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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