Using Mini UART on RPI CM

Stefan Wahren info at
Wed Sep 30 11:13:46 PDT 2015

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 

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?


[1] -

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