[Intel-gfx] [PATCH] drm/edid: Fix DDC probe for passive DP dongles

[Daniel Vetter]

On Thu, May 21, 2015 at 11:28:48AM +0300, Jani Nikula wrote:
> On Thu, 21 May 2015, Todd Previte <tprevite at gmail.com> wrote:
> > Passive DP->DVI/HDMI dongles show up to the system as HDMI devices, as they
> > do not have a sink device in them to respond to any AUX traffic. When
> > probing these dongles over the DDC, sometimes they will NAK the first attempt
> > even though the transaction is valid and they support the DDC protocol. The
> > retry loop inside of drm_do_probe_ddc_edid() would normally catch this case
> > and try the transaction again, resulting in success.
> >
> > That, however, was thwarted by the fix for fdo.org bug #41059. The patch is:
> > 	commit 9292f37e1f5c79400254dca46f83313488093825
> > 	Author: Eugeni Dodonov <eugeni.dodonov at intel.com>
> > 	Date:   Thu Jan 5 09:34:28 2012 -0200
> >
> > 	drm: give up on edid retries when i2c bus is not responding
> 
> Some extra background:
> 
> That commit refers to the i2c bit banging code, while i915 now prefers
> gmbus, and only falls back to big banging on certain failures. (See
> gmbux_xfer() in i915/intel_i2c.c). This means that in most cases i915 is
> no longer susceptible to the 5*3 timeout loops, but it also means we
> don't have the i2c bit banging retry at all on -ENXIO, like Todd notes.
> 
> The questions are, is one retry after -ENXIO in drm_do_probe_ddc_edid
> enough now? Should we revert the original commit instead since the
> underlying algorithm has changed? Or should we return something other
> than -ENXIO from our gmbus code to not hit this exit with no retries
> path?

Jani&I dug around a bit on this on irc and it looks like a proper i2c
implementation should indeed retry on the first ENXIO, except when the
I2C_M_IGNORE_NAK flag is set. There's piles of examples how to do it, but
it's clear that we've never done this properly in our gmbus driver.

Can you please test whether fixing that in intel_i2c.c does resolve the
issue too? It would at least explain a lot of the fallback troubles we've
had with gmbus and i2c slaves not waking up in time, which all got
magically resolved by falling back to the bit-banging interface (which
does this retrying already).

Thanks, Daniel

> 
> > This added code to exit immediately if the return code from the
> > i2c_transfer function was -ENXIO in order to reduce the amount of time spent
> > in waiting for unresponsive or disconnected devices. For the DP dongles,
> > this means that the second retry never happens which results in a failed
> > EDID probe and a black screen.
> >
> > To work around this problem without undoing the fix for bug #41059, the
> > number of retries is checked along with the return code. This allows for a
> > device to NAK once and still continue operations. A second NAK will result
> > in breaking the loop as it would have before and stopping the DDC probe.
> 
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=85924
> 
> Maybe throw this at other dongle bugs you can find too?
> 
> We're going to need Tested-bys though.
> 
> BR,
> Jani.
> 
> 
> > Signed-off-by: Todd Previte <tprevite at gmail.com>
> > Cc: intel-gfx at lists.freedesktop.org
> > ---
> >  drivers/gpu/drm/drm_edid.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
> > index 7087da3..e8047bd 100644
> > --- a/drivers/gpu/drm/drm_edid.c
> > +++ b/drivers/gpu/drm/drm_edid.c
> > @@ -1238,7 +1238,10 @@ drm_do_probe_ddc_edid(void *data, u8 *buf, unsigned int block, size_t len)
> >  		 */
> >  		ret = i2c_transfer(adapter, &msgs[3 - xfers], xfers);
> >  
> > -		if (ret == -ENXIO) {
> > +		/* Passive DP->DVI/HDMI dongles sometimes NAK the first probe
> > +		 * Try to probe again but if it NAKs, stop trying
> > +		 */
> > +		if (ret == -ENXIO && retries < 5) {
> >  			DRM_DEBUG_KMS("drm: skipping non-existent adapter %s\n",
> >  					adapter->name);
> >  			break;
> > -- 
> > 1.9.1
> >
> > _______________________________________________
> > Intel-gfx mailing list
> > Intel-gfx at lists.freedesktop.org
> > http://lists.freedesktop.org/mailman/listinfo/intel-gfx
> 
> -- 
> Jani Nikula, Intel Open Source Technology Center
> _______________________________________________
> Intel-gfx mailing list
> Intel-gfx at lists.freedesktop.org
> http://lists.freedesktop.org/mailman/listinfo/intel-gfx

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch