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

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Message-ID: <CAGb2v64U-vOW4K2sBk0ipM4JCgmKyZq5R+9STkU9-rTOFqV1eQ@mail.gmail.com>
Date:   Thu, 27 Oct 2016 14:52:42 +0800
From:   Chen-Yu Tsai <wens@...e.org>
To:     Archit Taneja <architt@...eaurora.org>
Cc:     Chen-Yu Tsai <wens@...e.org>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Maxime Ripard <maxime.ripard@...e-electrons.com>,
        David Airlie <airlied@...ux.ie>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        devicetree <devicetree@...r.kernel.org>,
        linux-sunxi <linux-sunxi@...glegroups.com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2 1/8] drm/bridge: rgb-to-vga: Support an enable GPIO

On Thu, Oct 27, 2016 at 2:40 PM, Archit Taneja <architt@...eaurora.org> wrote:
>
>
> On 10/25/2016 02:29 PM, Chen-Yu Tsai wrote:
>>
>> On Tue, Oct 25, 2016 at 4:09 PM, Archit Taneja <architt@...eaurora.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?

Not on this particular board. The gpio controls the external LDO that
supplies 3.3V to VDD.

>
>> 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.

Yes, that it what I plan to do. I'll drop the enable-gpios property,
and add a power-supply property for the regulator.

Regards
ChenYu

>
> Archit
>
>
>>
>>
>> Thanks
>> ChenYu
>>
>>>
>>> Thanks,
>>> Archit
>>>
>>>
>>>>
>>>> Signed-off-by: Chen-Yu Tsai <wens@...e.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