On 08/04/2016 05:15 PM, Alexander Graf wrote:
On 04 Aug 2016, at 20:11, Stephen Warren <swar...@wwwdotorg.org> wrote:
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.
I guess I should’ve put that in a comment somewhere. Unfortunately we can’t. If
I just return an error on probe, U-Boot will panic because we tell it in a
CONFIG define that we require a serial port (grep for
CONFIG_REQUIRE_SERIAL_CONSOLE).
We could maybe try to unset that define instead?
Yes, assuming that U-Boot runs just fine with HDMI console only, I think
it's fine to unset CONFIG_REQUIRE_SERIAL_CONSOLE.
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.
We can do that if we can fail at probe time. If we absolutely must have a
serial driver to work in the first place, that doesn’t work. I can try to poke
at it, but it’ll be a few days I think :).
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