[PATCH] i915: Add QUIRK_EDP_CHANNEL_EQ for Dell 7200 2-in-1

[Jason Andryuk]

On Fri, Oct 23, 2020 at 8:48 AM Jason Andryuk <jandryuk at gmail.com> wrote:
>
> We're seeing channel equalization "fail" consistently coming out of DPMS
> on the eDP of a Dell Latitude 7200 2-in-1.  When the display tries to
> come out of DPMS, it briefly flashes on before going dark.  This repeats
> once per second, and the system is unusable.  ssh-ing into the system,
> it also seems to be sluggish when in this state.  You have to reboot to
> get the display back.
>
> In intel_dp_link_training_channel_equalization, lane 0 can get to state
> 0x7 by the 3rd pattern, but lane 1 never gets further than 0x1.
> [drm] ln0_1:0x0 ln2_3:0x0 align:0x0 sink:0x0 adj_req0_1:0x0 adj_req2_3:0x0
> [drm] ln0_1:0x11 ln2_3:0x0 align:0x80 sink:0x0 adj_req0_1:0x44 adj_req2_3:0x0
> [drm] ln0_1:0x11 ln2_3:0x0 align:0x80 sink:0x0 adj_req0_1:0x88 adj_req2_3:0x0
> [drm] ln0_1:0x71 ln2_3:0x0 align:0x80 sink:0x0 adj_req0_1:0x84 adj_req2_3:0x0
> [drm] ln0_1:0x71 ln2_3:0x0 align:0x0 sink:0x0 adj_req0_1:0x84 adj_req2_3:0x0
> [drm] ln0_1:0x71 ln2_3:0x0 align:0x0 sink:0x0 adj_req0_1:0x84 adj_req2_3:0x0
>
> Narrow fast vs. wide slow is not an option because the max clock is
> 270000 and the 1920x1280 resolution requires 2x270000.
> [drm] DP link computation with lane count min/max 1/2 270000/270000 bpp
> min/max 18/24 pixel clock 164250KHz
>
> The display is functional even though lane 1 is in state 0x1, so just
> return success for channel equalization on eDP.
>
> Introduce QUIRK_EDP_CHANNEL_EQ and match the DMI for a Dell Latitude
> 7200 2-in-1.  This quirk allows channel equalization to succeed even
> though it failed.
>
> Workaround for https://gitlab.freedesktop.org/drm/intel/-/issues/1378

CI reported the patch doesn't apply to drm-tip.  It was developed
against 5.4 and forward ported to 5.10-rc1-ish when I submitted it.
It applied there but not to drm-tip.

5.4 & 5.6.6 is fine until DPMS.  Then when it tries to come out, it
fails link training and gives the flashing.
5.8.16 starts flashing during boot.  I guess the driver now runs link
training during boot?

drm-tip doesn't have the flashing issue.  I guess "drm/i915: Switch to
LTTPR non-transparent mode link training"  or some of the other link
training change lets the hardware succeed?

Oh, this is interesting:
kernel: i915 0000:00:02.0: [drm:hsw_set_signal_levels [i915]] Using
signal levels 02000000
kernel: [drm:intel_dp_link_train_phy [i915]] ln0_1:0x71 ln2_3:0x0
align:0x0 sink:0x0 adj_req0_1:0x84 adj_req2_3:0x0
kernel: i915 0000:00:02.0: [drm:intel_dp_link_train_phy [i915]]
Channel equalization failed 5 times
kernel: i915 0000:00:02.0: [drm:intel_dp_link_train_phy [i915]]
[CONNECTOR:95:eDP-1] Link Training failed at link rate = 270000, lane
count = 2, at DPRX
kernel: i915 0000:00:02.0: [drm:intel_enable_pipe [i915]] enabling pipe A

Note DPRX above, so not using LTTPR.

Looks like the link training logic is wrong. :

intel_dp_link_training_channel_equalization fails, so
intel_dp_link_train_phy fails, but:

intel_dp_link_train_all_phys(struct intel_dp *intel_dp,
                             const struct intel_crtc_state *crtc_state,
                             int lttpr_count)
{
        bool ret = true;
        int i;

        intel_dp_prepare_link_train(intel_dp, crtc_state);

        for (i = lttpr_count - 1; i >= 0; i--) {
                enum drm_dp_phy dp_phy = DP_PHY_LTTPR(i);

                ret = intel_dp_link_train_phy(intel_dp, crtc_state, dp_phy);
                intel_dp_disable_dpcd_training_pattern(intel_dp, dp_phy);

                if (!ret)
                        break;
        }

        if (ret)
                intel_dp_link_train_phy(intel_dp, crtc_state, DP_PHY_DPRX);

Here we don't update ret, so linking training "succeeds" for DPRX.

This does let the 7200 display "work", but it's probably not what you
want in general.

Regards,
Jason