[U-Boot] [PATCH v2] serial: bcm283x_mu: Detect disabled serial device

[Stephen Warren <swarren@wwwdotorg.org>]

On 08/04/2016 01:11 AM, Alexander Graf wrote:
> On the raspberry pi, you can disable the serial port to gain dynamic frequency
> scaling which can get handy at times.
>
> However, in such a configuration the serial controller gets its rx queue filled
> up with zero bytes which then happily get transmitted on to whoever calls
> getc() today.
>
> This patch adds detection logic for that case by checking whether the RX pin is
> mapped to GPIO15 and disables the mini uart if it is not mapped properly.
>
> That way we can leave the driver enabled in the tree and can determine during
> runtime whether serial is usable or not, having a single binary that allows for
> uart and non-uart operation.

> diff --git a/drivers/serial/serial_bcm283x_mu.c b/drivers/serial/serial_bcm283x_mu.c

> @@ -72,9 +87,18 @@ static int bcm283x_mu_serial_probe(struct udevice *dev)
>  {
>  	struct bcm283x_mu_serial_platdata *plat = dev_get_platdata(dev);
>  	struct bcm283x_mu_priv *priv = dev_get_priv(dev);
> +	struct bcm283x_gpio_regs *gpio = (struct bcm283x_gpio_regs *)plat->gpio;
>
>  	priv->regs = (struct bcm283x_mu_regs *)plat->base;
>
> +	/*
> +	 * The RPi3 disables the mini uart by default. The easiest way to find
> +	 * out whether it is available is to check if the pin is muxed.
> +	 */
> +	if (((readl(&gpio->gpfsel1) >> BCM283X_GPIO_GPFSEL1_F15_SHIFT) &
> +	    BCM283X_GPIO_ALTFUNC_MASK) != BCM283X_GPIO_ALTFUNC_5)
> +		priv->disabled = true;
> +
>  	return 0;

Comment on the current implementation: Can't probe() return an error if 
the device should be disabled? That would avoid the need to check 
priv->disabled in all the other functions.

Overall comment: I'd rather not put this logic into the UART driver 
itself; it is system-specific rather than device-specific. I'd also 
rather not have the UART driver touching GPIO registers; that's not very 
modular, and could cause problems if the Pi is converted to use DT to 
instantiate devices.

Instead, can we put the logic into board/raspberrypi/rpi/rpi.c? I.e. 
have some early function come along and enable/disable the 
bcm2837_serials device object as appropriate? That way it isolates the 
code to the Pi specifically, and not any other bcm283x board. We'd want 
to wrap that code in #ifdef CONFIG_PL01X_SERIAL.