[Intel-gfx] [PATCH] drm/i915/dp: wait on timeout before retry

[Shankar, Uma]

> -----Original Message-----
> From: Murthy, Arun R <arun.r.murthy at intel.com>
> Sent: Monday, July 18, 2022 4:49 PM
> To: Ville Syrjälä <ville.syrjala at linux.intel.com>
> Cc: intel-gfx at lists.freedesktop.org; Zanoni, Paulo R <paulo.r.zanoni at intel.com>;
> tprevite at gmail.com; Shankar, Uma <uma.shankar at intel.com>; Nikula, Jani
> <jani.nikula at intel.com>
> Subject: RE: [PATCH] drm/i915/dp: wait on timeout before retry
> 
> > -----Original Message-----
> > From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > Sent: Monday, July 4, 2022 7:41 PM
> > To: Murthy, Arun R <arun.r.murthy at intel.com>
> > Cc: intel-gfx at lists.freedesktop.org; Zanoni, Paulo R
> > <paulo.r.zanoni at intel.com>; tprevite at gmail.com; Shankar, Uma
> > <uma.shankar at intel.com>; Nikula, Jani <jani.nikula at intel.com>
> > Subject: Re: [PATCH] drm/i915/dp: wait on timeout before retry
> >
> > On Mon, Jul 04, 2022 at 12:53:52PM +0530, Arun R Murthy wrote:
> > > On linktraining error/timeout before retry need to wait for 400usec
> > > as per the DP CTS spec1.2
> >
> > s/CTS//
> >
> > > Under section 2.7.2 AUX Transaction Response/Reply Time-outs AUX
> > > Replier (the uPacket RX) must start sending the reply back to the
> > > AUX requester (the uPacket TX) within the response period of 300μs.
> > > The timer for Response Time-out starts ticking after the uPacket RX
> > > has finished receiving the AUX STOP condition which ends the AUX
> > > Request
> > transaction.
> > > The timer is reset either when the Response Time-out period has
> > > elapsed or when the uPacket RX has started to send the AUX Sync
> > > pattern (which follows
> > > 10 to 16 active pre-charge pulses) for the Reply transaction. If the
> > > uPacket TX does not receive a reply from the uPacket RX it must wait
> > > for a Reply Time-out period of 400us before initiating the next AUX
> > > Request transaction. The timer for the Reply Time-out starts ticking
> > > after the uPacket TX has finished sending the AUX STOP condition.
> > >
> > > The patch with commit 74ebf294a1dd ("drm/i915: Add a delay in
> > > Displayport AUX transactions for compliance testing") removes this
> > > delay mentioning the hardware already meets this requirement, but as
> > > per the spec the delay mentioned in the spec specifies how long to
> > > wait for the receiver response before timeout. So the delay here to
> > > wait for timeout and not a delay after timeout. The DP spec
> > > specifies a delay after timeout and hence adding this delay.
> >
> > Not sure what you're saying here. The spec states the reply timeout
> > should start counting once the TX has sent the AUX STOP, and gets
> > reset when the reply AUX SYNC is detected.
> >
> > If that doesn't match what the hardware is doing then we really need
> > to get bspec updated to say what is actually happening.
> >
> > Oh, and the reply timeout has been increased to 3.2ms in later
> > revisions of the spec to deal with LTTPRs. We should adjust the code to match.
> >
> Will take this separately!

Hi Arun,
I would suggest to create an issue to track the LTTPR work and validate. Also get the bspec
updated to match the hardware behavior.

With above done, we can take this patch to unblock multiple CI issues.
@Ville Syrjälä Hope this is ok.

Regards,
Uma Shankar

> Thanks and Regards,
> Arun R Murthy
> --------------------