linux-kernel - Re: [PATCH v4 2/6] dmaengine: Add dma_chan_is_in

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Message-ID: <e9c3a7c20807011831n74eb69b8r3095b62515fe5c9@mail.gmail.com>
Date:	Tue, 1 Jul 2008 18:31:25 -0700
From:	"Dan Williams" <dan.j.williams@...el.com>
To:	"Haavard Skinnemoen" <haavard.skinnemoen@...el.com>
Cc:	"Pierre Ossman" <drzeus-list@...eus.cx>,
	linux-kernel@...r.kernel.org, linux-embedded@...r.kernel.org,
	kernel@...32linux.org, shannon.nelson@...el.com,
	"David Brownell" <david-b@...bell.net>
Subject: Re: [PATCH v4 2/6] dmaengine: Add dma_chan_is_in_use() function

On Thu, Jun 26, 2008 at 6:23 AM, Haavard Skinnemoen
<haavard.skinnemoen@...el.com> wrote:
> This moves the code checking if a DMA channel is in use from
> show_in_use() into an inline helper function, dma_is_in_use(). DMA
> controllers can use this in order to give clients exclusive access to
> channels (usually necessary when setting up slave DMA.)
>
> I have to admit that I don't really understand the channel refcounting
> logic at all... dma_chan_get() simply increments a per-cpu value. How
> can we be sure that whatever CPU calls dma_chan_is_in_use() sees the
> same value?

As Chris noted in the comments at the top of dmaengine.c this is an
implementation Rusty's 'bigref'.  It seeks to avoid the
cache-line-bouncing overhead of maintaining a single global refcount
in hot paths like tcp_v{4,6}_rcv().  When the channel is being
removed, a rare event, we transition to the accurate, yet slow, global
method.

Your observation is correct, dma_chan_is_in_use() may lie in the case
when the current cpu is not using the channel.  For this particular
test I think you can look to see if this channel's resources are
already allocated.  If they are then some other client got a hold of
this channel before the current attempt.  Hmm... that would also
require that we free the channel's resources in the case where the
client replies with DMA_NAK, probably something we should do anyways.

Thoughts?

--
Dan
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