[v4] USB: serial: ftdi_sio: implement GPIO support for FT-X devices

[Johan Hovold <johan@kernel.org>]

On Sun, Sep 16, 2018 at 07:58:47PM +0200, Karoly Pados wrote:
> This patch allows using the CBUS pins of FT-X devices as GPIO in CBUS
> bitbanging mode. There is no conflict between the GPIO and VCP
> functionality in this mode. Tested on FT230X and FT231X.
> 
> As there is no way to request the current CBUS register configuration
> from the device, all CBUS pins are set to a known state when the first
> GPIO is requested. This allows using libftdi to set the GPIO pins
> before loading this module for UART functionality, a behavior that
> existing applications might be relying upon (though no specific case
> is known to the authors of this patch).
> 
> Signed-off-by: Karoly Pados <pados@pados.hu>
> ---
> Changelog:
> - v2: Fix compile error when CONFIG_GPIOLIB is not defined.
> - v3: Incorporate review feedback.
> - v4: Include linux/gpio/driver.h unconditionally.
>       Replace and invert gpio_input with gpio_output.
>       Make ftdi_gpio_direction_get return 0/1.
>       Change dev_err msg in ftdi_set_bitmode_req.
>       Change formatting of error checking in ftdi_gpio_get.
>       Drop dev_err in ftdi_gpio_set.
>       Remove some line breaks and empty lines.
>       Change error handling in ftdi_read_eeprom (and adjust caller).
>       Replace SIO->FTX in FTDI_SIO_CBUS_MUX_GPIO macro name.

Really nice job with this; looks nice and clean over all. Just a few
comments below.

> +static int ftx_gpioconf_init(struct usb_serial_port *port)
> +{
> +	struct ftdi_private *priv = usb_get_serial_port_data(port);
> +	struct usb_serial *serial = port->serial;
> +	const u16 cbus_cfg_addr = 0x1a;
> +	const u16 cbus_cfg_size = 8;

Looks like you only need to read four bytes.

> +	u8 *cbus_cfg_buf;
> +	int result;
> +	u8 i;
> +
> +	/* Read a part of device EEPROM */
> +	cbus_cfg_buf = kmalloc(cbus_cfg_size, GFP_KERNEL);
> +	if (!cbus_cfg_buf)
> +		return -ENOMEM;
> +
> +	result = ftdi_read_eeprom(serial, cbus_cfg_buf,
> +				  cbus_cfg_addr, cbus_cfg_size);
> +	if (result != 0)

result < 0 would be more informative here.

> +		goto out_free;
> +
> +	/* Chip-type guessing logic based on libftdi. */
> +	priv->gc.ngpio = 4;  /* FT230X, FT231X */
> +	if (le16_to_cpu(serial->dev->descriptor.bcdDevice) != 0x1000)
> +		priv->gc.ngpio = 1;  /* FT234XD */

As I mentioned in my last mail: I've asked FTDI about this, but I fear
that FTX234XD has bcdDevice 0x1000 and we may need to just always
register all four pins after all.

> +	/* Determine which pins are configured for CBUS bitbanging */
> +	priv->gpio_altfunc = 0xff;
> +	for (i = 0; i < priv->gc.ngpio; ++i) {
> +		if (cbus_cfg_buf[i] == FTDI_FTX_CBUS_MUX_GPIO)
> +			priv->gpio_altfunc &= ~BIT(i);
> +	}
> +
> +out_free:
> +	kfree(cbus_cfg_buf);
> +
> +	return result;
> +}

> +static void ftdi_gpio_remove(struct usb_serial_port *port)
> +{
> +	struct ftdi_private *priv = usb_get_serial_port_data(port);
> +
> +	if (priv->gpio_used) {
> +		/* Remark: Exiting CBUS-mode does not reset pin states too */
> +		ftdi_exit_cbus_mode(port);
> +		priv->gpio_used = false;
> +	}

This should go after deregistration or we have a tiny race window here.

> +	if (priv->gpio_registered) {
> +		gpiochip_remove(&priv->gc);
> +		priv->gpio_registered = false;
> +	}
> +}

> diff --git a/drivers/usb/serial/ftdi_sio.h b/drivers/usb/serial/ftdi_sio.h
> index dcd0b6e05baf..6856ccdac9b4 100644
> --- a/drivers/usb/serial/ftdi_sio.h
> +++ b/drivers/usb/serial/ftdi_sio.h
> @@ -35,7 +35,10 @@
>  #define FTDI_SIO_SET_EVENT_CHAR		6 /* Set the event character */
>  #define FTDI_SIO_SET_ERROR_CHAR		7 /* Set the error character */
>  #define FTDI_SIO_SET_LATENCY_TIMER	9 /* Set the latency timer */
> -#define FTDI_SIO_GET_LATENCY_TIMER	10 /* Get the latency timer */
> +#define FTDI_SIO_GET_LATENCY_TIMER	0x0a /* Get the latency timer */
> +#define FTDI_SIO_SET_BITMODE		0x0b /* Set GPIO mode */

Nit: "Set bitbang mode", I think would be more correct here as this
request is also used to configure the asynchronous bitbang mode, etc.

Thanks,
Johan