[PATCH v4 0/9] MediaTek DisplayPort: support eDP and aux-bus

[AngeloGioacchino Del Regno]

Changes in v4:
 - Set data lanes to idle to prevent stalls if bootloader didn't
   properly close the eDP port
 - Now using the .done_probing() callback for AUX bus to prevent
   probe deferral loops in case the panel-edp driver is a module
   as previously seen with another bridge driver (ANX7625) on
   some other SoCs (MT8192 and others)
 - Rebased over next-20230706
 - Dropped Chen-Yu's T-b tag on last patch as some logic changed
   (before, I wasn't using the .done_probing() callback).

Changes in v3:
 - Added DPTX AUX block initialization before trying to communicate
   to stop relying on the bootloader keeping it initialized before
   booting Linux.
 - Fixed commit description for patch [09/09] and removed commented
   out code (that slipped from dev phase.. sorry!).

This series adds "real" support for eDP in the mtk-dp DisplayPort driver.

Explaining the "real":
Before this change, the DisplayPort driver did support eDP to some
extent, but it was treating it entirely like a regular DP interface
which is partially fine, after all, embedded DisplayPort *is* actually
DisplayPort, but there might be some differences to account for... and
this is for both small performance improvements and, more importantly,
for correct functionality in some systems.

Functionality first:

One of the common differences found in various boards implementing eDP
and machines using an eDP panel is that many times the HPD line is not
connected. This *must* be accounted for: at startup, this specific IP
will raise a HPD interrupt (which should maybe be ignored... as it does
not appear to be a "real" event...) that will make the eDP panel to be
detected and to actually work but, after a suspend-resume cycle, there
will be no HPD interrupt (as there's no HPD line in my case!) producing
a functionality issue - specifically, the DP Link Training fails because
the panel doesn't get powered up, then it stays black and won't work
until rebooting the machine (or removing and reinserting the module I
think, but I haven't tried that).

Now for.. both:
eDP panels are *e*DP because they are *not* removable (in the sense that
you can't unplug the cable without disassembling the machine, in which
case, the machine shall be powered down..!): this (correct) assumption
makes us able to solve some issues and to also gain a little performance
during PM operations.

What was done here is:
 - Caching the EDID if the panel is eDP: we're always going to read the
   same data everytime, so we can just cache that (as it's small enough)
   shortening PM resume times for the eDP driver instance;
 - Always return connector_status_connected if it's eDP: non-removable
   means connector_status_disconnected can't happen during runtime...
   this also saves us some time and even power, as we won't have to
   perform yet another power cycle of the HW;
 - Added aux-bus support!
   This makes us able to rely on panel autodetection from the EDID,
   avoiding to add more and more panel timings to panel-edp and, even
   better, allowing to use one panel node in devicetrees for multiple
   variants of the same machine since, at that point, it's not important
   to "preventively know" what panel we have (eh, it's autodetected...!).

This was tested on a MT8195 Cherry Tomato Chromebook (panel-edp on aux-bus)


P.S.: For your own testing commodity, here's a reference devicetree:
&edp_tx {
	status = "okay";

	pinctrl-names = "default";
	pinctrl-0 = <&edptx_pins_default>;

	ports {
		#address-cells = <1>;
		#size-cells = <0>;

		port at 0 {
			reg = <0>;
			edp_in: endpoint {
				remote-endpoint = <&dp_intf0_out>;
			};
		};

		port at 1 {
			reg = <1>;
			edp_out: endpoint {
				data-lanes = <0 1 2 3>;
				remote-endpoint = <&panel_in>;
			};
		};
	};

	aux-bus {
		panel: panel {
			compatible = "edp-panel";
			power-supply = <&pp3300_disp_x>;
			backlight = <&backlight_lcd0>;
			port {
				panel_in: endpoint {
					remote-endpoint = <&edp_out>;
				};
			};
		};
	};
};


AngeloGioacchino Del Regno (9):
  drm/mediatek: dp: Cache EDID for eDP panel
  drm/mediatek: dp: Move AUX and panel poweron/off sequence to function
  drm/mediatek: dp: Always return connected status for eDP in .detect()
  drm/mediatek: dp: Always set cable_plugged_in at resume for eDP panel
  drm/mediatek: dp: Change logging to dev for mtk_dp_aux_transfer()
  drm/mediatek: dp: Enable event interrupt only when bridge attached
  drm/mediatek: dp: Use devm variant of drm_bridge_add()
  drm/mediatek: dp: Move AUX_P0 setting to
    mtk_dp_initialize_aux_settings()
  drm/mediatek: dp: Add support for embedded DisplayPort aux-bus

 drivers/gpu/drm/mediatek/mtk_dp.c | 197 +++++++++++++++++++-----------
 1 file changed, 127 insertions(+), 70 deletions(-)

-- 
2.40.1