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

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Message-ID: <556EF4EA.50606@hurleysoftware.com>
Date:	Wed, 03 Jun 2015 08:36:58 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	"Anderson, Stuart R" <stuart.r.anderson@...el.com>,
	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>,
	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

Hi Stuart,

On 06/02/2015 08:16 PM, Anderson, Stuart R wrote:
> 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).

The patch title and commit log are somewhat misleading.
What this patch actually does is re-implement pciserial earlyprintk
in terms of earlycon instead, so the functionality is retained.

Patch 1/2 adds pciserial support to earlycon with command line parameters
of the forms

	earlycon=uart8250,pci[32],<B:D.F>,<options>

This patch 2/2
1. removes the x86-only pciserial bootconsole implementation, and
2. wires that behavior up to command line parameters of the forms

	earlyprintk=uart8250,pci,<B:D.F>,<options>

which is not exactly what was suggested; rather that the existing
earlyprintk= command line format for pciserial should be transparently
handled by earlycon.


Regards,
Peter Hurley

> 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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