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

From: Stefano Stabellini <sstabellini@kernel.org>
To: Julien Grall <julien.grall@arm.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>,
	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 13:05:43 -0700 (PDT)	[thread overview]
Message-ID: <alpine.DEB.2.10.1707051303480.2919@sstabellini-ThinkPad-X260> (raw)
In-Reply-To: <9da1b0dc-af82-d270-0118-347e1aff8afc@arm.com>

On Wed, 5 Jul 2017, Julien Grall wrote:
> 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).

I see now that I missed an important part of the thread. Just follow
Lazlo's suggestion.

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