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Re: [Xen-devel] [PATCH 00/17 v5] SBSA UART emulation support in Xen

Hi Julien,

On 23 June 2017 at 16:12, Julien Grall <julien.grall@xxxxxxx> wrote:
> Hi Bhupinder,
> On 22/06/17 08:38, Bhupinder Thakur wrote:
>> There are some TBD items which need to be looked at in the future:
>> 1. Currently UEFI firmware logs the output to hvc console only. How can
>>    UEFI firmware be made aware of pl011 console and how it can use it
>>    as a console instead of hvc.
> Would it be possible to summarize the discussions we had a couple of weeks
> ago here?

Currently, UEFI firmware uses hvc as the console for input/output. Now
with the support
of SBSA UART in Xen, it is preferrable that UEFI firmware should be
able to the uart
as well.

One option which was discussed was to use pl011 purely as a debug
port. Currently the debug
prints are intermixed with the normal console output. Now with uart
port becoming available
the debug prints can be redirected to pl011 thus cleaning up the console output.

Other option is to output everything on both HVC and pl011 both but it
takes away the advantage
of separating out the debug and normal console prints. However, pl011
can be used as debug
port based on a compile time flag. If this compile-time is off, then
the output can be sent to both
HVC and pl011.

Based on this discussion I feel that:
- the default behaviour should be writing the output to both HVC and pl011.
- pl011 can be used as a pure debug port based on a compile-time flag.

>> 2. Linux seems to have hvc console as the default console i.e. if no
>>    console is specified then it uses hvc as the console. How can an
>>    option be provided in Linux to select either hvc or pl011 as the
>>    default console.
> I am wondering what would happen if you use stdout-path in the device-tree.
> Does it override the default console?
I tried adding a "chosen" node in the DT to select "sbsa-pl011" as the
stdout-path. I added the
 following code in make_chosen_node() function:

 fdt_property_string(fdt, "stdout-path", "sbsa-pl011");

However, I still see the initial console output going to hvc only.

> IHMO, the best way to select the default console would be using either the
> SPCR (for ACPI) or stdout-path (for DT). But the HVC console does not have
> any description in the firmware. It might be worth considering adding
> description.
> The drawback is the user would always have to specify the console on the
> command line. I think this is not too bad for a first approach.
Do we plan to address this requirement in the current patch series?

>> 3. ACPI support for pl011 device.
> I would be ok to defer this after PL011 series is merged.


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