The ti-sn65dsi86 MIPI DSI to eDP bridge chip has a dedicated hardware HPD (Hot Plug Detect) pin on it, but it's mostly useless for eDP because of excessive debouncing in hardware. Specifically there is no way to disable the debouncing and for eDP debouncing hurts you because HPD is just used for knowing when the panel is ready, not for detecting physical plug events.
Currently the driver in Linux just assumes that nobody has HPD hooked up. It relies on folks setting the "no-hpd" property in the panel node to specify that HPD isn't hooked up and then the panel driver using this to add some worst case delays when turning on the panel.
Apparently it's also useful to specify "no-hpd" in the bridge node so that the bridge driver can make sure it's doing the right thing without peeking into the panel [1]. This would be used if anyone ever found it useful to implement support for the HW HPD pin on the bridge. Let's add this property to the bindings.
NOTES: - This is somewhat of a backward-incompatible change. All current known users of ti-sn65dsi86 didn't have "no-hpd" specified in the bridge node yet none of them had HPD hooked up. This worked because the current Linux driver just assumed that HPD was never hooked up. We could make it less incompatible by saying that for this bridge it's assumed HPD isn't hooked up _unless_ a property is defined, but "no-hpd" is much more standard and it's unlikely to matter unless someone quickly goes and implements HPD in the driver. - It is sensible to specify "no-hpd" at the bridge chip level and specify "hpd-gpios" at the panel level. That would mean HPD is hooked up to some other GPIO in the system, just not the hardware HPD pin on the bridge chip.
[1] https://lore.kernel.org/r/20200417180819.GE5861@pendragon.ideasonboard.com
Signed-off-by: Douglas Anderson dianders@chromium.org Reviewed-by: Stephen Boyd swboyd@chromium.org Reviewed-by: Linus Walleij linus.walleij@linaro.org ---
Changes in v4: - Tacked on "or is otherwise unusable." to description.
Changes in v3: - useful implement => useful to implement
Changes in v2: - ("dt-bindings: drm/bridge: ti-sn65dsi86: Document no-hpd") new for v2.
.../devicetree/bindings/display/bridge/ti,sn65dsi86.yaml | 8 ++++++++ 1 file changed, 8 insertions(+)
diff --git a/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml b/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml index 6d7d40ad45ac..75c4e8b8e4b7 100644 --- a/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml +++ b/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml @@ -28,6 +28,12 @@ properties: maxItems: 1 description: GPIO specifier for GPIO1 pin on bridge (active low).
+ no-hpd: + type: boolean + description: + Set if the HPD line on the bridge isn't hooked up to anything or is + otherwise unusable. + vccio-supply: description: A 1.8V supply that powers the digital IOs.
@@ -207,6 +213,8 @@ examples: clocks = <&rpmhcc RPMH_LN_BB_CLK2>; clock-names = "refclk";
+ no-hpd; + ports { #address-cells = <1>; #size-cells = <0>;