linux-kernel - console output duplicated when registering additional consoles

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Date:   Wed, 6 Nov 2019 17:09:15 -0800
From:   Jonathan Richardson <jonathan.richardson@...adcom.com>
To:     gregkh@...uxfoundation.org, jslaby@...e.com
Cc:     linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
        Scott Branden <scott.branden@...adcom.com>,
        Ray Jui <ray.jui@...adcom.com>,
        Srinath Mannam <srinath.mannam@...adcom.com>
Subject: console output duplicated when registering additional consoles

Hi,

I have a console driver that registers an additional boot console and
"normal" console. The command line is:
earlycon=uart8250,mmio32,0x68A10000 console=ttyS0,115200n8
earlylog=0x8f110000,0x10000

When the second bootconsole is registered via register_console(), the
output in the log buffer is written a second time to the uart8250
console resulting in duplicate output. When the second "normal"
console is registered the complete buffer is written again to
uart8250. It looks like a bug in printk but I'm still trying to figure
out how it's keeping track of what has been written to a console.

I've attached the driver and uart8250 output.

Thanks.

View attachment "console-output.txt" of type "text/plain" (48107 bytes)

Download attachment "log_console.c" of type "application/octet-stream" (4418 bytes)