Re: [Xen-devel] Xen ARM - Exposing a PL011 to the guest

[Julien Grall <julien.grall@xxxxxxx>]

On 03/02/17 13:53, Bhupinder Thakur wrote:
> Hi,
Hi Bhupinder,

> > > 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.
So if I understand correctly, you don't receive anymore output. Correct?
Have you tried to see whether the pl011 driver is receiving interrupt or 
even Linux calling it.
Assuming you have Xen built with debug enabled (see CONFIG_DEBUG), you 
could use xen_raw_printk in the pl011 driver to see what's going on. 
This will issue an hypercall and the message will be directly printed on 
Xen console.
If you compile out the HVC_XEN support, you could directly use 
HYPERVISOR_console_io(CONSOLEIO_write, len, str).
> Another observation is that unless you compile out HVC_XEN support,
> the hvc0 console also keeps receiving the data even if the
> console=ttyAMA0.
HVC is the preferred console when using Xen, I think you would also need 
to add "earlycon=pl0111,addr" on the kernel commandline where addr is 
the base address of the PL011 to get Linux using the serial port as boot 
console.
Cheers,

--
Julien Grall

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