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From:   John Ogness <john.ogness@linutronix.de>
To:     Marek Szyprowski <m.szyprowski@samsung.com>,
        Petr Mladek <pmladek@suse.com>
Cc:     Sergey Senozhatsky <senozhatsky@chromium.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel@vger.kernel.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-amlogic@lists.infradead.org
Subject: Re: [PATCH printk v5 1/1] printk: extend console_lock for
 per-console locking
In-Reply-To: <32bba8f8-dec7-78aa-f2e5-f62928412eda@samsung.com>
References: <20220421212250.565456-1-john.ogness@linutronix.de>
 <20220421212250.565456-15-john.ogness@linutronix.de>
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 <CGME20220427070833eucas1p27a32ce7c41c0da26f05bd52155f0031c@eucas1p2.samsung.com>
 <2a82eae7-a256-f70c-fd82-4e510750906e@samsung.com>
 <Ymjy3rHRenba7r7R@alley>
 <b6c1a8ac-c691-a84d-d3a1-f99984d32f06@samsung.com>
 <87fslyv6y3.fsf@jogness.linutronix.de>
 <51dfc4a0-f6cf-092f-109f-a04eeb240655@samsung.com>
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Date:   Mon, 02 May 2022 15:17:29 +0206
Message-ID: <87y1zkkrjy.fsf@jogness.linutronix.de>
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On 2022-05-02, Marek Szyprowski <m.szyprowski@samsung.com> wrote:
> Data written to /dev/kmsg and all kernel logs were always displayed
> correctly. Also data written directly to /dev/ttyAML0 is displayed
> properly on the console. The latter doesn't however trigger the input
> related activity.
>
> It looks that the data read from the uart is delivered only if other
> activity happens on the kernel console. If I type 'reboot' and press
> enter, nothing happens immediately. If I type 'date >/dev/ttyAML0' via
> ssh then, I only see the date printed on the console. However if I
> type 'date >/dev/kmsg', the the date is printed and reboot happens.

I suppose if you login via ssh and check /proc/interrupts, then type
some things over serial, then check /proc/interrupts again, you will see
there have been no interrupts for the uart. But interrupts for other
devices are happening. Is this correct?

> For comparison, here is a 't' sysrq result from the 'working' serial
> console (next-20220429), which happens usually 1 of 4 boots:
>
> https://pastebin.com/mp8zGFbW

This still looks odd to me. We should be seeing a trace originating from
ret_from_fork+0x10/0x20 and kthread+0x118/0x11c.

I wonder if the early creation of the thread is somehow causing
problems. Could you try the following patch to see if it makes a
difference? I would also like to see the sysrq-t output with this patch
applied:

---------------- BEGIN PATCH ---------------
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 2311a0ad584a..c4362d25de22 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3837,7 +3837,7 @@ static int __init printk_activate_kthreads(void)
 
 	return 0;
 }
-early_initcall(printk_activate_kthreads);
+late_initcall(printk_activate_kthreads);
 
 #if defined CONFIG_PRINTK
 /* If @con is specified, only wait for that console. Otherwise wait for all. */
---------------- END PATCH ---------------

Thanks for your help with this!

John