RE: [PATCH 3/3] drm/i915/dp: Include the time taken by AUX Tx for timeout

[Jani Nikula <jani.nikula@linux.intel.com>]

On Mon, 23 Sep 2024, "Srikanth V, NagaVenkata" <nagavenkata.srikanth.v@intel.com> wrote:
>> > -----Original Message-----
>> > From: Intel-xe <intel-xe-bounces@lists.freedesktop.org> On Behalf Of
>> > Arun R Murthy
>> > Sent: Thursday, September 12, 2024 10:36 AM
>> > To: intel-xe@lists.freedesktop.org; intel-gfx@lists.freedesktop.org
>> > Cc: Murthy, Arun R <arun.r.murthy@intel.com>; Srikanth V, NagaVenkata
>> > <nagavenkata.srikanth.v@intel.com>
>> > Subject: [PATCH 3/3] drm/i915/dp: Include the time taken by AUX Tx for
>> > timeout
>> >
>> > As per DP spec the timeout for LANE_CHANNEL_EQ_DONE is 400ms. But this
>> 
>> Adding where in DP spec example dp2.1 section x.x is a good idea
>> 
> Please refer to section 3.5.2.16.1 128b/132b DP LANEx_CHANNEL_EQ_DONE Sequnce
> and Figure3-51: 128b/132b DP EQ Time Budget Illustration of DP2.1a spec.
>
>> > timeout value is exclusively for the Aux RD Interval and excludes the
>> > time consumed for the AUX Tx (i.e reading/writing FFE presets). Add
>> > another 50ms for these AUX Tx to the 400ms timeout.
>> 
>> Is this something we came up with by trial and error or is this also a part of spec
>> 
> It's not by trail and erros. Timeout value of 450ms is part of DP.

It's a very poorly written spec, and contradicts itself. The only place
where I see 450 ms even mentioned is Figure 3-52. But everywhere else it
seems to say many times 400 ms is the max, without a hint that it would
be exclusive of aux. It does not say anything about the sum of rd
interval being related to the max of 400 ms either.

Regardless, I have no qualms about relaxing the timeout by 50 ms.

With the commit message explaining the above, with references,

Acked-by: Jani Nikula <jani.nikula@intel.com>


>
>> Regards,
>> Suraj Kandpal
>> >
>> > Signed-off-by: Srikanth V NagaVenkata
>> > <nagavenkata.srikanth.v@intel.com>
>> > Signed-off-by: Arun R Murthy <arun.r.murthy@intel.com>
>> > ---
>> >  drivers/gpu/drm/i915/display/intel_dp_link_training.c | 2 +-
>> >  1 file changed, 1 insertion(+), 1 deletion(-)
>> >
>> > diff --git a/drivers/gpu/drm/i915/display/intel_dp_link_training.c
>> > b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
>> > index ca179bed46ad..b6573934c6dd 100644
>> > --- a/drivers/gpu/drm/i915/display/intel_dp_link_training.c
>> > +++ b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
>> > @@ -1414,7 +1414,7 @@ intel_dp_128b132b_lane_eq(struct intel_dp
>> > *intel_dp,
>> >  	}
>> >
>> >  	/* Time budget for the LANEx_EQ_DONE Sequence */
>> > -	deadline = jiffies + msecs_to_jiffies_timeout(400);
>> > +	deadline = jiffies + msecs_to_jiffies_timeout(450);
>> >
>> >  	for (try = 0; try < max_tries; try++) {
>> >  		fsleep(delay_us);
>> > --
>> > 2.25.1
>

-- 
Jani Nikula, Intel