linux-kernel - Re: [PATCH 1/4 v2] dmaengine: add a tx_free method to struct dma_async_tx

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Date:	Thu, 11 Dec 2008 09:43:05 -0700
From:	"Dan Williams" <dan.j.williams@...el.com>
To:	"Guennadi Liakhovetski" <g.liakhovetski@....de>
Cc:	linux-kernel@...r.kernel.org,
	linux-fbdev-devel@...ts.sourceforge.net, adaplas@...il.com,
	"Sascha Hauer" <s.hauer@...gutronix.de>,
	linux-arm-kernel@...ts.arm.linux.org.uk
Subject: Re: [PATCH 1/4 v2] dmaengine: add a tx_free method to struct dma_async_tx_descriptor

On Thu, Dec 11, 2008 at 8:55 AM, Guennadi Liakhovetski
<g.liakhovetski@....de> wrote:
> How does the struct's size impact their cache utilisation then?

Increasing the size may push some fields across a cache-line boundary
leading to an additional cache-lines that need to
filled/kept-coherent/etc...  See pahole:
http://lwn.net/Articles/206805/

>
>> > And you mean, that increasing the size
>> > of the struct by one pointer and letting users explicitly free those
>> > descriptors when they want is worse than introducing a tasklet that will
>> > have to periodically scan the list of descriptors while other hot paths
>> > will move elements to and from this list, look for acked elements, lock
>> > the list and free those elements? Periodically, because although we have
>> > an event when to free them - on ioctl - there is no API to trigger that
>> > tasklet.
>>
>> There are a few events that trigger this: completion interrupt,
>> someone polls is_tx_complete, we run out of descriptors.
>
> Sorry, actually, it's not VIDIOC_DQBUF where we have to free buffer(s),
> it's VIDIOC_STREAMOFF. And in a normal case there are no more completion
> interrupts, no allocation requests, and noone is interested in
> is_tx_complete. Normally you would just get a dma_release_channel after
> that. Or, of course, the application may decide to restart capture, then
> we get requests, completions, etc. again. So, yes, I could do this, it
> just seems a bit unnatural to me.

Could the application tolerate another call to dma_request_channel
when it restarts?  I.e. just call dma_release_channel from the ioctl
to clean everything up and not worry about another free() mechanism?

Thanks,
Dan
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