linux-kernel - Re: dmaengine: Can we schedule new transfer from dma callback routine??

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20110411085603.GA13041@n2100.arm.linux.org.uk>
Date:	Mon, 11 Apr 2011 09:56:03 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	viresh kumar <viresh.kumar@...com>
Cc:	"Koul, Vinod" <vinod.koul@...el.com>,
	Dan Williams <dan.j.williams@...el.com>,
	"linus.walleij@...ricsson.com" <linus.walleij@...ricsson.com>,
	Amit GOEL <amit.goel@...com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Armando VISCONTI <armando.visconti@...com>,
	Shiraz HASHIM <shiraz.hashim@...com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: dmaengine: Can we schedule new transfer from dma callback
	routine??

On Mon, Apr 11, 2011 at 01:25:04PM +0530, viresh kumar wrote:
> 
> Hello,
> 
> In dw_dmac.c driver, dwc_descriptor_complete() routine, following is
> mentioned before calling callback:
> 
> 	/*
> 	 * The API requires that no submissions are done from a
> 	 * callback, so we don't need to drop the lock here
> 	 */
> 	if (callback)
> 		callback(param);
> 
> Does this hold true for dmaengine??

Not for slave devices - see Dan's reply:

http://lists.arm.linux.org.uk/lurker/message/20101223.005313.a38d7bf0.en.html

As the slave API hasn't been well documented, there's a lot of
inconsistency of behaviour between DMA engine slave implementations.
I'd suggest at least fixing slave DMA engine drivers to ensure that:

(a) the callback is always called in tasklet context
(b) the callback can submit new slave transactions (iow, the spinlock
    which prep_slave_sg takes must not be held during the callback.)

The way that others solve this is to move the completed txd structures
to a local 'completed' list, and then walk this list after the spinlocks
have been dropped.

IOW, something like this:

my_tasklet()
{
	INIT_LIST_HEAD(completed);

	spin_lock_irqsave(my_chan->lock);
	for_each_txd(my_txd, my_chan) {
		if (has_completed(my_txd))
			list_add_tail(my_txd->node, &completed);
	}
	spin_unlock_irqrestore(my_chan->lock);

	list_for_each_entry_safe(my_txd, next, &completed, node) {
		void *callback_param = my_txd->txd.callback_param;
		void (*fn)(void *) = my_txd->txd.callback;

		my_txd_free(my_chan, my_txd);

		fn(callback_param);
	}	
}
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/