[PATCHv2 03/22] drm/bridge: tc358767: fix ansi 8b10b use

[Laurent Pinchart]

Hi Tomi,

On Fri, May 03, 2019 at 02:43:51PM +0300, Tomi Valkeinen wrote:
> On 23/04/2019 17:56, Laurent Pinchart wrote:
> 
> >> During initial driver development I had one eDP display that reports 0 in Bit 0
> >> (ANSI 8B/10B) of DPCD reg 0x0006 (MAIN_LINK_CHANNEL_CODING).
> >> Also it does not react on setting Bit 0 (SET_ANSI 8B10B) in 0x0108
> >> (MAIN_LINK_CHANNEL_CODING_SET) - after reading back it was 0 again.
> >> So I had to disable 8B10 encoding on tc358767 side to make this display
> >> work.
> > 
> > Out of curiosity, how does the eDP display recover the clock without
> > 8B/10B encoding ?
> > 
> >> On other hand if there are displays that report zero bit 0 in
> >> MAIN_LINK_CHANNEL_CODING while needing 8b10b then this patch looks
> >> reasonable.
> >>
> >> May be driver should read back MAIN_LINK_CHANNEL_CODING_SET
> >> register after setting it and check if 8b10b actually enabled?
> > 
> > This sounds like a reasonable thing to try. Tomi, do you still have
> > accesss to those faulty monitors ?
> 
> On my monitor it does not seem to matter whether I write 0 or 1 to
> MAIN_LINK_CHANNEL_CODING_SET, as long as I have 8b10b enabled on
> TC358767 side. The writes do go through, and I can read the written bit
> back.
> 
> So... I guess when we talk about eDP panels, there may be all kinds of
> oddities, as they're usually meant to be used with a certain configuration.
> 
> But if everyone agrees that 8B10B is a normal, required feature of DP,
> and there is an eDP panel that needs 8B10B disabled, maybe that panel
> has to be handled separately as a special case? A dts entry to disable
> 8B10B? Or something. But it does not sound like a good idea for the
> driver to try to guess this.

As reported by Andrey, there is at least one panel that would break with
this patch. I agree 8b10b should be the default, but if the above
procedure works for all the panels we know about, is there an issue
implementing it ?

-- 
Regards,

Laurent Pinchart