Date: Tue, 23 Jun 2009 10:21:29 +0100
From: Alan Cox <alan@lxorguk.ukuu.org.uk>
To: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: "'David Brownell'" <david-b@pacbell.net>,
       "'Alan Stern'" <stern@rowland.harvard.edu>,
       "'Peter Korsgaard'" <jacmet@sunsite.dk>,
       "'USB list'" <linux-usb@vger.kernel.org>,
       "'Kernel development list'" <linux-kernel@vger.kernel.org>,
       kyungmin.park@samsung.com,
       "'Marek Szyprowski'" <m.szyprowski@samsung.com>
Subject: Re: PROBLEM: kernel oops with g_serial USB gadget on 2.6.30
Message-ID: <20090623102129.3b58adbf@lxorguk.ukuu.org.uk>
In-Reply-To: <002001c9f3dd$d7a24df0$86e6e9d0$%szyprowski@samsung.com>
References: <001201c9f341$20b8b710$622a2530$%szyprowski@samsung.com>
	<200906222026.51511.david-b@pacbell.net>
	<001c01c9f3cd$ad02b1d0$07081570$%szyprowski@samsung.com>
	<200906230022.39040.david-b@pacbell.net>
	<002001c9f3dd$d7a24df0$86e6e9d0$%szyprowski@samsung.com>
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> that g_serial driver interacts with tty layer in that packet_done callback,
> so this is the source of the problems. I noticed that some other UDC
> drivers also does all its job from an interrupt, so they also might be
> affected. How this bug should be properly resolved?

Either by not setting ->low_latency or by running the data paths from a
non IRQ context.

Basically: don't set tty->low_latency if you are handling the processing
from the IRQ path. The only case low_latency is useful is handling data
from a non-IRQ path where you have latency concerns for tx/rx switching.

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