linux-kernel - Re: [PATCH 1/2] tty: serial: 8250

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Date:	Mon, 29 Dec 2014 20:08:59 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Eddie Huang <eddie.huang@...iatek.com>
Cc:	Rob Herring <robherring2@...il.com>,
	Mark Rutland <mark.rutland@....com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Matthias Brugger <matthias.bgg@...il.com>,
	srv_heupstream@...iatek.com, Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>, Jiri Slaby <jslaby@...e.cz>,
	Alan Cox <alan@...ux.intel.com>,
	"devicetree@...r.kernel.org" <devicetree@...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>,
	"linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
	Sascha Hauer <kernel@...gutronix.de>,
	yingjoe.chen@...iatek.com, yh.chen@...iatek.com
Subject: Re: [PATCH 1/2] tty: serial: 8250_mtk: Add earlycon

On Monday 29 December 2014 12:24:56 Eddie Huang wrote:
> Hi Rob,
> 
> On Fri, 2014-12-26 at 15:24 -0600, Rob Herring wrote:
> > On Thu, Dec 18, 2014 at 2:33 AM, Eddie Huang <eddie.huang@...iatek.com> wrote:
> > > Mediatek UART has highspeed register, but 8250_early.c doesn't
> > > support this, so add earlycon in 8250_mtk.c
> > 
> > I don't see any highspeed register setup here. More generically,
> > aren't you just skipping any UART setup? That may be useful on other
> > platforms with 8250s, too. With the kernel command line version, you
> > could perhaps add a "noinit" flag. The DT case is harder, and I'm not
> > sure how we should handle that. We could perhaps add a
> > "stdout-path-initialized" flag to chosen.
> > 
> 
> Yes, I skipped UART setup same as msm_serial.c and amba-pl011.c
> (although they are standalone serial driver). Just like earlyprintk, I
> think earlycon should reuse UART setting in loader. Since some other
> platforms with 8250 already depend on this, it's ok to add flags to
> distinguish whether 8250 earlycon driver should init hw or not. As you
> said, add "noinit" flag is simple, but "stdout-path-initialized" need
> more discussion. 

It would definitely be nice to have consistent behavior, and I also
think it would be good to default to noinit for the console, because that
is the more likely case. Basically anything that can do debug_ll on ARM
already depends on this behavior because the debug_ll infrastructure
does not initialize the console before using it. I believe the same
is true for powerpc udbg.

	Arnd
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