linux-kernel - Re: [PATCH 6/8] tty/serial: pl011: add generic earlycon support

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Message-ID: <20140423163954.GA14426@localhost>
Date:	Wed, 23 Apr 2014 17:39:56 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Rob Herring <robherring2@...il.com>
Cc:	"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>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.cz>,
	Russell King <linux@....linux.org.uk>,
	Will Deacon <Will.Deacon@....com>,
	"x86@...nel.org" <x86@...nel.org>, Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH 6/8] tty/serial: pl011: add generic earlycon support

On Wed, Apr 16, 2014 at 11:14:28PM +0100, Rob Herring wrote:
> On Mon, Mar 24, 2014 at 6:28 AM, Catalin Marinas
> <catalin.marinas@....com> wrote:
> > On Fri, Mar 21, 2014 at 09:08:46PM +0000, Rob Herring wrote:
> >> From: Rob Herring <robh@...nel.org>
> >>
> >> Add earlycon support for the pl011 serial port. This allows enabling
> >> the pl011 for console when early_params are processed. This is based
> >> on the arm64 earlyprintk support and is intended to replace it.
> >>
> >> Signed-off-by: Rob Herring <robh@...nel.org>
> >> Cc: Russell King <linux@....linux.org.uk>
> >> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> >> Cc: Jiri Slaby <jslaby@...e.cz>
> >> ---
> >>  Documentation/kernel-parameters.txt |  5 +++--
> >>  drivers/tty/serial/amba-pl011.c     | 30 +++++++++++++++++++++++++++++-
> >>  2 files changed, 32 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
> >> index 5ce8b7a..81bdd52 100644
> >> --- a/Documentation/kernel-parameters.txt
> >> +++ b/Documentation/kernel-parameters.txt
> >> @@ -887,8 +887,9 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
> >>               uart[8250],io,<addr>[,options]
> >>               uart[8250],mmio,<addr>[,options]
> >>               uart[8250],mmio32,<addr>[,options]
> >> -                     Start an early, polled-mode console on the 8250/16550
> >> -                     UART at the specified I/O port or MMIO address.
> >> +             pl011,<addr>
> >> +                     Start an early, polled-mode console on a serial port
> >> +                     at the specified I/O port or MMIO address. 8250
> >>                       MMIO inter-register address stride is either 8-bit
> >>                       (mmio) or 32-bit (mmio32).
> >>                       The options are the same as for ttyS, above.
> >
> > I think the last line is a bit misleading. Or did you intend to leave it
> > with the uart[8250] parameter? See below:
> 
> How about this (excuse the gmail lack of tabs):
> 
> earlycon= [KNL] Output early console device and options.
> 
>  uart[8250],io,<addr>[,options]
>  uart[8250],mmio,<addr>[,options]
>  uart[8250],mmio32,<addr>[,options]
>   Start an early, polled-mode console on an 8250 serial
>   port at the specified I/O port or MMIO address. 8250
>   MMIO inter-register address stride is either 8-bit
>   (mmio) or 32-bit (mmio32).
> 
>   The options are the same as for ttyS, above.
> 
>  pl011,<addr>
>   Start an early, polled-mode console on a pl011 serial
>   port at the specified address. The pl011 serial port
>   must already be setup and configured. Options are not
>   yet supported.
> 
>  smh Use ARM semihosting calls for early console.

It looks fine.

> >> diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
> >> index d4eda24..4227c0a 100644
> >> --- a/drivers/tty/serial/amba-pl011.c
> >> +++ b/drivers/tty/serial/amba-pl011.c
> > [...]
> >> +static int __init pl011_early_console_setup(struct earlycon_device *device,
> >> +                                         const char *opt)
> >> +{
> >> +     if (!device->port.membase)
> >> +             return -ENODEV;
> >> +
> >> +     device->con->write = pl011_early_write;
> >> +     return 0;
> >> +}
> >> +EARLYCON_DECLARE(pl011, pl011_early_console_setup);
> >
> > Here we expect the PL011 to be already initialised by the boot loader
> > and the kernel continues using the same settings. So maybe clarify this
> > in the pl011 kernel parameter doc and we can add proper configuration
> > using a separate patch.
> 
> Enabling and setup would not be too hard, but either baud rate will
> always have to be configured or we'll have to specify the input clock
> rate too. The 8250 driver basically does the former or assumes a fixed
> clock.
> 
> Adding any setup will also break any non-pl011 based SBSA compliant
> uart since the configuration registers are not standardized. I guess
> we can add "sbsauart" when/if that happens.

I think for now we can assume that the pl011 is initialised at the
right baud rate prior to starting the kernel.

-- 
Catalin
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