[v3,1/6] drm/bridge: ti-sn65dsi86: Export bridge GPIOs to Linux

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

Let's expose these pins as GPIOs.  A few notes:
- Access to ti-sn65dsi86 is via i2c so we set "can_sleep".
- These pins can't be configured for IRQ.
- There are no programmable pulls or other fancy features.
- Keeping the bridge chip powered might be expensive.  The driver is
  setup such that if all used GPIOs are only inputs we'll power the
  bridge chip on just long enough to read the GPIO and then power it
  off again.  Setting a GPIO as output will keep the bridge powered.
- If someone releases a GPIO we'll implicitly switch it to an input so
  we no longer need to keep the bridge powered for it.

Because of all of the above limitations we just need to implement a
bare-bones GPIO driver.  The device tree bindings already account for
this device being a GPIO controller so we only need the driver changes
for it.

NOTE: Despite the fact that these pins are nominally muxable I don't
believe it makes sense to expose them through the pinctrl interface as
well as the GPIO interface.  The special functions are things that the
bridge chip driver itself would care about and it can just configure
the pins as needed.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Bartosz Golaszewski <bgolaszewski@baylibre.com>
---

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.

Changes in v2:
- ("Export...GPIOs") is 1/2 of replacement for ("Allow...bridge GPIOs")

 drivers/gpu/drm/bridge/ti-sn65dsi86.c | 194 ++++++++++++++++++++++++++
 1 file changed, 194 insertions(+)

Quoting Douglas Anderson (2020-04-23 09:25:43)
> 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
> 
> Let's expose these pins as GPIOs.  A few notes:
> - Access to ti-sn65dsi86 is via i2c so we set "can_sleep".
> - These pins can't be configured for IRQ.
> - There are no programmable pulls or other fancy features.
> - Keeping the bridge chip powered might be expensive.  The driver is
>   setup such that if all used GPIOs are only inputs we'll power the
>   bridge chip on just long enough to read the GPIO and then power it
>   off again.  Setting a GPIO as output will keep the bridge powered.
> - If someone releases a GPIO we'll implicitly switch it to an input so
>   we no longer need to keep the bridge powered for it.
> 
> Because of all of the above limitations we just need to implement a
> bare-bones GPIO driver.  The device tree bindings already account for
> this device being a GPIO controller so we only need the driver changes
> for it.
> 
> NOTE: Despite the fact that these pins are nominally muxable I don't
> believe it makes sense to expose them through the pinctrl interface as
> well as the GPIO interface.  The special functions are things that the
> bridge chip driver itself would care about and it can just configure
> the pins as needed.
> 
> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> Cc: Linus Walleij <linus.walleij@linaro.org>
> Cc: Bartosz Golaszewski <bgolaszewski@baylibre.com>
> ---

Reviewed-by: Stephen Boyd <swboyd@chromium.org>

On Thu, Apr 23, 2020 at 6:26 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
>
> Let's expose these pins as GPIOs.  A few notes:
> - Access to ti-sn65dsi86 is via i2c so we set "can_sleep".
> - These pins can't be configured for IRQ.
> - There are no programmable pulls or other fancy features.
> - Keeping the bridge chip powered might be expensive.  The driver is
>   setup such that if all used GPIOs are only inputs we'll power the
>   bridge chip on just long enough to read the GPIO and then power it
>   off again.  Setting a GPIO as output will keep the bridge powered.
> - If someone releases a GPIO we'll implicitly switch it to an input so
>   we no longer need to keep the bridge powered for it.
>
> Because of all of the above limitations we just need to implement a
> bare-bones GPIO driver.  The device tree bindings already account for
> this device being a GPIO controller so we only need the driver changes
> for it.
>
> NOTE: Despite the fact that these pins are nominally muxable I don't
> believe it makes sense to expose them through the pinctrl interface as
> well as the GPIO interface.  The special functions are things that the
> bridge chip driver itself would care about and it can just configure
> the pins as needed.
>
> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> Cc: Linus Walleij <linus.walleij@linaro.org>
> Cc: Bartosz Golaszewski <bgolaszewski@baylibre.com>

Pretty cool.

