linux-kernel - RE: [PATCH v5 2/2] arch/x86: remove pci uart early console from early

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Date:	Wed, 3 Jun 2015 00:16:36 +0000
From:	"Anderson, Stuart R" <stuart.r.anderson@...el.com>
To:	Bin Gao <bin.gao@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>
CC:	Ingo Molnar <mingo@...nel.org>, Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Peter Hurley <peter@...leysoftware.com>,
	Jiri Slaby <jslaby@...e.cz>, Borislav Petkov <bp@...e.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v5 2/2] arch/x86: remove pci uart early console from
 early_prink.c

Bin, et al,

What we are losing here is the ability to specify a UART by its bus address instead of just supplying the memory or io address. There are some cases where this is useful, though I admit it is probably not going to be widely used. I have seen a platform where the location of the UART moves depending on the firmware version, but the bus address (B:D.F) did not change. There are also some platforms where you do not know the address until you boot the OS and can use the UART to login and find the address of the UART (oops. Chicken and egg problem).

Also, I was going to soon send a patch to allow "pciserial32" for the case where the UART registers are 32-bit aligned instead of 8-bit aligned.

Stuart

-----Original Message-----
From: Bin Gao [mailto:bin.gao@...ux.intel.com] 
Sent: Tuesday, June 02, 2015 3:43 PM
To: Thomas Gleixner
Cc: Ingo Molnar; Ingo Molnar; H. Peter Anvin; Greg Kroah-Hartman; One Thousand Gnomes; Peter Hurley; Jiri Slaby; Borislav Petkov; Anderson, Stuart R; linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 2/2] arch/x86: remove pci uart early console from early_prink.c

On Tue, Jun 02, 2015 at 11:07:39PM +0200, Thomas Gleixner wrote:
> What about the memory mapped uarts which have been source of trouble 
> in the past?
> 
> Thanks,
> 
> 	tglx

Not sure which specific early console you are referring to.
Currently we have serial, ttyS, vga, dbgp, xen, efi and pciserial in arch/x86/kernel/early_printk.c, and only pciserial is memory mapped (but it's being removed).

And this patch only touches pciserial/serial parts so ideally shouldn't have impact on other early consoles.

-Bin
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