[RFC] drm/bridge: megachips-stdpxxxx-ge-b850v3-fw: switch to drm_do_get_edid()

[Peter Senna Tschudin]

On Mon, Sep 4, 2023 at 12:16 PM Jani Nikula <jani.nikula at intel.com> wrote:
>
> On Sat, 02 Sep 2023, Peter Senna Tschudin <peter.senna at gmail.com> wrote:
> > Good morning Jani,
> >
> > It has been a long time since I wrote the driver, and many many years
> > since I sent my last kernel patch, so my memory does not serve me very
> > well, but I will try to shed some light.
> >
> > On Fri, Sep 1, 2023 at 12:24 PM Jani Nikula <jani.nikula at intel.com> wrote:
> >>
> >> The driver was originally added in commit fcfa0ddc18ed ("drm/bridge:
> >> Drivers for megachips-stdpxxxx-ge-b850v3-fw (LVDS-DP++)"). I tried to
> >> look up the discussion, but didn't find anyone questioning the EDID
> >> reading part.
> >>
> >> Why does it not use drm_get_edid() or drm_do_get_edid()?
> >>
> >> I don't know where client->addr comes from, so I guess it could be
> >> different from DDC_ADDR, rendering drm_get_edid() unusable.
> >>
> >> There's also the comment:
> >>
> >>         /* Yes, read the entire buffer, and do not skip the first
> >>          * EDID_LENGTH bytes.
> >>          */
> >>
> >> But again, there's not a word on *why*.
> >
> > The video pipeline has two hardware bridges between the LVDS from the
> > SoC and DP+ output. For reasons, we would get hot plug events from one
> > of these bridges, and EDID from the other. If I am not mistaken, I
> > documented this strangeness in the DTS readme file.
> >
> > Did this shed any light on the *why* or did I tell you something you
> > already knew?
>
> I guess that answers the question why it's necessary to specify the ddc
> to use, but not why drm_do_get_edid() could not be used. Is it really
> necessary to read the EDID in one go?

I have a very weak recollection about hotplug and EDID issues with the
LVDS driver. I am not very confident about this, but maybe I needed to
find ways to read EDID early enough to please the LVDS display driver
during the LVDS driver startup.