I wonder if this chip could use the generic regmap GPIO helpers
that we are working on when they come around?
https://lore.kernel.org/linux-gpio/20200423174543.17161-11-michael@walle.cc/

> +#include <linux/gpio/driver.h>
> +#include <linux/gpio.h>

Only <linux/gpio/driver.h> should be needed else you are doing
something wrong.

> + * @gchip:        If we expose our GPIOs, this is used.
> + * @gchip_output: A cache of whether we've set GPIOs to output.  This
> + *                serves double-duty of keeping track of the direction and
> + *                also keeping track of whether we've incremented the
> + *                pm_runtime reference count for this pin, which we do
> + *                whenever a pin is configured as an output.

That sounds a bit hairy but I guess it's fine.

> + */
>  struct ti_sn_bridge {
>         struct device                   *dev;
>         struct regmap                   *regmap;
> @@ -102,6 +136,9 @@ struct ti_sn_bridge {
>         struct gpio_desc                *enable_gpio;
>         struct regulator_bulk_data      supplies[SN_REGULATOR_SUPPLY_NUM];
>         int                             dp_lanes;
> +
> +       struct gpio_chip                gchip;
> +       DECLARE_BITMAP(gchip_output, SN_NUM_GPIOS);

Do you really need a bitmap for 4 bits? Can't you just have something
like an u8 and check bit 0,1,2,3 ... well I suppose it has some elegance to
it as well but... hm.

> +static struct ti_sn_bridge *gchip_to_pdata(struct gpio_chip *chip)
> +{
> +       return container_of(chip, struct ti_sn_bridge, gchip);
> +}
> +
> +static int ti_sn_bridge_gpio_get_direction(struct gpio_chip *chip,
> +                                          unsigned int offset)
> +{
> +       struct ti_sn_bridge *pdata = gchip_to_pdata(chip);

Is there some specific reason why you don't just use
gpiochip_get_data()?

> +       /*
> +        * We already have to keep track of the direction because we use
> +        * that to figure out whether we've powered the device.  We can
> +        * just return that rather than (maybe) powering up the device
> +        * to ask its direction.
> +        */
> +       return test_bit(offset, pdata->gchip_output) ?
> +               GPIOF_DIR_OUT : GPIOF_DIR_IN;
> +}

Don't use these legacy defines, they are for consumers.
Use GPIO_LINE_DIRECTION_IN  and GPIO_LINE_DIRECTION_OUT.
from <linux/gpio/driver.h>

> +       ret = regmap_read(pdata->regmap, SN_GPIO_IO_REG, &val);
> +       pm_runtime_put(pdata->dev);
> +
> +       if (ret)
> +               return ret;
> +
> +       return (val >> (SN_GPIO_INPUT_SHIFT + offset)) & 1;

My preferred way to do this is:

#include <linux/bits.h>

return !!(val & BIT(SN_GPIO_INPUT_SHIFT + offset));

> +static void ti_sn_bridge_gpio_set(struct gpio_chip *chip, unsigned int offset,
> +                                 int val)
> +{
> +       struct ti_sn_bridge *pdata = gchip_to_pdata(chip);
> +       int ret;
> +
> +       if (!test_bit(offset, pdata->gchip_output)) {
> +               dev_err(pdata->dev, "Ignoring GPIO set while input\n");
> +               return;
> +       }
> +
> +       val &= 1;
> +       ret = regmap_update_bits(pdata->regmap, SN_GPIO_IO_REG,
> +                                BIT(SN_GPIO_OUTPUT_SHIFT + offset),
> +                                val << (SN_GPIO_OUTPUT_SHIFT + offset));

Looks like a job for the generic helper library.

> +static int ti_sn_bridge_gpio_direction_input(struct gpio_chip *chip,
> +                                            unsigned int offset)
> +{
> +       struct ti_sn_bridge *pdata = gchip_to_pdata(chip);
> +       int shift = offset * 2;
> +       int ret;
> +
> +       if (!test_and_clear_bit(offset, pdata->gchip_output))
> +               return 0;
> +
> +       ret = regmap_update_bits(pdata->regmap, SN_GPIO_CTRL_REG,
> +                                0x3 << shift, SN_GPIO_MUX_INPUT << shift);

But this 0x03 does not look very generic, it's not just 1 bit but 2.

Overall it looks good, just the minor things above need fixing or
looking into.

