Re: [Intel-gfx] [PATCH v2 5/8] drm/i915/dp: rewrite DP 2.0 128b/132b link training based on errata

Jani Nikula
2022-02-08T09:17:28Z

On Fri, 04 Feb 2022, Ville Syrjälä <ville.syrjala@linux.intel.com> wrote:
> On Thu, Feb 03, 2022 at 11:03:54AM +0200, Jani Nikula wrote:
>> The DP 2.0 errata completely overhauls the 128b/132b link training, with
>> no provisions for backward compatibility with the original DP 2.0
>> specification.
>>
>> The changes are too intrusive to consider reusing the same code for both
>> 8b/10b and 128b/132b, mainly because the LTTPR channel equalisation is
>> done concurrently instead of serialized.
>>
>> NOTES:
>>
>> * It's a bit unclear when to wait for DP_INTERLANE_ALIGN_DONE and
>> per-lane DP_LANE_SYMBOL_LOCKED. Figure xx4 in the SCR implies the
>> LANEx_CHANNEL_EQ_DONE sequence may end with either 0x77,0x77,0x85 *or*
>> 0x33,0x33,0x84 (for four lane configuration in DPCD 0x202..0x204)
>> i.e. without the above bits set. Text elsewhere seems contradictory or
>> incomplete.
>>
>> * We read entire link status (6 bytes) everywhere instead of individual
>> DPCD addresses.
>>
>> * There are some subtle ambiguities or contradictions in the order of
>> some DPCD access and TPS signal enables/disables. It's also not clear
>> whether these are significant.
>>
>> v2:
>> - Always try one last time after timeouts to avoid races (Ville)
>> - Extend timeout to cover the entire LANEx_EQ_DONE sequence (Ville)
>> - Also check for eq interlane align done in LANEx_CDS_DONE Sequence (Ville)
>> - Check for Intra-hop status before link training
>>
>> Cc: Uma Shankar <uma.shankar@intel.com>
>> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
>> Signed-off-by: Jani Nikula <jani.nikula@intel.com>
>> ---
>> .../drm/i915/display/intel_dp_link_training.c | 279 +++++++++++++++++-
>> 1 file changed, 278 insertions(+), 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 4e507aa75a03..cc2b82d9114c 100644
>> --- a/drivers/gpu/drm/i915/display/intel_dp_link_training.c
>> +++ b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
>> @@ -1102,6 +1102,277 @@ intel_dp_link_train_all_phys(struct intel_dp *intel_dp,
>> return ret;
>> }
>>
>> +
>> +/*
>> + * 128b/132b DP LANEx_EQ_DONE Sequence (DP 2.0 E11 3.5.2.16.1)
>> + */
>> +static bool
>> +intel_dp_128b132b_lane_eq(struct intel_dp *intel_dp,
>> + const struct intel_crtc_state *crtc_state)
>> +{
>> + struct intel_encoder *encoder = &dp_to_dig_port(intel_dp)->base;
>> + struct drm_i915_private *i915 = to_i915(encoder->base.dev);
>> + u8 link_status[DP_LINK_STATUS_SIZE];
>> + int delay_us;
>> + int try, max_tries = 20;
>> + unsigned long deadline;
>> + bool timeout = false;
>> +
>> + /*
>> + * Reset signal levels. Start transmitting 128b/132b TPS1.
>> + *
>> + * Put DPRX and LTTPRs (if any) into intra-hop AUX mode by writing TPS1
>> + * in DP_TRAINING_PATTERN_SET.
>> + */
>> + if (!intel_dp_reset_link_train(intel_dp, crtc_state, DP_PHY_DPRX,
>> + DP_TRAINING_PATTERN_1)) {
>> + drm_err(&i915->drm,
>> + "[ENCODER:%d:%s] Failed to start 128b/132b TPS1\n",
>> + encoder->base.base.id, encoder->base.name);
>> + return false;
>> + }
>> +
>> + delay_us = drm_dp_128b132b_read_aux_rd_interval(&intel_dp->aux);
>> +
>> + /* Read the initial TX FFE settings. */
>> + if (drm_dp_dpcd_read_link_status(&intel_dp->aux, link_status) < 0) {
>> + drm_err(&i915->drm,
>> + "[ENCODER:%d:%s] Failed to read TX FFE presets\n",
>> + encoder->base.base.id, encoder->base.name);
>> + return false;
>> + }
>> +
>> + /* Update signal levels and training set as requested. */
>> + intel_dp_get_adjust_train(intel_dp, crtc_state, DP_PHY_DPRX, link_status);
>> + if (!intel_dp_update_link_train(intel_dp, crtc_state, DP_PHY_DPRX)) {
>> + drm_err(&i915->drm,
>> + "[ENCODER:%d:%s] Failed to set initial TX FFE settings\n",
>> + encoder->base.base.id, encoder->base.name);
>> + return false;
>> + }
>> +
>> + /* Start transmitting 128b/132b TPS2. */
>> + if (!intel_dp_set_link_train(intel_dp, crtc_state, DP_PHY_DPRX,
>> + DP_TRAINING_PATTERN_2)) {
>> + drm_err(&i915->drm,
>> + "[ENCODER:%d:%s] Failed to start 128b/132b TPS2\n",
>> + encoder->base.base.id, encoder->base.name);
>> + return false;
>> + }
>> +
>> + /* Time budget for the LANEx_EQ_DONE Sequence */
>> + deadline = jiffies + msecs_to_jiffies(400);
>
> Didn't we have a msecs_to_jiffies_timeout() that adds an extra
> jiffy to make sure we don't bail too early?

