Re: Xen ARM - Exposing a PL011 to the guest

[Christoffer Dall <cdall@linaro.org>]

On Fri, Feb 3, 2017 at 2:53 PM, Bhupinder Thakur
<bhupinder.thakur@linaro.org> wrote:
> Hi,
>
>>> Hi,
>>>
>>> I have done the changes for emulating pl011 in Xen. Currently, I have
>>> verified the emulation code by manually reading/writing data to
>>> /dev/ttyAMA0 which is the device file for pl011 device. The data is
>>> flowing fine between xenconsoled and the guest domain.
>>>
>>> As a next step, I wanted to use /dev/ttyAMA0 as a console.
>>>
>>> For that I tried adding console=ttyAMA0 instead of console=hvc0 in the
>>> "extra" directive in the domU configuration file. However, I do not
>>> see the output on the console once I attached the console using "xl
>>> console <domain_name>". I tried using "xl console -t serial
>>> <domain_name>" also but that shows the tty1 console and not the
>>> ttyAMA0 one.
>>
>> I would try to verify that when Linux uses the pl011 as a console (as
>> opposed to just writing some characters into /dev/ttyAMA0), that it
>> doesn't error out somewhere.  So like Stefano suggests, make sure you
>> check the guest kernel log after boot (assuming you can SSH or something
>> into the box) and look at 'dmesg' to see if the pl011 driver failed.
>>
>> You could also try adding some log info in you Xen pl011 emulation code
>> to see if the guest kernel driver prods the device in the sequence you
>> expect, based on looking at the linux driver for the pl011.
>
> I tested with the following changes:
>
> 1. Compiled out CONFIG_HVC_XEN
> 2. Specified "console=ttyAMA0" in the guest configuration
>
> With some more changes/fixes, the guest console is working partially
> over ttyAMA0. I can see the guest boot prints on the console. However,
> the prints stop just towards the end when the init scripts are
> executed.
>
> So it seems that during boot, initially ttyAMA0 is used as a
> write-only port, where the console driver keeps writing the output to
> the port. Till this point, only tx has been enabled on ttyAMA0 (driver
> probe() is called at this point). No irq/rx has been enabled yet. Just
> as the init scripts are executed, the irq/rx is also enabled on
> ttyAMA0 (driver startup() is called at this time which enables rx and
> registers IRQ) and then the boot prints stop.
>
> Another observation is that unless you compile out HVC_XEN support,
> the hvc0 console also keeps receiving the data even if the
> console=ttyAMA0.
>

What's your guest distro and file system settings?

This sounds to me like the console bringup job is hard-wired to
/dev/hvc0 as opposed to using the console.  I think most ubuntu
versions prior to using systemd needs the right files in /etc/init.
Check if you have /etc/init/hvc.conf or something like that and if you
have /etc/init/ttyAMA0.conf.  If you're using systemd or a different
distro, figure out what the equivalent settings there are.

-Christoffer

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