Yours,
Linus Walleij

Hi,

On Tue, Apr 28, 2020 at 5:44 AM Linus Walleij <linus.walleij@linaro.org> wrote:
>
> On Thu, Apr 23, 2020 at 6:26 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
> >
> > Let's expose these pins as GPIOs.  A few notes:
> > - Access to ti-sn65dsi86 is via i2c so we set "can_sleep".
> > - These pins can't be configured for IRQ.
> > - There are no programmable pulls or other fancy features.
> > - Keeping the bridge chip powered might be expensive.  The driver is
> >   setup such that if all used GPIOs are only inputs we'll power the
> >   bridge chip on just long enough to read the GPIO and then power it
> >   off again.  Setting a GPIO as output will keep the bridge powered.
> > - If someone releases a GPIO we'll implicitly switch it to an input so
> >   we no longer need to keep the bridge powered for it.
> >
> > Because of all of the above limitations we just need to implement a
> > bare-bones GPIO driver.  The device tree bindings already account for
> > this device being a GPIO controller so we only need the driver changes
> > for it.
> >
> > NOTE: Despite the fact that these pins are nominally muxable I don't
> > believe it makes sense to expose them through the pinctrl interface as
> > well as the GPIO interface.  The special functions are things that the
> > bridge chip driver itself would care about and it can just configure
> > the pins as needed.
> >
> > Signed-off-by: Douglas Anderson <dianders@chromium.org>
> > Cc: Linus Walleij <linus.walleij@linaro.org>
> > Cc: Bartosz Golaszewski <bgolaszewski@baylibre.com>
>
> Pretty cool.
>
> I wonder if this chip could use the generic regmap GPIO helpers
> that we are working on when they come around?
> https://lore.kernel.org/linux-gpio/20200423174543.17161-11-michael@walle.cc/

An important part of my patch is the handling of power management.
Specifically:
* If the GPIO is an input we don't need to keep the device powered,
just power it temporarily to read the pin.
* If the GPIO is an output we do need to keep the device powered.

I suppose that could be common for other similar devices so as long as
the generic interfaces can handle this concept we can try to use it.


> > +#include <linux/gpio/driver.h>
> > +#include <linux/gpio.h>
>
> Only <linux/gpio/driver.h> should be needed else you are doing
> something wrong.

It's because I needed GPIOF_DIR_OUT / GPIOF_DIR_IN which was
apparently wrong.  See below.


> > + * @gchip:        If we expose our GPIOs, this is used.
> > + * @gchip_output: A cache of whether we've set GPIOs to output.  This
> > + *                serves double-duty of keeping track of the direction and
> > + *                also keeping track of whether we've incremented the
> > + *                pm_runtime reference count for this pin, which we do
> > + *                whenever a pin is configured as an output.
>
> That sounds a bit hairy but I guess it's fine.
>
> > + */
> >  struct ti_sn_bridge {
> >         struct device                   *dev;
> >         struct regmap                   *regmap;
> > @@ -102,6 +136,9 @@ struct ti_sn_bridge {
> >         struct gpio_desc                *enable_gpio;
> >         struct regulator_bulk_data      supplies[SN_REGULATOR_SUPPLY_NUM];
> >         int                             dp_lanes;
> > +
> > +       struct gpio_chip                gchip;
> > +       DECLARE_BITMAP(gchip_output, SN_NUM_GPIOS);
>
> Do you really need a bitmap for 4 bits? Can't you just have something
> like an u8 and check bit 0,1,2,3 ... well I suppose it has some elegance to
> it as well but... hm.

Doing so requires adding a lock to this driver to handle concurrent
users of the different GPIOs.  I can go back and do that but I'd
rather not.

Some prior discussion:

https://lore.kernel.org/r/CAD=FV=WJONhm4ukwZa2vGtozrz_SmLuTCLxVimnGba7wRPPzgQ@mail.gmail.com

...if you want me to change this to a u8 + a mutex then please let me
know, otherwise I'll assume keeping it a bitmap is fine.


> > +static struct ti_sn_bridge *gchip_to_pdata(struct gpio_chip *chip)
> > +{
> > +       return container_of(chip, struct ti_sn_bridge, gchip);
> > +}
> > +
> > +static int ti_sn_bridge_gpio_get_direction(struct gpio_chip *chip,
> > +                                          unsigned int offset)
> > +{
> > +       struct ti_sn_bridge *pdata = gchip_to_pdata(chip);
>
> Is there some specific reason why you don't just use
> gpiochip_get_data()?

