diff mbox series

[v6,3/3] dt-bindings: drm/bridge: ti-sn65dsi86: Document no-hpd

Message ID 20200513145807.v6.3.I72892d485088e57378a4748c86bc0f6c2494d807@changeid
State New
Headers show
Series drm: Prepare to use a GPIO on ti-sn65dsi86 for Hot Plug Detect | expand

Commit Message

Doug Anderson May 13, 2020, 9:59 p.m. UTC
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>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
---

Changes in v6: None
Changes in v5: None
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(+)

Comments

Rob Herring May 26, 2020, 10:22 p.m. UTC | #1
On Wed, May 13, 2020 at 02:59:02PM -0700, Douglas Anderson wrote:
> 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.

I would say 'no-hpd' belongs wherever HPD is broken. So it may still 
make sense in the panel. (Otherwise, it needs to be removed from 
panel-common.yaml and some panel bindings, right?)
 
> [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>
> Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
> ---

In any case,

Reviewed-by: Rob Herring <robh@kernel.org>
diff mbox series

Patch

diff --git a/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml b/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml
index 07d26121afca..be10e8cf31e1 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.
 
@@ -213,6 +219,8 @@  examples:
         clocks = <&rpmhcc RPMH_LN_BB_CLK2>;
         clock-names = "refclk";
 
+        no-hpd;
+
         ports {
           #address-cells = <1>;
           #size-cells = <0>;