serial: clk: bcm2835: Strange effects when using aux-uart in console

Martin Sperl kernel at
Wed Feb 10 07:12:05 PST 2016


I got a strange effect when using the auxiliar UART as the logging console:

The kernel is configured for drivers/tty/serial/8250/8250_bcm2835aux.c to
get linked into the main kernel (requires also 8250.c to be linked in).

As long as I boot using the main UART (PL011) in the kernel parameters:
  earlyprintk consoleblank=0 console=ttyAMA0 root=/dev/mmcblk0p2 rootwait

Everything works fine - early_printk works and normal logging works as well.
Both configured ttys (/dev/ttyAMA0 and /dev/ttyS0) in /etc/initttab
work fine.

Now when I switch the console to use the auxiliar UART
(which right now does not support early_printk) like this:
  earlyprintk consoleblank=0 console=ttyS0,115200n1 root=/dev/mmcblk0p2 rootwait

I get the situation where:
* u-boot starts and writes to the main UART
* Kernel boots using early_printk logging to the main UART until we get to:
  [    2.657090] bootconsole [earlycon0] disabled
* then we see the earlyprintk buffer dumped on the aux-uart
* then booting continues and logging happens on the aux-uart:
  [    2.657090] bootconsole [earlycon0] disabled
  [    2.662673] 20201000.serial: ttyAMA0 at MMIO 0x20201000 (irq = 81, base_baud = 0) is a PL011 rev2
  [    2.663793] console [ttyS0] disabled
  [    2.663899] 20215040.serial: ttyS0 at MMIO 0x0 (irq = 53, base_baud = 31224999) is a 16550
  [    3.384166] console [ttyS0] enabled
* everything works fine and I get networking and everything up
  [....] Starting NTP server: ntpd. ok
  [....] Starting OpenBSD Secure Shell server: sshd. ok
  My IP address is ::

  Raspbian GNU/Linux 7 ttyS0
* I can even log in via SSH and networking works.
  Note that I have the getty for ttyAMA0 disabled in inittab
* when I then enable the tty on the main UART like this:
  root at raspcm:~# /sbin/getty -a root -L ttyAMA0 115200 vt100
* I get the following:
  [   72.962845] uart-pl011 20201000.serial: no DMA platform data
* the system freezes…
* sometimes I still get the following on the auxiliar UART before the system “crashes":
  (at least when starting the tty from inittab)
  [   73.072985] bcm2835-clk 20101000.cprman: plld: couldn't lock PLL
  [   73.079132] ------------[ cut here ]------------
  [   73.083848] WARNING: CPU: 0 PID: 2329 at drivers/clk/clk.c:680 clk_core_disable+0x34/0xf0()
  [   73.093067] ---[ end trace 416e78cea88f5fb5 ]---
  [   73.097848] ------------[ cut here ]------------
  [   73.102616] WARNING: CPU: 0 PID: 2329 at drivers/clk/clk.c:575 clk_core_unprepare+0x34/0x110()
  [   73.112219] ---[ end trace 416e78cea88f5fb6 ]---

A similar effect I get when using stty: 
  root at raspcm:~# stty -F /dev/ttyAMA0
  [  128.878127] uart-pl011 20201000.serial: no DMA platform data
  speed 9600 baud; line = 0;
  -brkint -imaxbel
Then the system freezes.

As if the plld does not lock - even if it (probably) is already locked.

Or the message actually comes from bcm2835_clock_wait_busy which also
produces this message - so maybe we should better identify the message.

Note that the same also applies if I remove earlyprintk with
the only difference being that I do not get the initial boot
messages on the main UART.
(at that time only: "Uncompressing Linux... done, booting the kernel.”
shows on the main uart after u-boot start..)

Unfortunately it is only possible to really test this on a
rpi-Compute module, as only there we can expose both uarts
on distinct GPIOs at the same time:
  GPIO 14: level=1 fsel=4 alt=0 func=TXD0
  GPIO 15: level=1 fsel=4 alt=0 func=RXD0
  GPIO 32: level=1 fsel=2 alt=5 func=TXD1
  GPIO 33: level=1 fsel=2 alt=5 func=RXD1

Here the commits I have used based on spi/for-next (based on 4.4.0)
with the patches (* prefix) for the driver for aux-uart
added separately as they are not in the tree I have used:
* a96f6c7 serial: bcm2835: add driver for bcm2835-aux-uart
* 464e876 clk: bcm2835: Add bindings for the auxiliary peripheral clock gates.
* 1af1b28 clk: bcm2835: Add a driver for the auxiliary peripheral clock gates.
ece19b8 Merge remote-tracking branch 'spi/fix/loopback' into spi-linus
887e6a3 Merge tag 'spi-v4.5' into spi-linus
ebea7c0 spi: fix counting in spi-loopback-test code
cabeea9 Merge remote-tracking branches 'spi/topic/sun4i', 'spi/topic/topcliff-pc
4f95307 Merge remote-tracking branches 'spi/topic/overlay', 'spi/topic/pxa2xx',
41d5a70 Merge remote-tracking branches 'spi/topic/lm70llp', 'spi/topic/loopback'
635b9b2 Merge remote-tracking branches 'spi/topic/dw', 'spi/topic/dw-mid', 'spi/
3c27892 Merge remote-tracking branches 'spi/topic/bcm63xx', 'spi/topic/butterfly
9b17e80 Merge remote-tracking branch 'spi/topic/sunxi' into spi-next
174c211 Merge remote-tracking branch 'spi/topic/core' into spi-next
f30f072 Merge remote-tracking branch 'spi/fix/mtk' into spi-linus
afd2ff9 Linux 4.4

Maybe someone has gotten any idea?

In the meantime I will try to reproduce on a 4.5-rc3 based branch.



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