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

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Message-ID: <5695584E.3040902@hurleysoftware.com>
Date:	Tue, 12 Jan 2016 11:47:26 -0800
From:	Peter Hurley <peter@...leysoftware.com>
To:	Sebastian Frias <sf84@...oste.net>
Cc:	Mason <slash.tmp@...e.fr>,
	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 06:22 AM, Sebastian Frias wrote:
> 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.

Got it, thanks for the info.
Please test the series I just cc'd you on plus the patch I sent
you yesterday.

That should get you an earlycon up and running on that simulator;
let me know if it doesn't and we'll go from there.

> 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.

I expect that patch to go into -next sometime during the 4.5-rc cycle.


Regards,
Peter Hurley

>> 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.
>>