linux-kernel - Re: [PATCH] Fix data loss in cdc-acm

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Date:	Thu, 23 Jul 2015 00:53:18 +0200
From:	Sven Brauch <mail@...nbrauch.de>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Oliver Neukum <oneukum@...e.com>, Johan Hovold <johan@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Toby Gray <toby.gray@...lvnc.com>, linux-usb@...r.kernel.org,
	linux-serial@...r.kernel.org
Subject: Re: [PATCH] Fix data loss in cdc-acm

Hi,

On 23/07/15 00:12, Peter Hurley wrote:
> The premature unthrottle actually leads to the data loss but the throttling
> with a mere 2K left is _way too late_.
Ok, yes, I think so too.

> 10ms is a _really_ long time for a cpu not to attend to a kworker.
> Which raises 2 questions:
> 1. What are the termios settings of the tty receiving input? Is it 'raw'
>    mode or typical terminal mode (icanon, echo, etc.) or something else?
In my test code, I open the tty like
  fd = open("/dev/ttyACM0", O_RDWR | O_NOCTTY | O_NONBLOCK);
I don't make any other changes to the default settings. To be honest,
I'm not sure in which mode it is operating then (I was assuming raw, but
I might be wrong?).

> 2. Are there RT threads that are hogging cpu time?
I can't see any, I think the only thing which occasionally goes to RT is
pulseaudio (but during at least some of the tests I wasn't even playing
audio, so that sounds very unplausible to me). I also cannot see a
correlation between failure rate and CPU load.

Best regards,
Sven


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