linux-kernel - Re: [PATCH] serial: msm: Add magic SysRq support in dmmode

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Date:	Thu, 30 Oct 2014 12:00:52 +0000
From:	Daniel Thompson <daniel.thompson@...aro.org>
To:	Stephen Boyd <sboyd@...eaurora.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:	David Brown <davidb@...eaurora.org>,
	Daniel Walker <dwalker@...o99.com>,
	Bryan Huntsman <bryanh@...eaurora.org>,
	Jiri Slaby <jslaby@...e.cz>, linux-arm-msm@...r.kernel.org,
	linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
	patches@...aro.org, linaro-kernel@...ts.linaro.org,
	John Stultz <john.stultz@...aro.org>,
	Sumit Semwal <sumit.semwal@...aro.org>
Subject: Re: [PATCH] serial: msm: Add magic SysRq support in dmmode

On 29/10/14 18:01, Stephen Boyd wrote:
> On 10/29/2014 09:38 AM, Daniel Thompson wrote:
>> Currently the _dm RX handler detects breaks but does not pass any
>> characters to uart_handle_sysrq_char().
>>
>> The _dm optimizations combined with the port's spin lock make if
>> difficult to pass all characters to the sysrq logic because we cannot
>> safely call uart_handle_sysrq_char() when the lock is held without
>> deadlock (the console handler also takes the lock).
>>
>> Rather than passing all characters via uart_handle_sysrq_char() this patch
>> only passes the last few characters in the FIFO. This should include all
>> characters typed as a slow (human) rate. This makes the problem much
>> simpler and allows us to move the handling of these characters outside
>> of the port lock. This makes magic SysRq work if there is a human at
>> the keyboard (or a short delay in a script).
>>
>> Signed-off-by: Daniel Thompson <daniel.thompson@...aro.org>
>> ---
> 
> I have a patch that I was going to send for this (Frank R. and I have
> been looking at it since a month or two ago). This patch doesn't look
> correct given that the SR register is not actually accurate and doesn't
> indicate that a break is there in the fifo. I'll Cc you on the patch,
> please test it.

Testing on IFC6410 is mostly OK.

Most sysrq actions work fine but resume from kdb is extremely
unreliable; in most cases is kills the serial RX ('echo g >
/proc/sysrq-trigger' shows that characters are still going into the FIFO
but never get to the interrupt handler).

However, this is not a regression versus my patch, resume was not
reliable with my patch either.
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