Hrmh, still local in i915_utils.h after all these years.

>
>> +
>> + for (try = 0; try < max_tries; try++) {
>> + usleep_range(delay_us, 2 * delay_us);
>> +
>> + /*
>> + * The delay may get updated. The transmitter shall read the
>> + * delay before link status during link training.
>> + */
>> + delay_us = drm_dp_128b132b_read_aux_rd_interval(&intel_dp->aux);
>> +
>> + if (drm_dp_dpcd_read_link_status(&intel_dp->aux, link_status) < 0) {
>> + drm_err(&i915->drm,
>> + "[ENCODER:%d:%s] Failed to read link status\n",
>> + encoder->base.base.id, encoder->base.name);
>> + return false;
>> + }
>> +
>> + if (drm_dp_128b132b_link_training_failed(link_status)) {
>> + intel_dp_dump_link_status(intel_dp, DP_PHY_DPRX, link_status);
>> + drm_err(&i915->drm,
>> + "[ENCODER:%d:%s] Downstream link training failure\n",
>> + encoder->base.base.id, encoder->base.name);
>> + return false;
>> + }
>> +
>> + if (drm_dp_128b132b_lane_channel_eq_done(link_status, crtc_state->lane_count)) {
>> + drm_dbg_kms(&i915->drm,
>> + "[ENCODER:%d:%s] Lane channel eq done\n",
>> + encoder->base.base.id, encoder->base.name);
>> + break;
>> + }
>
> The state diagrame has thow two steps in reverse order. I suppose it
> doens't matter but probably better if we don't deviate too much without
> a good reason.

I thought the fail bit should probably have priority. What if you have
both eq done *and* lt failed set, for whatever reason?

>
>> +
>> + if (timeout) {
>> + intel_dp_dump_link_status(intel_dp, DP_PHY_DPRX, link_status);
>> + drm_err(&i915->drm,
>> + "[ENCODER:%d:%s] Lane channel eq timeout\n",
>> + encoder->base.base.id, encoder->base.name);
>> + return false;
>> + }
>> +
>> + if (time_after(jiffies, deadline))
>> + timeout = true; /* try one last time after deadline */
>
> Is there a reason we can't do this just before drm_dp_dpcd_read_link_status()
> so we don't have to pass the timeout status from one loop iteration to
> the next?

The point is to check one last time after timeout has passed, like you
suggested in previous review, and I agreed.

>
>> +
>> + /* Update signal levels and training set as requested. */
>> + intel_dp_get_adjust_train(intel_dp, crtc_state, DP_PHY_DPRX, link_status);
>> + if (!intel_dp_update_link_train(intel_dp, crtc_state, DP_PHY_DPRX)) {
>> + drm_err(&i915->drm,
>> + "[ENCODER:%d:%s] Failed to update TX FFE settings\n",
>> + encoder->base.base.id, encoder->base.name);
>> + return false;
>> + }
>> + }
>> +
>> + if (try == max_tries) {
>> + intel_dp_dump_link_status(intel_dp, DP_PHY_DPRX, link_status);
>> + drm_err(&i915->drm,
>> + "[ENCODER:%d:%s] Max loop count reached\n",
>> + encoder->base.base.id, encoder->base.name);
>> + return false;
>> + }
>> +
>> + for (;;) {
>> + if (drm_dp_128b132b_eq_interlane_align_done(link_status)) {
>> + drm_dbg_kms(&i915->drm,
>> + "[ENCODER:%d:%s] Interlane align done\n",
>> + encoder->base.base.id, encoder->base.name);
>> + break;
>> + }
>
> Not a big fan of pasing the link_status between loops. Can't
> we just read the status here at the start of the loop always?

