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

Ville Syrjälä
2022-02-04T11:37:38Z

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?

> +
> + 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.

> +
> + 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?

> +
> + /* 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?

> +
> + 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.

> +
> + 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

--
Ville Syrjälä
Intel