Messages in this thread | | | Date | Tue, 1 Jul 2008 18:31:25 -0700 | From | "Dan Williams" <> | Subject | Re: [PATCH v4 2/6] dmaengine: Add dma_chan_is_in_use() function |
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On Thu, Jun 26, 2008 at 6:23 AM, Haavard Skinnemoen <haavard.skinnemoen@atmel.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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