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

From: Julien Grall <julien.grall@arm.com>
To: Stefano Stabellini <sstabellini@kernel.org>
Cc: Wei Liu <wei.liu2@citrix.com>,
	Ian Jackson <ian.jackson@eu.citrix.com>,
	Bhupinder Thakur <bhupinder.thakur@linaro.org>,
	xen-devel@lists.xenproject.org, nd@arm.com
Subject: Re: [PATCH 00/17 v5] SBSA UART emulation support in Xen
Date: Wed, 5 Jul 2017 20:43:12 +0100	[thread overview]
Message-ID: <9da1b0dc-af82-d270-0118-347e1aff8afc@arm.com> (raw)
In-Reply-To: <alpine.DEB.2.10.1707051151080.2919@sstabellini-ThinkPad-X260>

On 05/07/2017 20:06, Stefano Stabellini wrote:
> On Wed, 5 Jul 2017, Julien Grall wrote:
>> On 07/04/2017 08:31 AM, Bhupinder Thakur wrote:
>>> Hi Julien,
>>
>> Hi Bhupinder,
>>
>> Thank you for the summary!
>>
>> [...]
>>>
>>> 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.
>>
>> Hmmm. If I remember correctly this was suggested but ruled out. It was
>> considered that pl011 and PV console should not be treated equal. PL011 would
>> be used for boot diagnostics (i.e imagine an Image with no Xen support).
>
> Actually I remember the opposite:
> afd2e931-706b-6e25-1f0e-feee16e83c88@redhat.com (this was a private
> reply though).

This was an answer to my question whether a user could select the serial 
by himself. To this reply, you asked whether it was feasible to output 
on all the serials console, but I don't see any yes/no answer.

On the rest of the thread, it has been mentioned it was difficult to 
multiplex to serial console (I forwarded you the thread). Christoffer, 
Laszlo and Ard agreed that PL011 should only be used as boot diagnostics 
and debug (if selected at compile time).

Cheers,

-- 
Julien Grall

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