linux-kernel - Re: [PATCH] ARM: tegra: Make earlyprintk choose a UART at runtime.

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Message-ID: <CAD=FV=WxqKX62vM4r=qtQKVmNKbNP2u4jXd35iF4raveOudUqA@mail.gmail.com>
Date:	Mon, 3 Oct 2011 13:03:00 -0700
From:	Doug Anderson <dianders@...omium.org>
To:	Stephen Warren <swarren@...dia.com>
Cc:	Olof Johansson <olof@...om.net>, Colin Cross <ccross@...roid.com>,
	Erik Gilling <konkers@...roid.com>,
	Russell King <linux@....linux.org.uk>,
	Tony Lindgren <tony@...mide.com>,
	Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
	Jeremy Kerr <jeremy.kerr@...onical.com>,
	Simon Glass <sjg@...omium.org>,
	Allen Martin <AMartin@...dia.com>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ARM: tegra: Make earlyprintk choose a UART at runtime.

Stephen,

OK, thanks for testing!  I will rev the patch and resend in a little bit.

-Doug

---

On Fri, Sep 30, 2011 at 3:11 PM, Stephen Warren <swarren@...dia.com> wrote:
> Doug Anderson wrote at Monday, September 26, 2011 12:24 PM:
>> With this change we automatically detect which UART to use for
>> earlyprintk (and for printing during decompression).  The
>> detection involves coordination with the bootloader: it's expected
>> that the bootloader will leave a 'D' (for [D]ebug) in the UART
>> scratchpad register for whichever UART we should use for debugging.
>
> This works fine on Tegra20; there, even if the UART clock is off and/or
> the UART is in reset, you can still read the UART_SCR register (and get
> a bogus value) without the system hanging.
>
> However, I just tested on Tegra30, and the behavior has changed; if either
> a UART is in reset /or/ the UART clock is stopped, then the system hangs
> when UART_SCR is read.
>
> As such, this change will cause Tegra30 to fail to boot. I'd rather it
> was augmented to check the clock enable and module reset bits as well
> as the scratch register value in all cases.
>
> Yes, I know mainline doesn't support Tegra30 yet, but we're very close
> to posting the first few patches to start enabling it, and I wouldn't
> want to break it right before that!
>
> --
> nvpublic
>
>
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