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

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Date:	Fri, 19 Apr 2013 10:39:53 +0100
From:	Will Deacon <will.deacon@....com>
To:	Pranavkumar Sawargaonkar <pranavkumar@...aro.org>
Cc:	Peter Maydell <peter.maydell@...aro.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	Patch Tracking <patches@...aro.org>,
	Marc Zyngier <Marc.Zyngier@....com>,
	"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 10:34:56AM +0100, Pranavkumar Sawargaonkar wrote:
> On 19 April 2013 15:00, Peter Maydell <peter.maydell@...aro.org> wrote:
> > On 19 April 2013 10:27, Will Deacon <will.deacon@....com> wrote:
> >> On Fri, Apr 19, 2013 at 10:25:35AM +0100, Pranavkumar Sawargaonkar wrote:
> >>> 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.
> >
> > I don't think avoiding writing a spec is necessarily a good reason
> > for insisting on emulation of a lump of hardware 95% of whose
> > capabilities you aren't going to use...
> 
> True.
> Also 8250 will require emulation of registers, and i am not sure about
> if mach-virt will have any emulation of real hw ?

The point of mach-virt is that it is completely parameterised. So, if you're
not emulating an 8250, then don't tell the kernel that you have one!
Similarly, if you *do* emulate it, then either create a device-tree node for
it or pass the appropriate earlyprintk= string on the command line.

As far as kvmtool is concerned, we'd probably have a new command-line option
for arm64, allowing you to specify the early console device.

Will
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