Mini UART on RaspberryPi CM3/CM3+

[Rouven Czerwinski]

On Tue, 2020-10-20 at 00:29 +0200, Robert Carnecky (Neopsis) wrote:
> On Mon, Oct 19, 2020 at 11:28 AM Ahmad Fatoum <a.fatoum at pengutronix.de> wrote:
> > Hello,
> > 
> > On 10/18/20 1:17 AM, Robert Carnecky (Neopsis) wrote:
> > > On Sat, Oct 17, 2020 at 11:29 PM Ahmad Fatoum <a.fatoum at pengutronix.de> wrote:
> > > > Hi,
> > > > 
> > > > On 10/17/20 1:31 PM, Robert Carnecky (Neopsis) wrote:
> > > > > Finally, I managed to boot into barebox menu using UART1 (/dev/ttyS0)
> > > > > as the console. The solution was to remove all UART related overlays
> > > > > from config.txt and only activate UART1.
> > > > > 
> > > > >    enable_uart=1
> > > > > 
> > > > > Anyway, another problem occurred. When booting via barebox the CPU
> > > > > serial number is wrong. Obviously, barebox somehow manipulates the
> > > > > internal structures and
> > > > > 
> > > > >    cat /proc/cpuinfo
> > > > > 
> > > > > returns always
> > > > > 
> > > > >    Hardware        : BCM2835
> > > > >    Revision          : 0000
> > > > >    Serial              : 0000000000000000
> > > > >    Model              : Raspberry Pi Compute Module 3 IO board V3.0
> > > > > 
> > > > > When I boot the same image directly without barebox boot loader, the
> > > > > info is OK.  I found some discussions about U-Boot and wrong serial
> > > > > number here
> > > > > 
> > > > >   https://www.raspberrypi.org/forums/viewtopic.php?t=183474#p1162778
> > > > > 
> > > > > maybe that's barebox related too. Anyway, that is a definitive no go
> > > > > with barebox for us because parts of our software are expecting the
> > > > > unique serial number.
> > > > 
> > > > I believe you can boot Linux with /vc.dtb and you'll get the same
> > > > device tree passed to Linux as if you booted without passing through
> > > > barebox.
> > > 
> > > Yes, but then I lose my Linux console! A summary, in all cases Barebox device
> > > tree in arch/arm/dts/bcm2837-rpi-cm3.dts was updated following your hint, e.g.
> > > 
> > > / {
> > >   chosen {
> > >      stdout-path = "&uart1";
> > >   };
> > >  };
> > > 
> > > &uart1 {
> > >         pinctrl-names = "default";
> > >         pinctrl-0 = <&uart1_gpio14>;
> > >         status = "okay";
> > >  };
> > > 
> > > &uart0 {
> > >      pinctrl-names = "default";
> > >      pinctrl-0 = <&uart0_gpio32>;
> > >      status = "okay";
> > >  };
> > > 
> > > 1. Booting kernel direct, works as expected (console on uart1),
> > > expected behavior
> > > 
> > > kernel=zImage
> > > enable_uart=1
> > > dtoverlay=uart0,txd0_pin=32,rxd0_pin=33,pin_func=7
> > > dtoverlay=uart1,txd1_pin=14,rxd1_pin=15
> > > 
> > > 2. With Barebox loader. The same config.txt as in case 1),
> > >     - no Barebox console
> > >     - kernel boots with the console on uart1
> > > 
> > > kernel=barebox-raspberry-pi-cm3.img
> > > enable_uart=1
> > > dtoverlay=uart0,txd0_pin=32,rxd0_pin=33,pin_func=7
> > > dtoverlay=uart1,txd1_pin=14,rxd1_pin=15
> > > 
> > > 3.  With Barebox loader, no uart overlays in config.txt.
> > >     - Barebox console on uart1 ok,
> > >     - kernel console on uart1
> > >     - almost done), but ... no cpuinfo
> > > 
> > > kernel=barebox-raspberry-pi-cm3.img
> > > enable_uart=1
> > > 
> > > 4. With Barebox loader, no uart overlays in config.txt, Linux boots with vc.dtb.
> > >     - Barebox console on uart1 ok
> > >     - no Linux console, Linux boots with cpuinfo ok
> > > 
> > > kernel=barebox-raspberry-pi-cm3.img
> > > enable_uart=1
> > > 
> > > I did not found any combination when I get the Barebox console and
> > > Linux console on uart1
> > > and with cpuinfo ok. If the serial number is not needed, case 3) should be ok.
> > 
> > What happens if you combine 2. and 4. ?
> > So you have enable_uart=1, the uart overlays _and_ Linux boots with vc.dtb?
> 
> Negativ. Whenever I enable overlays in the config.txt that map uart0
> to pin32/33 and
> uart1 to pin 14/15 (console), I do not get Barebox console. It does
> not depend on the device
> tree passed to the kernel. This interaction between Barebox device
> tree and Raspberry config.txt
> overlays is a bit strange for me.
> 
> Just an idea - does Barebox fully support this hardware? From the 2835
> ARM Peripherals Guide,
> chapter 2.2 miniUART :
> 
> > The implemented UART is not a 16650 compatible UART However as far as possible
> > the first 8 control and status registers are laid out like a 16550 UART. Al 16550 register
> > bits which are not supported can be written but will be ignored and read back as 0.
> > All control bits for simple UART receive/transmit operations are available.
> 
> Barebox has support for NS16650 ( CONFIG_DRIVER_SERIAL_NS16550) but not for
> Raspberry miniUART.
> 
> Anyway, I am giving up. I am sure I tested all combinations of
> overlays, I have written my
> own dts, nothing works as I need (console=uart1, incl. proper serial
> number handling).
> There is no problem to get the console on uart0/ttyAMA0, but not on uart1/ttyS0.
> The Raspberry way how they are handling uarts lacks all logic and is completely
> unclear.

Hi,

looking into arch/arm/dts/bcm2837-rpi-cm3.dts:
/ {
        chosen {
                stdout-path = &uart0;
        };
};

The CM3 module is currently default configured to use uart0 as the
stdout path, while bcm2837-rpi-3.dts is configured for uart1.
This was done in ab76f9d09d3b7b77fc00d84bc2fac6bfbf82c69c after the
miniuart support was added to the NS16550 driver
in 77de547cd179b910cdcb8530df15d77a0fc66a3c.

Changing the stdout-path for the barebox internal device tree should
output barebox correctly on the miniuart console.

Regards,
Rouven Czerwinski