I guess. It can lead to an extra read if interlane align gets done
already in the previous loop.

Combined with the timeout flag from the previous loop, the
implementation proposed here passes CDS if the last attempt after
timeout has passed has interlane align done, but does not read anything
more here.

>
>> +
>> + if (timeout) {
>> + intel_dp_dump_link_status(intel_dp, DP_PHY_DPRX, link_status);
>> + drm_err(&i915->drm,
>> + "[ENCODER:%d:%s] Interlane align timeout\n",
>> + encoder->base.base.id, encoder->base.name);
>> + return false;
>> + }
>> +
>> + if (time_after(jiffies, deadline))
>> + timeout = true; /* try one last time after deadline */
>> +
>> + usleep_range(2000, 3000);
>> +
>> + if (drm_dp_dpcd_read_link_status(&intel_dp->aux, link_status) < 0) {
>> + drm_err(&i915->drm,
>> + "[ENCODER:%d:%s] Failed to read link status\n",
>> + encoder->base.base.id, encoder->base.name);
>> + return false;
>> + }
>> +
>> + if (drm_dp_128b132b_link_training_failed(link_status)) {
>> + intel_dp_dump_link_status(intel_dp, DP_PHY_DPRX, link_status);
>> + drm_err(&i915->drm,
>> + "[ENCODER:%d:%s] Downstream link training failure\n",
>> + encoder->base.base.id, encoder->base.name);
>> + return false;
>> + }
>> + }
>> +
>> + return true;
>> +}
>> +
>> +/*
>> + * 128b/132b DP LANEx_CDS_DONE Sequence (DP 2.0 E11 3.5.2.16.2)
>> + */
>> +static bool
>> +intel_dp_128b132b_lane_cds(struct intel_dp *intel_dp,
>> + const struct intel_crtc_state *crtc_state,
>> + int lttpr_count)
>> +{
>> + struct intel_encoder *encoder = &dp_to_dig_port(intel_dp)->base;
>> + struct drm_i915_private *i915 = to_i915(encoder->base.dev);
>> + u8 link_status[DP_LINK_STATUS_SIZE];
>> + unsigned long deadline;
>> + bool timeout = false;
>> +
>> + if (drm_dp_dpcd_writeb(&intel_dp->aux, DP_TRAINING_PATTERN_SET,
>> + DP_TRAINING_PATTERN_2_CDS) != 1) {
>> + drm_err(&i915->drm,
>> + "[ENCODER:%d:%s] Failed to start 128b/132b TPS2 CDS\n",
>> + encoder->base.base.id, encoder->base.name);
>> + return false;
>> + }
>> +
>> + /* Time budget for the LANEx_CDS_DONE Sequence */
>> + deadline = jiffies + msecs_to_jiffies((lttpr_count + 1) * 20);
>> +
>> + for (;;) {
>> + usleep_range(2000, 3000);
>> +
>> + if (drm_dp_dpcd_read_link_status(&intel_dp->aux, link_status) < 0) {
>> + drm_err(&i915->drm,
>> + "[ENCODER:%d:%s] Failed to read link status\n",
>> + encoder->base.base.id, encoder->base.name);
>> + return false;
>> + }
>> +
>> + if (drm_dp_128b132b_eq_interlane_align_done(link_status) &&
>> + drm_dp_128b132b_cds_interlane_align_done(link_status) &&
>> + drm_dp_128b132b_lane_symbol_locked(link_status, crtc_state->lane_count)) {
>> + drm_dbg_kms(&i915->drm,
>> + "[ENCODER:%d:%s] CDS interlane align done\n",
>> + encoder->base.base.id, encoder->base.name);
>> + break;
>> + }
>> +
>> + if (drm_dp_128b132b_link_training_failed(link_status)) {
>> + intel_dp_dump_link_status(intel_dp, DP_PHY_DPRX, link_status);
>> + drm_err(&i915->drm,
>> + "[ENCODER:%d:%s] Downstream link training failure\n",
>> + encoder->base.base.id, encoder->base.name);
>> + return false;
>> + }
>> +
>> + if (timeout) {
>> + intel_dp_dump_link_status(intel_dp, DP_PHY_DPRX, link_status);
>> + drm_err(&i915->drm,
>> + "[ENCODER:%d:%s] CDS timeout\n",
>> + encoder->base.base.id, encoder->base.name);
>> + return false;
>> + }
>> +
>> + if (time_after(jiffies, deadline))
>> + timeout = true; /* try one last time after deadline */
>> + }
>> +
>> + /* FIXME: Should DP_TRAINING_PATTERN_DISABLE be written first? */
>> + if (intel_dp->set_idle_link_train)
>> + intel_dp->set_idle_link_train(intel_dp, crtc_state);
>> +
>> + return true;
>> +}
>> +
>> +/*
>> + * 128b/132b link training sequence. (DP 2.0 E11 SCR on link training.)
>> + */
>> +static bool
>> +intel_dp_128b132b_link_train(struct intel_dp *intel_dp,
>> + const struct intel_crtc_state *crtc_state,
>> + int lttpr_count)
>> +{
>> + struct drm_i915_private *i915 = dp_to_i915(intel_dp);
>> + struct intel_connector *connector = intel_dp->attached_connector;
>> + struct intel_encoder *encoder = &dp_to_dig_port(intel_dp)->base;
>> + bool passed = false;
>> + u8 sink_status;
>> +
>> + /* FIXME: We should possibly do this earlier. */
>> + if (drm_dp_dpcd_readb(&intel_dp->aux, DP_SINK_STATUS, &sink_status) != 1) {
>> + drm_dbg_kms(&i915->drm, "Failed to read sink status\n");
>> + return false;
>> + }
>> +
>> + if (sink_status & DP_INTRA_HOP_AUX_REPLY_INDICATION) {
>> + drm_dbg_kms(&i915->drm, "Previous link training in progress\n");
>> + return false;
>> + }
>> +
>> + if (intel_dp_128b132b_lane_eq(intel_dp, crtc_state) &&
>> + intel_dp_128b132b_lane_cds(intel_dp, crtc_state, lttpr_count))
>> + passed = true;
>
> I think we are supposed to wait for the intra-hop AUX bit to go low
> here.

