On Fri, Nov 18, 2016 at 03:22:49PM +0200, Jani Nikula wrote:
> On Fri, 18 Nov 2016, Manasi Navare <manasi.d.navare@xxxxxxxxx> wrote:
> > If link training fails, then we need to fallback to lower
> > link rate first and if link training fails at RBR, then
> > fallback to lower lane count.
> > This function finds the next lower link rate/lane count
> > value after link training failure.
> >
> > v4:
> > * Remove the redundant variable link_train_failed
> > v3:
> > * Remove fallback_link_rate_index variable, just obtain
> > that using the helper intel_dp_link_rate_index (Jani Nikula)
> > v2:
> > Squash the patch that returns the link rate index (Jani Nikula)
> >
> > Acked-by: Tony Cheng <tony.cheng@xxxxxxx>
> > Acked-by: Harry Wentland <harry.wentland@xxxxxxx>
> > Cc: Ville Syrjala <ville.syrjala@xxxxxxxxxxxxxxx>
> > Cc: Jani Nikula <jani.nikula@xxxxxxxxxxxxxxx>
> > Cc: Daniel Vetter <daniel.vetter@xxxxxxxxx>
> > Signed-off-by: Manasi Navare <manasi.d.navare@xxxxxxxxx>
> > ---
> > drivers/gpu/drm/i915/intel_dp.c | 40 ++++++++++++++++++++++++++++++++++++++++
> > drivers/gpu/drm/i915/intel_drv.h | 4 ++++
> > 2 files changed, 44 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> > index 90283ed..4fb89e1 100644
> > --- a/drivers/gpu/drm/i915/intel_dp.c
> > +++ b/drivers/gpu/drm/i915/intel_dp.c
> > @@ -288,6 +288,46 @@ static int intel_dp_common_rates(struct intel_dp *intel_dp,
> > common_rates);
> > }
> >
> > +static int intel_dp_link_rate_index(struct intel_dp *intel_dp,
> > + int *common_rates, int link_rate)
> > +{
> > + int common_len;
> > + int index;
> > +
> > + common_len = intel_dp_common_rates(intel_dp, common_rates);
> > + for (index = 0; index < common_len; index++) {
> > + if (link_rate == common_rates[common_len - index - 1])
> > + return common_len - index - 1;
> > + }
> > +
> > + return -1;
> > +}
> > +
> > +int intel_dp_get_link_train_fallback_values(struct intel_dp *intel_dp,
> > + int link_rate, uint8_t lane_count)
> > +{
> > + int common_rates[DP_MAX_SUPPORTED_RATES] = {};
> > + int common_len;
> > + int link_rate_index = -1;
> > +
> > + common_len = intel_dp_common_rates(intel_dp, common_rates);
> > + link_rate_index = intel_dp_link_rate_index(intel_dp,
> > + common_rates,
> > + link_rate);
> > + if (link_rate_index > 0) {
> > + intel_dp->fallback_link_rate = common_rates[link_rate_index - 1];
> > + intel_dp->fallback_lane_count = intel_dp_max_lane_count(intel_dp);
>
> So you first try lower and lower link rates, until you're at the
> lowest...
>
Yes so first it tries to lower the link rate until it goes to the lowes i.e RBR.
All this link rate reduction happens with lane count set to maximum.
> > + } else if (lane_count > 1) {
> > + intel_dp->fallback_link_rate = common_rates[common_len - 1];
> > + intel_dp->fallback_lane_count = lane_count >> 1;
>
> ...and then go to highest rate, double lane count, and go back back to
> reducing link rate. Rinse and repeat.
>
So after link rate reaches RBR, it checks if lane count is > 1 and if it is then
it jumps the link rate back to highest supported and reduces the link rate from
4 to 2 to 1.
> Problem is, lane_count will always be > 1, and you'll keep doubling lane
> count without bounds if link training persistently fails, and I don't
> think you'll reach the below else branch.
>
So since I am reducing the lane count in each iteration, lane_count >> 1, at some point
it will go to 0 and that point it will exit with DRM_ERROR that link train failed
because at thatpoint we have exhausted trying all the link raterate and lane count
combinations.
I am never doubling the link rate for it to reach out of bounds, infact I am reducing
the link rate to half each time (right shifting lane count) so at some point lane count will
fall to less than 0 when it will fall into the last else part.
Regards
Manasi
> I regret that I haven't caught all of these issues all at once; it is in
> part testament to the fact that the state machine here is not easy to
> follow.
>
> BR,
> Jani.
>
> > + } else {
> > + DRM_ERROR("Link Training Unsuccessful\n");
> > + return -1;
> > + }
> > +
> > + return 0;
> > +}
> > +
> > static enum drm_mode_status
> > intel_dp_mode_valid(struct drm_connector *connector,
> > struct drm_display_mode *mode)
> > diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
> > index cd132c2..e1c43a9 100644
> > --- a/drivers/gpu/drm/i915/intel_drv.h
> > +++ b/drivers/gpu/drm/i915/intel_drv.h
> > @@ -887,6 +887,8 @@ struct intel_dp {
> > uint32_t DP;
> > int link_rate;
> > uint8_t lane_count;
> > + int fallback_link_rate;
> > + uint8_t fallback_lane_count;
> > uint8_t sink_count;
> > bool link_mst;
> > bool has_audio;
> > @@ -1383,6 +1385,8 @@ bool intel_dp_init_connector(struct intel_digital_port *intel_dig_port,
> > void intel_dp_set_link_params(struct intel_dp *intel_dp,
> > int link_rate, uint8_t lane_count,
> > bool link_mst);
> > +int intel_dp_get_link_train_fallback_values(struct intel_dp *intel_dp,
> > + int link_rate, uint8_t lane_count);
> > void intel_dp_start_link_train(struct intel_dp *intel_dp);
> > void intel_dp_stop_link_train(struct intel_dp *intel_dp);
> > void intel_dp_sink_dpms(struct intel_dp *intel_dp, int mode);
>
> --
> Jani Nikula, Intel Open Source Technology Center
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