linux-kernel - Re: [RFC] arm64: Early printk support for virtio-mmio console devices.

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Message-ID: <20130419092752.GB13283@mudshark.cambridge.arm.com>
Date:	Fri, 19 Apr 2013 10:27:52 +0100
From:	Will Deacon <will.deacon@....com>
To:	Pranavkumar Sawargaonkar <pranavkumar@...aro.org>
Cc:	Marc Zyngier <Marc.Zyngier@....com>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	Anup Patel <anup.patel@...aro.org>,
	Patch Tracking <patches@...aro.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC] arm64: Early printk support for virtio-mmio console
 devices.

On Fri, Apr 19, 2013 at 10:25:35AM +0100, Pranavkumar Sawargaonkar wrote:
> On 19 April 2013 14:35, Will Deacon <will.deacon@....com> wrote:
> > On Thu, Apr 18, 2013 at 09:48:49AM +0100, Pranavkumar Sawargaonkar wrote:
> >> Actually i thought adding a config register will be easier to add a
> >> code than writing entire emulation as 8250 emulation will require to
> >> deal with dealing with more registers and more code.
> >
> > kvmtool already has an 8250 emulation! All we need to do is hack together
> > something which will allow us to instantiate those ioport devices in a more
> > flexible manner (namely, inside the MMIO space for ARM). For earlyprintk, I
> > suspect we can get away without an interrupt too, which should simplify
> > things a bit to start with.
> >
> > Regardless of the outcome of this discussion, I think getting the 8250
> > working on ARM is definitely something worth doing. If I get time, I'll take
> > a look.
> 
> I am not against using 8250 emulation (as far as it solves printk
> issues for kernel booting logs), but my point is why not to add early
> read-write support for virtio console, which also can be useful in
> emulation less mach-virt environment also ?

We can have both, but only one of those requires a change to the virtio
specification.

Will
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