gpio-omap: handle bias flag for gpio line

Tony and Linus -

I'm hoping you could give me some feedback on this approach. My goal is
to allow a userspace library (like libgpiod) to be able to set pull-up
and pull-downs on the AM3358.

I've changed gpio-omap.c so omap_gpio_set_config() will call
omap_gpio_bias() when the parameter is one of the PIN_CONFIG_BIAS_xxx
flags. That function will call gpiochip_generic_config() and the rest
proceeds normally without further changes.

However, this does require the following:
1) patch to fix return code in pcs_parse_pinconf() [0]
2) patch to add the gpio-ranges mappings [1]
2) change the compatible for the am33xx_pinmux node to "pinconf-single"
4) add "pinctrl-single,bias-pulldown" or "pinctrl-single,bias-pullup"
   property to a pin node.

I found the binding documentation [2] for the bias properties to be very
confusing as to how those 4 values work:

> - pinctrl-single,bias-pullup : array of value that are used to configure the
>   input bias pullup in the pinmux register.
>
> 		/* input, enabled pullup bits, disabled pullup bits, mask */
> 		pinctrl-single,bias-pullup = <0 1 0 1>;
> 
> - pinctrl-single,bias-pulldown : array of value that are used to configure the
>   input bias pulldown in the pinmux register.

>		/* input, enabled pulldown bits, disabled pulldown bits, mask */
>		pinctrl-single,bias-pulldown = <2 2 0 2>;

For AM3358, the pin conf register has the format [3]:

bit	attribute      value
  6	slew           { 0: fast, 1: slow }
  5     rx_active      { 0: rx disable, 1: rx enabled }
  4     pu_typesel     { 0: pulldown select, 1: pullup select }
  3     puden          { 0: pud enable, 1: disabled }
  2     mode           3 bits to selec mode 0 to 7
  1     mode
  0     mode

I figured out the values for bias-pull{up,down} properties based on:

        16      8       4       2       1
        2^4     2^3     2^2     2^1     2^0
mask    1       1       0       0       0       24
pull-up 1       1       0       0       0       24
pull-dn 0       1       0       0       0       8
none    0       0       0       0       0       0

Thus the properties are:

	pinctrl-single,bias-pulldown = < 8  8 0 24>;
	pinctrl-single,bias-pullup   = <24 24 0 24>;

For an example, I added "pinctrl-single,bias-pulldown" to the node
pinmux-ehrpwm1-pins in am335x-pocketbeagle.dts:

ehrpwm1_pins: pinmux-ehrpwm1-pins {
	pinctrl-single,pins = <
		AM33XX_PADCONF(AM335X_PIN_GPMC_A2, PIN_OUTPUT_PULLDOWN, MUX_MODE6)
		/* (U14) gpmc_a2.ehrpwm1A */
	>;
	pinctrl-single,bias-pulldown = <8 8 0 24>;
}

AM335X_PIN_GPMC_A2 is PIN18 based on the memory offset and corresponds
to gpiochip 1 line 18 (MUX_MODE1).

Here is function graph trace (note: I used -fno-inline) when gpiomon
runs with pull-down bias flag on gpiochip 1 line 18:

debian@beaglebone:~$ ./libgpiod/tools/gpiomon -B pull-down 1 18

 0)               |  gpio_ioctl() { 
 0)   2.708 us    |    gpiochip_get_desc();
 0)               |    gpiod_request() {
 0)               |      gpiod_request_commit() {
 0)   2.834 us    |        gpiochip_line_is_valid();
 0)               |        omap_gpio_request() {
 0)   2.708 us    |          omap_enable_gpio_module();
 0) + 15.375 us   |        }
 0)               |        gpiod_get_direction() {
 0)   2.500 us    |          gpiod_to_chip();
 0)   3.459 us    |          omap_gpio_get_direction();
 0) + 16.125 us   |        }
 0) + 52.875 us   |      }
 0) + 60.750 us   |    }
 0)               |    gpiod_direction_input() {
 0)               |      omap_gpio_input() {
 0)   2.500 us    |        omap_set_gpio_direction();
 0)   8.125 us    |      }
 0)               |      gpio_set_bias() {
 0)               |        gpio_set_config() {
 0)               |          gpio_do_set_config() {
 0)               |            omap_gpio_set_config() {
 0)               |              omap_gpio_bias() {
 0)               |                gpiochip_generic_config() {
 0)               |                  pinctrl_gpio_set_config() {
 0)               |                    pinctrl_get_device_gpio_range() {
 0)   3.292 us    |                      pinctrl_match_gpio_range();
 0)   8.667 us    |                    }
 0)               |                    pinconf_set_config() {
 0)               |                      pcs_pinconf_set() {
 0)   5.250 us    |                        pcs_get_function();
 0)   3.417 us    |                        pcs_readl();
 0)               |                        pcs_pinconf_clear_bias() {
 0)               |                          pcs_pinconf_set() {
 0)   3.125 us    |                            pcs_get_function();
 0)   2.833 us    |                            pcs_readl();
 0)   3.792 us    |                            pcs_writel();
 0) + 20.958 us   |                          }
 0)               |                          pcs_pinconf_set() {
 0)   2.750 us    |                            pcs_get_function();
 0)   8.083 us    |                          }
 0) + 37.333 us   |                        }
 0)   2.708 us    |                        pcs_writel();
 0) + 65.916 us   |                      }
 0) + 71.375 us   |                    }
 0) + 89.083 us   |                  }
 0) + 95.292 us   |                }
 0) ! 100.750 us  |              }
 0) ! 106.667 us  |            }
 0) ! 111.791 us  |          }
 0) ! 117.167 us  |        }
 0) ! 122.917 us  |      }
 0)   2.750 us    |      desc_to_gpio();
 0) ! 146.459 us  |    }

Thus, all it seems I can do is set a bias flag on a gpio line if the
corresponding bias property is already set for that pin in dts.  This
does not accomplish anything.

The ideal capability in my mind would be for pinctrl-single to
understand that a pin defined by a "pinctrl-single,pins" property has
both a configuration value and a mux mode value.  The current pinconf
support is based on "pinctrl-single,bias-pull{up,down}" which does not
seem useful.

I would appreciate any feedback.

Thanks,
Drew

[0] https://lore.kernel.org/linux-omap/20200608125143.GA2789203@x1/
[1] https://lore.kernel.org/linux-omap/20200602131428.GA496390@x1/
[2] Documentation/devicetree/bindings/pinctrl/pinctrl-single.txt
[3] https://www.ti.com/lit/ug/spruh73q/spruh73q.pdf (see Figure 9-51)
---
 arch/arm/boot/dts/am335x-pocketbeagle.dts |  2 ++
 arch/arm/boot/dts/am33xx-l4.dtsi          |  2 +-
 drivers/gpio/Makefile                     |  2 +-
 drivers/gpio/gpio-omap.c                  | 37 ++++++++++++++++++++---
 drivers/pinctrl/Makefile                  |  2 +-
 5 files changed, 38 insertions(+), 7 deletions(-)

Message ID	20200625002736.GA24954@x1
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Series	gpio-omap: handle bias flag for gpio line \| expand gpio-omap: handle bias flag for gpio line

gpio-omap: handle bias flag for gpio line

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