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

[Stephen Warren]

On 08/04/2016 05:15 PM, Alexander Graf wrote:
>
>> On 04 Aug 2016, at 20:11, Stephen Warren <swarren at 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 :).