linux-kernel - Re: [PATCH 0/6] get rid of on-stack dma buffers

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Message-Id: <201103221412.46507.oneukum@suse.de>
Date:	Tue, 22 Mar 2011 14:12:46 +0100
From:	Oliver Neukum <oneukum@...e.de>
To:	Florian Mickler <florian@...kler.org>
Cc:	"Roedel, Joerg" <Joerg.Roedel@....com>,
	"Greg Kroah-Hartman" <greg@...ah.com>,
	"janne-dvb@...nau.be" <janne-dvb@...nau.be>,
	"g.marco@...enet.de" <g.marco@...enet.de>,
	"tskd2@...oo.co.jp" <tskd2@...oo.co.jp>,
	"liplianin@...by" <liplianin@...by>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"pb@...uxtv.org" <pb@...uxtv.org>,
	"linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
	"max@...eto.com" <max@...eto.com>,
	"mchehab@...radead.org" <mchehab@...radead.org>,
	"aet@...terburn.org" <aet@...terburn.org>,
	"mkrufky@...uxtv.org" <mkrufky@...uxtv.org>,
	James Bottomley <James.Bottomley@...senpartnership.com>,
	"js@...uxtv.org" <js@...uxtv.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Andy Walls <awalls@...metrocast.net>,
	"nick@...k-andrew.net" <nick@...k-andrew.net>
Subject: Re: [PATCH 0/6] get rid of on-stack dma buffers

Am Dienstag, 22. März 2011, 14:08:17 schrieb Florian Mickler:
> Am 22.03.2011 12:10 schrieb "Roedel, Joerg" <Joerg.Roedel@....com>:
> >
> > On Mon, Mar 21, 2011 at 05:03:15PM -0400, Florian Mickler wrote:
> > > I guess (not verified), that the dma api takes sufficient precautions
> > > to abort the dma transfer if a timeout happens.  So freeing _should_
> > > not be an issue. (At least, I would expect big fat warnings everywhere
> > > if that were the case)
> >
> > Freeing is very well an issue. All you can expect from the DMA-API is to
> > give you a valid DMA handle for your device. But it can not prevent that
> > a device uses this handle after you returned it. You need to make sure
> > yourself that any pending DMA is canceled before calling kfree().
> 
> Does usb_control_msg do this? It waits for completion but takes also a
> timeout parameter. I will recheck this once I'm home.

It uses usb_start_wait_urb() which upon a timeout kills the URB. The
buffer is unused after usb_control_msg() returns.

	HTH
		Oliver
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