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

Laurent Pinchart
2018-09-06T12:25:56Z

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.

--
Regards,

Laurent Pinchart