From mboxrd@z Thu Jan  1 00:00:00 1970
From: panand@redhat.com (Pratyush Anand)
Date: Fri, 22 Jul 2016 19:26:30 +0530
Subject: [PATCH v1 3/4] arm64: Add arm64 kexec support
In-Reply-To: <5791EF62.5070006@arm.com>
References: <cover.1468970114.git.geoff@infradead.org>
 <9ed513fd44a19b2f49b4da99477cedfff7341bf0.1468970114.git.geoff@infradead.org>
 <20160720153859.GG24691@leverpostej>
 <1469042361.2977.90.camel@infradead.org>
 <20160721103120.GB20559@leverpostej> <5790A902.6070601@arm.com>
 <1469137776.2977.128.camel@infradead.org>
 <20160722040842.GA14911@localhost.localdomain>
 <5791EF62.5070006@arm.com>
Message-ID: <20160722135630.GC14911@localhost.localdomain>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

Hi Robin,

On 22/07/2016:11:03:14 AM, Robin Murphy wrote:
> On 22/07/16 05:08, Pratyush Anand wrote:
> > On 21/07/2016:02:49:36 PM, Geoff Levand wrote:
> >> On Thu, 2016-07-21 at 11:50 +0100, Robin Murphy wrote:
> >>> The Exynos UART (drivers/tty/serial/samsung.c) is one which comes to
> >>> mind as definitely existing, and on arm64 systems to boot. The TX
> >>> register is at offset 0x20 there.
> >>
> >> Here's what I came up with.
> >>
> >>
> >> +	struct data {const char *name; int tx_offset;};
> >> +	static const struct data ok_list[] = {
> >> +	/*	{"armada-3700-uart", ?},	*/
> >> +		{"exynos4210-uart", 0x20},
> >> +	/*	{"ls1021a-lpuart", ?},		*/
> >> +	/*	{"meson-uart", ?},		*/
> >> +	/*	{"mt6577-uart", ?},		*/
> >> +		{"ns16550", 0},
> >> +		{"ns16550a", 0},
> >> +		{"pl011", 0},
> >> +		{NULL, 0}
> >> +	};
> > 
> > sinc functionality is just to debug the scenario when something goes wrong in
> > purgatory. IMHO, it should be disabled by default. So, why not to keep it as
> > simple as possible. Its a low level debugging mainly for developer, so user
> > should know the absolute address. Therefore, I think no need to parse earlycon
> > or earlyprintk from command line.  Whatever user passes in --port can be treated
> > as address of TX register. If TX offset is 0x20, then user can pass --port as
> > base+0x20. Additionally, we can pass TX register width as well. So what about
> > something like "--port=0x1c020000,1" where 0x1c020000 is TX register address and
> > 1 says about it's width in bytes.
> 
> I don't think even that is worthwhile, since without any polling it
> still relies on the UART having FIFOs, someone having already enabled
> the FIFOs, the FIFOs being deep enough and/or the output being short
> enough. In short, it's fragile enough that I'm not convinced it's even
> useful as a debug option. I suggest we simply copy the purgatory console
> implementation from, say, ARM or Alpha.

May be I am missing, but deep TX FIFO should not be an issue. Whatever, we write
in TX register, that will go to the port eventually. However, I do agree that
short FIFO could be an issue and overflow is quite possible in that case. I had
been trying to convince Geoff to take [1], which will help to resolve it.

[1] https://github.com/pratyushanand/kexec-tools/commit/0d49d420810f070bcff9c96e377477d96f909d11

~Pratyush
> 
> Robin.
> 
> > 
> > ~Pratyush
> >