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

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Message-ID: <56950C21.3010805@laposte.net>
Date:	Tue, 12 Jan 2016 15:22:25 +0100
From:	Sebastian Frias <sf84@...oste.net>
To:	Mason <slash.tmp@...e.fr>, Peter Hurley <peter@...leysoftware.com>
CC:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-serial@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
	Mans Rullgard <mans@...sr.com>
Subject: Re: [RFC PATCH] always probe UART HW when options are not specified

On 01/12/2016 10:37 AM, Mason wrote:
> On 11/01/2016 20:06, Peter Hurley wrote:
>
>> 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?
>
> I'm not sure exactly which board Sebastian has in mind, but I've been
> using the SERIAL_8250_RT288X driver on my Tango4 board.

For the record, I'm using a SoC emulator, and thus do not have a 
bootloader per se and there are a bunch of other things that I cannot 
count on.
The emulator has the UART pre-setup, so I just need Linux to take over 
without changing the parameters.
Ideally, I would like to have the same image of Linux+DT to start in any 
instance of the emulator or real chips, regardless of the clock ratios, 
that's why I sort of need Linux to not change the UART speed, which is 
quite tricky because there are no clock generators in the emulator.

NOTE: on my tree I'm using the patch I previously submitted here, so 
Linux is probing the UART so it works for all my cases, but I would like 
to go back to a standard tree.

>
> Note: calling the UART driver "Au1x00/RT288x" is a bit of a misnomer,
> as these are names of SoCs using that distinct register layout.
> The actual IP is probably the 16550-compatible Palmchip BK-3103.
> https://sites.google.com/a/palmchiptech.com/palmchiptech/product-services/hardware-services/ip-cores/bk-3103
> (Not sure that this website is legitimate, though.)
>
> When I need earlyprintk support, I use this patch from Mans:
> http://thread.gmane.org/gmane.linux.kernel/2081016
>
> Regards.
>