Re: [PATCH 2/2] tty: serial: bcm63xx: Allow device nodes to be renamed to /dev/ttyBCM*

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On Mon, Nov 10, 2014 at 6:25 AM, Rob Herring <robh@xxxxxxxxxx> wrote:
> On Sun, Nov 9, 2014 at 2:55 AM, Kevin Cernekee <cernekee@xxxxxxxxx> wrote:
>> By default, bcm63xx_uart.c uses the standard 8250 device naming and
>> major/minor numbers.  There are at least two situations where this could
>> be a problem:
>>
>> 1) Multiplatform kernels that need to support some chips that have 8250
>> UARTs and other chips that have bcm63xx UARTs.
>>
>> 2) Some older chips like BCM7125 have a mix of both UART types.
>>
>> Add a new Kconfig option to tell the driver whether to register itself
>> as ttyS or ttyBCM.  By default it will retain the existing "ttyS"
>> behavior to avoid surprises.
>
> While I understand the desire to have stable names, this is the
> opposite direction we want to go. Per platform tty names complicates
> having a generic userspace. It is not so bad since most ARM platforms
> use ttyS or ttyAMA, but just think what the kernel and userspace side
> would look like if every single platform did this. We can't change
> everything to ttyS because the other names are already an ABI.
>
> This can be solved with a udev rule to create sym links.

Is it safe to register two console drivers named "ttyS" with the same
major/minor numbers?  Maybe there is a trick to making them coexist?

What I found is that everything worked fine on a system with
bcm63xx_uart hardware until I enabled the 8250 driver in .config.  At
that point the drivers clashed and I had no serial output
post-bootconsole.  It didn't even get to userland before it failed.
There are no DT entries mentioning 8250; the mere presence of the 8250
driver killed my console.

If this behavior is unexpected I can keep digging to find out what went wrong.

> Or if you
> just need to know which dev node is h/w uart X, you can get that from
> sysfs.

Right - I use busybox cttyhack in the init scripts to figure out the
console name, so the same rootfs image can be used for both ttyS0 and
ttyBCM0:

# Put a shell on the serial port
::respawn:/bin/cttyhack /bin/sh -l
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