can't change their meaning 
> like this.
> 
>> > 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?
>>
>> I just thought it was generally useful to have triggering edge encoded
>> into the bridge as it makes it clear that this edge is something
>> that is a delayed version of the driving edge which is subject to
>> clock skew caused by the speed of electrons in silicon and
>> copper and slew rate caused by parasitic capacitance.
> 
> I agree that we need both the driving and sampling edge. In many case they 
> will be opposite, and providing some kind of appropriate defaults in APIs is 
> fine by me, but we need a way to specify both when needed.

We do have pixel clock flags for displays, but also they are actually
controller oriented:

https://elixir.bootlin.com/linux/latest/source/include/video/display_timing.h#L15

I guess having different flags to denote driving and sampling edge
independently would be ideal. But then, is there really a use case where
it wouldn't be the exact opposite?

The other bus flags are actually fine as is. I suggest to just stick
with the bus flags as we have them now, at least for now.

Alternatively, we could provide "consumer/bridge" oriented flags which
use the same bit and just are the opposite of the controller oriented
flags, e.g.:

/* 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)
/* sample data on neg. edge */
#define DRM_BUS_FLAG_PIXDATA_SAMPLE_NEGEDGE	(1<<2)
/* sample data on pos. edge */
#define DRM_BUS_FLAG_PIXDATA_SAMPLE_POSEDGE	(1<<3)

--
Stefan