[U-Boot] [PATCH v2] serial: bcm283x_mu: Detect disabled serial device
Stephen Warren
swarren at wwwdotorg.org
Thu Aug 4 21:11:13 CEST 2016
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.
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