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

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Date:	Tue, 22 Mar 2016 10:50:20 +0100
From:	Matthias Schiffer <mschiffer@...verse-factory.net>
To:	Antony Pavlov <antonynpavlov@...il.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>,
	Peter Mamonov <pmamonov@...il.com>
Subject: Re: Nonterministic hang during bootconsole/console handover on ath79

On 03/22/2016 06:40 AM, Antony Pavlov wrote:
> On Tue, 22 Mar 2016 00:02:57 +0100
> Matthias Schiffer <mschiffer@...verse-factory.net> wrote:
> 
>> Hi,
>> we're experiencing weird nondeterministic hangs during bootconsole/console
>> handover on some ath79 systems on OpenWrt. I've seen this issue myself on
>> kernel 3.18.23~3.18.27 on a AR7241-based system, but according to other
>> reports ([1], [2]) kernel 4.1.x is affected as well, and other SoCs like
>> QCA953x likewise.
>>
>> See the log below for the exact place it hangs; the log was taken in during
>> a good boot; a bad boot will just hang forever at the marked location. The
>> issue is extremely hard to debug, as changing the timing in any way (like
>> adding additional printk) will usually make it work without problems. (Even
>> recompiling the kernel with the same config, but different uname timestamp
>> will make the occurence more or less likely)
>>
>> My theory is the following:
>>
>> As soon as ttyS0 is detected and installed as the console, there are two
>> console drivers active on the serial port at the same time: early0 and
>> ttyS0. I suspect that the hang occurs when the primitive early0
>> implementation prom_putchar_ar71xx waits indefinitely on THRE,
> 
> Can you use EJTAG to prove your theory?
> 
> -- 
> Best regards,
>   Antony Pavlov
> 

Unfortunately, the board I can reproduce this at the moment does not have
an accessible JTAG port. I hope I can reproduce it on other hardware which
is more developer-friendly, but no luck so far.

Matthias



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