RE: [PATCH V3 3/3] gpio: modepin: Add driver support for modepin GPIO controller

[Piyush Mehta <piyushm@xxxxxxxxxx>]

Hi Ahmad,

-----Original Message-----
From: Ahmad Fatoum <a.fatoum@xxxxxxxxxxxxxx> 
Sent: Wednesday, August 18, 2021 2:22 PM
To: Piyush Mehta <piyushm@xxxxxxxxxx>; arnd@xxxxxxxx; zou_wei@xxxxxxxxxx; gregkh@xxxxxxxxxxxxxxxxxxx; linus.walleij@xxxxxxxxxx; Michal Simek <michals@xxxxxxxxxx>; Jiaying Liang <jliang@xxxxxxxxxx>; iwamatsu@xxxxxxxxxxx; bgolaszewski@xxxxxxxxxxxx; robh+dt@xxxxxxxxxx; Rajan Vaja <RAJANV@xxxxxxxxxx>
Cc: linux-gpio@xxxxxxxxxxxxxxx; devicetree@xxxxxxxxxxxxxxx; git <git@xxxxxxxxxx>; Srinivas Goud <sgoud@xxxxxxxxxx>; linux-arm-kernel@xxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Pengutronix Kernel Team <kernel@xxxxxxxxxxxxxx>
Subject: Re: [PATCH V3 3/3] gpio: modepin: Add driver support for modepin GPIO controller

On 18.08.21 10:10, Piyush Mehta wrote:
> This patch adds driver support for the zynqmp modepin GPIO controller.
> GPIO modepin driver set and get the value and status of the PS_MODE 
> pin, based on device-tree pin configuration. These four mode pins are 
> configurable as input/output. The mode pin has a control register, 
> which have lower four-bits [0:3] are configurable as input/output, 
> next four-bits can be used for reading the data  as input[4:7], and 
> next setting the output pin state output[8:11].
> 
> Signed-off-by: Piyush Mehta <piyush.mehta@xxxxxxxxxx>
> Acked-by: Michal Simek <michal.simek@xxxxxxxxxx>
> Reviewed-by: Linus Walleij <linus.walleij@xxxxxxxxxx>
> ---

> +/**
> + * modepin_gpio_dir_in - Set the direction of the specified GPIO pin as input
> + * @chip:	gpio_chip instance to be worked on
> + * @pin:	gpio pin number within the device
> + *
> + * Return: 0 always
> + */
> +static int modepin_gpio_dir_in(struct gpio_chip *chip, unsigned int 
> +pin) {
> +	return 0;
> +}

You say the gpio controller can configure pins as inputs or outputs.
These pins are controller via firmware driver. We are updating BOOT_PIN_CTRL 	0xFF5E0250 register.
[0:3]  = When 0, the pins will be inputs from the board to the PS. When 1, the PS will drive these pins
[4:7] = Value captured from the mode pins
[8:11] = Value driven onto the mode pins, when control register bit set out = 1

The lower four-bits [0:3] we can set either input and output, based on configuration we read pin as for input [4:7]
and write on pin [8:11].
Example:
If we want to configure pin 1 as output, then we will configure as [0:3]=[0100], for access pin will trigger upper bit [8:11]=[0100].

Based on
https://www.xilinx.com/support/documentation/user_guides/ug1085-zynq-ultrascale-trm.pdf

page 46

PS_MODE Input/Output Dedicated 4-bit boot mode pins sampled on POR deassertion

Xilinx is using this pin for usb phy resets.

Yet, .direction_input is doing nothing. So, it's not clear to me, how this sequence could work:

 - set gpio output high (writes bootmode)
 - set gpio to input (no-op, pin will remain high, not high impedance)






I didn't check the previous discussions, but if this indeed works as intended, the how should be written here into the driver. That is a more useful comment than kernel doc for a stub function.

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Regards,
Piyush Mehta