Re: [PATCH v2 1/8] drm/bridge: rgb-to-vga: Support an enable GPIO

[Archit Taneja <architt@codeaurora.org>]

On 10/25/2016 02:29 PM, Chen-Yu Tsai wrote:
> On Tue, Oct 25, 2016 at 4:09 PM, Archit Taneja <architt@codeaurora.org> wrote:
>> Hi,
>>
>> On 10/20/2016 09:13 AM, Chen-Yu Tsai wrote:
>>>
>>> Some rgb-to-vga bridges have an enable GPIO, either directly tied to
>>> an enable pin on the bridge IC, or indirectly controlling a power
>>> switch.
>>>
>>> Add support for it.
>>
>>
>> Does the bridge on your platform have an active/passive DAC, or is it a
>> smarter encoder chip that is capable of doing more? If so, it might be
>> good to have a separate DT compatible string to it, like what's done
>> in the patch titled:
>>
>> drm: bridge: vga-dac: Add adi,adv7123 compatible string
>>
>> so that we can switch to a different driver later if needed.
>
> The chip is GM7123. It is not configurable. It just takes the LCD RGB/SYNC
> signals and converts them to analog. The only things you can change are
> putting it into sleep mode and tweaking the output drive strength by

Is sleep mode the thing that's controlled by this gpio?


> changing the external reference resistor. The latter would be a hardware
> design decision. I would say this qualifies as "dumb".

Yeah, I agree. I'd want feedback from Laurent too, since he had comments
on the usage of the original dumb-vga-dac driver.

>
> I revisited the board schematics, and the enable GPIO actually toggles
> an external LDO regulator. So this might be better modeled as a regulator
> supply?

If you model it as a regulator, how would you toggle the GPIO on your
platform?

Looking at the chip pin out, there is a 3.3V VDD supply needed for the
chip, so it would be good to have an optional 'power' regulator supply
anyway.

Archit

>
>
> Thanks
> ChenYu
>
>>
>> Thanks,
>> Archit
>>
>>
>>>
>>> Signed-off-by: Chen-Yu Tsai <wens@csie.org>
>>> ---
>>>  .../bindings/display/bridge/dumb-vga-dac.txt       |  2 ++
>>>  drivers/gpu/drm/bridge/dumb-vga-dac.c              | 28
>>> ++++++++++++++++++++++
>>>  2 files changed, 30 insertions(+)
>>>
>>> diff --git
>>> a/Documentation/devicetree/bindings/display/bridge/dumb-vga-dac.txt
>>> b/Documentation/devicetree/bindings/display/bridge/dumb-vga-dac.txt
>>> index 003bc246a270..d3484822bf77 100644
>>> --- a/Documentation/devicetree/bindings/display/bridge/dumb-vga-dac.txt
>>> +++ b/Documentation/devicetree/bindings/display/bridge/dumb-vga-dac.txt
>>> @@ -16,6 +16,8 @@ graph bindings specified in
>>> Documentation/devicetree/bindings/graph.txt.
>>>  - Video port 0 for RGB input
>>>  - Video port 1 for VGA output
>>>
>>> +Optional properties:
>>> +- enable-gpios: GPIO pin to enable or disable the bridge
>>>
>>>  Example
>>>  -------
>>> diff --git a/drivers/gpu/drm/bridge/dumb-vga-dac.c
>>> b/drivers/gpu/drm/bridge/dumb-vga-dac.c
>>> index afec232185a7..b487e5e9b56d 100644
>>> --- a/drivers/gpu/drm/bridge/dumb-vga-dac.c
>>> +++ b/drivers/gpu/drm/bridge/dumb-vga-dac.c
>>> @@ -10,6 +10,7 @@
>>>   * the License, or (at your option) any later version.
>>>   */
>>>
>>> +#include <linux/gpio/consumer.h>
>>>  #include <linux/module.h>
>>>  #include <linux/of_graph.h>
>>>
>>> @@ -23,6 +24,7 @@ struct dumb_vga {
>>>         struct drm_connector    connector;
>>>
>>>         struct i2c_adapter      *ddc;
>>> +       struct gpio_desc        *enable_gpio;
>>>  };
>>>
>>>  static inline struct dumb_vga *
>>> @@ -124,8 +126,26 @@ static int dumb_vga_attach(struct drm_bridge *bridge)
>>>         return 0;
>>>  }
>>>
>>> +static void dumb_vga_enable(struct drm_bridge *bridge)
>>> +{
>>> +       struct dumb_vga *vga = drm_bridge_to_dumb_vga(bridge);
>>> +
>>> +       if (vga->enable_gpio)
>>> +               gpiod_set_value_cansleep(vga->enable_gpio, 1);
>>> +}
>>> +
>>> +static void dumb_vga_disable(struct drm_bridge *bridge)
>>> +{
>>> +       struct dumb_vga *vga = drm_bridge_to_dumb_vga(bridge);
>>> +
>>> +       if (vga->enable_gpio)
>>> +               gpiod_set_value_cansleep(vga->enable_gpio, 0);
>>> +}
>>> +
>>>  static const struct drm_bridge_funcs dumb_vga_bridge_funcs = {
>>>         .attach         = dumb_vga_attach,
>>> +       .enable         = dumb_vga_enable,
>>> +       .disable        = dumb_vga_disable,
>>>  };
>>>
>>>  static struct i2c_adapter *dumb_vga_retrieve_ddc(struct device *dev)
>>> @@ -169,6 +189,14 @@ static int dumb_vga_probe(struct platform_device
>>> *pdev)
>>>                 return -ENOMEM;
>>>         platform_set_drvdata(pdev, vga);
>>>
>>> +       vga->enable_gpio = devm_gpiod_get_optional(&pdev->dev, "enable",
>>> +                                                  GPIOD_OUT_LOW);
>>> +       if (IS_ERR(vga->enable_gpio)) {
>>> +               ret = PTR_ERR(vga->enable_gpio);
>>> +               dev_err(&pdev->dev, "failed to request GPIO: %d\n", ret);
>>> +               return ret;
>>> +       }
>>> +
>>>         vga->ddc = dumb_vga_retrieve_ddc(&pdev->dev);
>>>         if (IS_ERR(vga->ddc)) {
>>>                 if (PTR_ERR(vga->ddc) == -ENODEV) {
>>>
>>
>> --
>> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
>> a Linux Foundation Collaborative Project

-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
a Linux Foundation Collaborative Project