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

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Date:   Tue, 25 Oct 2016 16:59:17 +0800
From:   Chen-Yu Tsai <wens@...e.org>
To:     Archit Taneja <architt@...eaurora.org>
Cc:     Chen-Yu Tsai <wens@...e.org>,
        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 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
changing the external reference resistor. The latter would be a hardware
design decision. I would say this qualifies as "dumb".

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?


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