linux-kernel - Re: Nonterministic hang during bootconsole/console handover on ath79

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Message-ID: <56F34C61.50505@universe-factory.net>
Date:	Thu, 24 Mar 2016 03:09:37 +0100
From:	Matthias Schiffer <mschiffer@...verse-factory.net>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Ralf Baechle <ralf@...ux-mips.org>, gregkh@...uxfoundation.org,
	jslaby@...e.com, linux-mips@...ux-mips.org,
	linux-serial@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Nonterministic hang during bootconsole/console handover on ath79

>> autoconfig_16550a() is doing all kinds of weird checks to detect different
>> hardware by writing a lot of register values which are documented as
>> reserved in the AR7242 datasheet (there's a leaked version going around
>> that can be easily googled...), no idea if any of those are problematic.
>> Just setting UPF_FIXED_TYPE as suggested by Peter would avoid that code
>> altogether.
> 
> That's just a debugging patch and not appropriate for permanent use,
> the reason being that this uart is _not_ 16550 compatible (or even 16450
> compatible).
> 
> The three options for 8250 driver support for this part are:
> 1. Similar to the debugging patch, set UPF_FIXED_TYPE but set port type
>    to PORT_8250 instead. This will lose FIFO support so 115K won't be
>    possible and likely neither will 38400.
> 
> 2. Set UPF_FIXED_TYPE but define a new PORT_* value and add support for
>    this PORT_* value to uart_config array, uapi headers, and anywhere
>    the scratch register is used.
> 
> 3. As with 2. above but don't set UPF_FIXED_TYPE and add a probe function
>    that detects ports of this type to autoconfig(). I don't recommend this
>    method.
> 
> This requirement is independent of fixing prom_putchar_ar71xx().
> 

I can send patches for all of this, and I think that 2. would be the nicest
solution. I've noticed though that include/uapi/linux/serial_core.h is
experiencing a little "overflow": PORT_MAX_8250 has grown just below the
first non-8250 entry. Should I just add the new entry at the bottom (and
thus grow the uart_config array by ~85 unused entries)? What about
PORT_MAX_8250 (used at least in drivers/tty/serial/8250/8250_of.c)?

Regards,
Matthias



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