Using Mini UART on RPI CM

[Eric Anholt]

Stefan Wahren <info at lategoodbye.de> writes:

> Hi Martin,
>
> i want to use both UARTs on a Raspberry Pi Compute Module. Yes, i know 
> you still working on the driver, but i hope you can help me with an issue.
>
> I took this branch [1] from your repository and add "ns16550" to the 
> compatible string of the uart1 node in bcm2835.dtsi
>
> Unfortunately i got an error during boot:
>
> [    0.738732] Serial: 8250/16550 driver, 1 ports, IRQ sharing enabled
> [    0.746685] of_serial 20215040.uart: clk or clock-frequency not defined
> ...
> [    3.448517] of_serial: probe of 20215040.uart failed with error -22
>
> I narrowed down the problem to this sequence in of_platform_serial_setup():
>
>    info->clk = devm_clk_get(&ofdev->dev, NULL);
>
>    ret = clk_prepare_enable(info->clk);
>
>    clk = clk_get_rate(info->clk);
>
> Finally the value of clk is zero, which leads to the error code 22 
> (EINVAL). In the bcm2835.dtsi file there are two clocks defined for uart1:
>
>    clocks = <&aux_clocks BCM2835_AUX_CLOCK_UART>, <&clocks 
> BCM2835_CLOCK_VPU>;
>
> After swapping both clocks in dtsi the problem disappears. But i think 
> this is not the right way to fix this issue.
>
> Did i miss something in my board dts file to enable the aux uart clock?

That branch looks like it needs the submitted version of "Add a driver
for the auxiliary peripheral clock gates" instead, which I think would
fix this problem.

https://github.com/anholt/linux/commit/c57cca7e0e96781ba3b9a0d0ab1069c0c20e0677

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