linux-kernel - Re: [RFC PATCH] always probe UART HW when options are not specified

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Message-ID: <5693FD3C.5080601@hurleysoftware.com>
Date:	Mon, 11 Jan 2016 11:06:36 -0800
From:	Peter Hurley <peter@...leysoftware.com>
To:	Sebastian Frias <sf84@...oste.net>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-serial@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
	mason <slash.tmp@...e.fr>,
	Måns Rullgård <mans@...sr.com>
Subject: Re: [RFC PATCH] always probe UART HW when options are not specified

On 01/11/2016 09:56 AM, Sebastian Frias wrote:
> Hi Peter,
> 
> On 01/11/2016 05:11 PM, Peter Hurley wrote:
>> On 01/11/2016 07:07 AM, Sebastian Frias wrote:
>>> On 12/22/2015 06:56 PM, Sebastian Frias wrote:
>>>>
>>>>       OF_EARLYCON_DECLARE(rt2880, "ralink,rt2880-uart",
>>>> early_serial8250_setup);
>>
>> There is no support for this uart in 8250 earlycon; the registers
>> need remapped.
> 
> Ok, two questions then:
> 1) If the UART is not supported on 8250 earlycon, what is the
> suggested/advised solution? Using just "earlyprintk"?

I don't have enough information to suggest what you "should" use
here.

Is this going to be a shipping product?
Is it single-core?
etc.

And what is your purpose for outputting early boot information
before loading the serial driver which does provide console output?

> 2) What would it take to make the "rt2880" work with the 8250
> earlycon? I mean, it is already pretty much supported in there, what
> would be missing? (I don't see why it blocks on earlycon_map) And
> would it be worth doing?

The rt2880 does not have the same register locations as a 8250.
The 8250 port driver remaps all register accesses with a LUT.

Adding support would be trivial.

>>>> at the end of the file, trying to mimic commit
>>>> d05f15707bb7659d2b863fafa1a918f286d74a63
>>>>
>>>> I'm still trying to figure out the right bootargs, so that's why both
>>>> "earlycon" and "console" are there. Suggestions welcome.
>>
>> Just 'earlycon' triggers the attempted registration of earlycon matching the
>> compatible string of the stdout-path node.
>>
>> The empty 'console' in bootargs is doing nothing.
> 
> Ok, thanks.
> 
> So, just to recap.
> We would like to understand what is the right way of doing this:
> 
> - we are using 8250 (rt288x variant) UART: CONFIG_SERIAL_8250_RT288X=y
> - the UART hardware is setup prior to Linux boot
> - we don't want Linux to change the UART settings, just to pick up
> whatever settings the UART has and take over the UART. 
> There were two replies to that, one by Greg Kroah-Hartman
> (http://www.spinics.net/lists/linux-serial/msg20278.html) and one by
> you, where you suggested we use "console=uart", but as I reported
> (http://www.spinics.net/lists/linux-serial/msg20307.html) it does not
> work, you replied that iotype and mmio are not optional but mandatory
> (http://www.spinics.net/lists/linux-serial/msg20310.html), and I
> wondered if it was really necessary to duplicate data that is already
> on the DT among other questions

At the time, I didn't know you were describing your h/w with DT.

If you use the console command line (console= or earlycon=) to start
an earlycon, then the uart address and iotype are mandatory.
For this usage, earlycon matching is attempted with every EARLYCON_DECLARE().

If you use plain "earlycon" on the command line, that will attempt to
register the uart described by stdout-path in DT. For this usage,
earlycon matching is attempted with the compatible string of every
OF_EARLYCON_DECLARE().

> (http://www.spinics.net/lists/linux-serial/msg20383.html), like how
> are DT described drivers supposed to interact with the
> "console="/"earlycon=" commandlines

They don't; those are orthogonal.

>, or, the contradiction between
> "console=ttyS0" means '9600n81' and "if unspecified [the uart
> options], the h/w is not re-initialized"> 

I thought I was clear on that: "console=ttyS0" initializes the h/w
to 9600n81 *because there are already existing users that must not break*.

"console=uart,..." probes the h/w
*because there are already existing users that must not break *

> So, for us, it is still not clear what is the recommended way of
> achieving our goal above, and it seems it is not clear what does
> "console=ttyS0" is supposed to do, hardcode ('9600n81') or probe
> ('the h/w is not re-initialized')

The DT way will be simplest at this point because you won't
have to write console handover matching for "console=rt288x,..."

With DT (ie, stdout-path) earlycon, when a serial driver loads,
an attempt is made to cross-reference any existing console with
the node that is loading and will do a console takeover from
a running earlycon for a matching uart node.

There is a bug with DT earlycon though.
If you have a dummy console that loads, the DT earlycon is
disabled at that point because boot consoles are disabled when
"real" consoles load.

> Any help will be appreciated.

Regards,
Peter Hurley