I guess I'm used to interfaces that don't have a data pointer.  I'll
change it to gpiochip_get_data() at your suggestion, though (I think)
it might be slightly less efficient (a function call and a pointer
dereference compared to a subtract operation).


> > +       /*
> > +        * We already have to keep track of the direction because we use
> > +        * that to figure out whether we've powered the device.  We can
> > +        * just return that rather than (maybe) powering up the device
> > +        * to ask its direction.
> > +        */
> > +       return test_bit(offset, pdata->gchip_output) ?
> > +               GPIOF_DIR_OUT : GPIOF_DIR_IN;
> > +}
>
> Don't use these legacy defines, they are for consumers.
> Use GPIO_LINE_DIRECTION_IN  and GPIO_LINE_DIRECTION_OUT.
> from <linux/gpio/driver.h>

That's what I get for reading the comments.  I'll change this in the
next version.  I've also sent the following patch to help keep other
people from falling into my trap:

https://lore.kernel.org/r/20200428172322.1.I396f351e364f3c09df7c7606e79abefb8682c092@changeid/


> > +       ret = regmap_read(pdata->regmap, SN_GPIO_IO_REG, &val);
> > +       pm_runtime_put(pdata->dev);
> > +
> > +       if (ret)
> > +               return ret;
> > +
> > +       return (val >> (SN_GPIO_INPUT_SHIFT + offset)) & 1;
>
> My preferred way to do this is:
>
> #include <linux/bits.h>
>
> return !!(val & BIT(SN_GPIO_INPUT_SHIFT + offset));

Somehow I think of "!!" as being a bool and this function as returning
something that's logically an int.  It really doesn't matter a whole
lot and I'm happy to change it, so I'll change it in the next version.


> > +static void ti_sn_bridge_gpio_set(struct gpio_chip *chip, unsigned int offset,
> > +                                 int val)
> > +{
> > +       struct ti_sn_bridge *pdata = gchip_to_pdata(chip);
> > +       int ret;
> > +
> > +       if (!test_bit(offset, pdata->gchip_output)) {
> > +               dev_err(pdata->dev, "Ignoring GPIO set while input\n");
> > +               return;
> > +       }
> > +
> > +       val &= 1;
> > +       ret = regmap_update_bits(pdata->regmap, SN_GPIO_IO_REG,
> > +                                BIT(SN_GPIO_OUTPUT_SHIFT + offset),
> > +                                val << (SN_GPIO_OUTPUT_SHIFT + offset));
>
> Looks like a job for the generic helper library.

I think that (for now) this comment is a no-op since the generic
helper library isn't landed yet, right?  ...and it wouldn't handle the
power management I need?  If I'm confused and I need to act on this
comment, please let me know.


> > +static int ti_sn_bridge_gpio_direction_input(struct gpio_chip *chip,
> > +                                            unsigned int offset)
> > +{
> > +       struct ti_sn_bridge *pdata = gchip_to_pdata(chip);
> > +       int shift = offset * 2;
> > +       int ret;
> > +
> > +       if (!test_and_clear_bit(offset, pdata->gchip_output))
> > +               return 0;
> > +
> > +       ret = regmap_update_bits(pdata->regmap, SN_GPIO_CTRL_REG,
> > +                                0x3 << shift, SN_GPIO_MUX_INPUT << shift);
>
> But this 0x03 does not look very generic, it's not just 1 bit but 2.

Sure, I can add #define SN_GPIO_MUX_MASK 0x3.  Basically the mux is:

* 0: input
* 1: output
* 2: special function

As talked about in the patch comments, I don't define this as an
official pinmux driver because that seems overkill.  I'll assume it's
OK to just do the #define and use it.  If you want something more, let
me know.


> Overall it looks good, just the minor things above need fixing or
> looking into.

Thank you very much for the review!  I'll plan to send a new patch out
in the next day or two with minor comments addressed and making the
assumptions I've documented above.  If I got something wrong then
please yell.  ...or yell after I send the next version and I'll send
yet another version after that!  :-)

-Doug