[v4,0/6] drm: Prepare to use a GPIO on ti-sn65dsi86 for Hot Plug Detect

As talked about in commit c2bfc223882d ("drm/bridge: ti-sn65dsi86:
Remove the mystery delay"), the normal HPD pin on ti-sn65dsi86 is
kinda useless, at least for embedded DisplayPort (eDP).  However,
despite the fact that the actual HPD pin on the bridge is mostly
useless for eDP, the concept of HPD for eDP still makes sense.  It
allows us to optimize out a hardcoded delay that many panels need if
HPD isn't hooked up.  Panel timing diagrams show HPD as one of the
events to measure timing from and we have to assume the worst case if
we can't actually read HPD.

One way to use HPD for eDP without using the mostly useless HPD pin on
ti-sn65dsi86 is to route the panel's HPD somewhere else in the system,
like to a GPIO.  This works great because eDP panels aren't physically
hotplugged.  That means the debouncing logic that caused us problems
wasn't really needed and a raw GPIO works great.

As per the above, a smart board designer would realize the value of
HPD and choose to route it to a GPIO somewhere on the board to avoid
the silly sn65dsi86 debouncing.  While said "smart designer" could
theoretically route HPD anywhere on the board, a really smart designer
would realize that there are several GPIOs on the bridge itself that
are nearly useless for anything but this purpose and route HPD to one
of those.

This series of patches is intended to allow the scenario described
above.

This patch has been tested on a board that is not yet mainline.  On
the hardware I have:
- Panel spec says HPD could take up to 200 ms to come up, so without
  HPD hooked up we need to delay 200 ms.
- On my board the panel is powered by the same rail as the
  touchscreen.  By chance of probe order the touchscreen comes up
  first.  This means by the time we check HPD in ti_sn_bridge_enable()
  it's already up.  Thus we can use the panel on 200 ms earlier.
- If I measure HPD on this pane it comes up ~56 ms after the panel is
  powered.  This means I can save 144 ms of delay.

Side effects (though not main goals) of this series are:
- ti-sn65dsi86 GPIOs are now exported in Linux.
- ti-sn65dsi86 bindings are converted to yaml.
- Common panel bindings now have "hpd-gpios" listed.
- The simple-panel driver in Linux can delay in prepare based on
  "hpd-gpios"
- ti-sn65dsi86 bindings (and current user) now specifies "no-hpd"
  if HPD isn't hooked up.

Changes in v4:
- Don't include gpio.h
- Use gpiochip_get_data() instead of container_of() to get data.
- GPIOF_DIR_XXX => GPIO_LINE_DIRECTION_XXX
- Use Linus W's favorite syntax to read a bit from a bitfield.
- Define and use SN_GPIO_MUX_MASK.
- Add a comment about why we use a bitmap for gchip_output.
- Tacked on "or is otherwise unusable." to description.

Changes in v3:
- Becaue => Because
- Add a kernel-doc to our pdata to clarify double-duty of gchip_output.
- More comments about how powering off affects us (get_dir, dir_input).
- Cleanup tail of ti_sn_setup_gpio_controller() to avoid one "return".
- Use a bitmap rather than rolling my own.
- Remind how gpio_get_optional() works in the commit message.
- useful implement => useful to implement

Changes in v2:
- ("Export...GPIOs") is 1/2 of replacement for ("Allow...bridge GPIOs")
- ("dt-bindings: display: Add hpd-gpios to panel-common...") new for v2
- ("simple...hpd-gpios") is 1/2 of replacement for ("Allow...bridge GPIOs")
- specification => specifier.
- power up => power.
- Added back missing suspend-gpios.
- data-lanes and lane-polarities are are the right place now.
- endpoints don't need to be patternProperties.
- Specified more details for data-lanes and lane-polarities.
- Added old example back in, fixing bugs in it.
- Example i2c bus is just called "i2c", not "i2c1" now.
- ("dt-bindings: drm/bridge: ti-sn65dsi86: Document no-hpd") new for v2.
- ("arm64: dts: sdm845: Add "no-hpd" to sn65dsi86 on cheza") new for v2.

Douglas Anderson (6):
  drm/bridge: ti-sn65dsi86: Export bridge GPIOs to Linux
  dt-bindings: display: Add hpd-gpios to panel-common bindings
  drm/panel-simple: Support hpd-gpios for delaying prepare()
  dt-bindings: drm/bridge: ti-sn65dsi86: Convert to yaml
  dt-bindings: drm/bridge: ti-sn65dsi86: Document no-hpd
  arm64: dts: sdm845: Add "no-hpd" to sn65dsi86 on cheza

 .../bindings/display/bridge/ti,sn65dsi86.txt  |  87 ------
 .../bindings/display/bridge/ti,sn65dsi86.yaml | 287 ++++++++++++++++++
 .../bindings/display/panel/panel-common.yaml  |   6 +
 arch/arm64/boot/dts/qcom/sdm845-cheza.dtsi    |   2 +
 drivers/gpu/drm/bridge/ti-sn65dsi86.c         | 195 ++++++++++++
 drivers/gpu/drm/panel/panel-simple.c          |  53 ++++
 6 files changed, 543 insertions(+), 87 deletions(-)
 delete mode 100644 Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.txt
 create mode 100644 Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml

Hi,

On Thu, Apr 30, 2020 at 12:46 PM Douglas Anderson <dianders@chromium.org> wrote:
>
> The ti-sn65dsi86 MIPI DSI to eDP bridge chip has 4 pins on it that can
> be used as GPIOs in a system.  Each pin can be configured as input,
> output, or a special function for the bridge chip.  These are:
> - GPIO1: SUSPEND Input
> - GPIO2: DSIA VSYNC
> - GPIO3: DSIA HSYNC or VSYNC
> - GPIO4: PWM

One suggestion that came off-list is to change the code to make the
numbering match up better with the datasheet.  Right now if you want
GPIO 2 you have to refer to it like:

hpd-gpios = <&sn65dsi86_bridge 1 GPIO_ACTIVE_HIGH>;

That's because the code right now numbers things starting at 0 even if
the datasheet numbers things starting at 1.

I'm planning to spin the series later today for this.  While at it,
I'll squash my yaml bindings fixup into the bindings patch in this
series.  If someone would rather I wait and not spin yet, please yell.

On Thu, May 7, 2020 at 4:39 PM Doug Anderson <dianders@chromium.org> wrote:

> One suggestion that came off-list is to change the code to make the
> numbering match up better with the datasheet.  Right now if you want
> GPIO 2 you have to refer to it like:
>
> hpd-gpios = <&sn65dsi86_bridge 1 GPIO_ACTIVE_HIGH>;
>
> That's because the code right now numbers things starting at 0 even if
> the datasheet numbers things starting at 1.

This is the hallmark of mixed-mode IC engineers at work.
They are at heart analog IC designers so of course they
enumerate everything starting at 1.

Digital IC designers are like programmers and start on 0.

Never the twain shall meet...

Yours,
Linus Walleij