Where's that said in the spec? I thought we're supposed to wait *before*
starting link training.

Also, I think the bit might only go low after we've stopped link
training, i.e. once the higher level calls intel_dp_stop_link_train()
which sets DP_TRAINING_PATTERN_DISABLE. So the wait, if any, would need
to be in that function.

>
>> +
>> + drm_dbg_kms(&i915->drm,
>> + "[CONNECTOR:%d:%s][ENCODER:%d:%s] 128b/132b Link Training %s at link rate = %d, lane count = %d\n",
>> + connector->base.base.id, connector->base.name,
>> + encoder->base.base.id, encoder->base.name,
>> + passed ? "passed" : "failed",
>> + crtc_state->port_clock, crtc_state->lane_count);
>> +
>> + return passed;
>> +}
>> +
>> /**
>> * intel_dp_start_link_train - start link training
>> * @intel_dp: DP struct
>> @@ -1115,6 +1386,7 @@ intel_dp_link_train_all_phys(struct intel_dp *intel_dp,
>> void intel_dp_start_link_train(struct intel_dp *intel_dp,
>> const struct intel_crtc_state *crtc_state)
>> {
>> + static bool passed;
>> /*
>> * TODO: Reiniting LTTPRs here won't be needed once proper connector
>> * HW state readout is added.
>> @@ -1127,6 +1399,11 @@ void intel_dp_start_link_train(struct intel_dp *intel_dp,
>>
>> intel_dp_prepare_link_train(intel_dp, crtc_state);
>>
>> - if (!intel_dp_link_train_all_phys(intel_dp, crtc_state, lttpr_count))
>> + if (intel_dp_is_uhbr(crtc_state))
>> + passed = intel_dp_128b132b_link_train(intel_dp, crtc_state, lttpr_count);
>> + else
>> + passed = intel_dp_link_train_all_phys(intel_dp, crtc_state, lttpr_count);
>> +
>> + if (!passed)
>> intel_dp_schedule_fallback_link_training(intel_dp, crtc_state);
>> }
>> --
>> 2.30.2

--
Jani Nikula, Intel Open Source Graphics Center