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dmaengine: up-level reference counting to the module level

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Simply, if a client wants any dmaengine channel then prevent all dmaengine
modules from being removed.  Once the clients are done re-enable module
removal.

Why?, beyond reducing complication:
1/ Tracking reference counts per-transaction in an efficient manner, as
   is currently done, requires a complicated scheme to avoid cache-line
   bouncing effects.
2/ Per-transaction ref-counting gives the false impression that a
   dma-driver can be gracefully removed ahead of its user (net, md, or
   dma-slave)
3/ None of the in-tree dma-drivers talk to hot pluggable hardware, but
   if such an engine were built one day we still would not need to notify
   clients of remove events.  The driver can simply return NULL to a
   ->prep() request, something that is much easier for a client to handle.

Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Maciej Sosnowski <maciej.sosnowski@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
This commit is contained in:
Dan Williams
2009-01-06 11:38:14 -07:00
parent 07f2211e4f
commit 6f49a57aa5
8 changed files with 132 additions and 111 deletions
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21
include/linux/dmaengine.h
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@ -165,7 +165,6 @@ struct dma_slave {
*/
struct dma_chan_percpu {
local_t refcount;
/* stats */
unsigned long memcpy_count;
unsigned long bytes_transferred;
@ -205,26 +204,6 @@ struct dma_chan {
void dma_chan_cleanup(struct kref *kref);
static inline void dma_chan_get(struct dma_chan *chan)
{
if (unlikely(chan->slow_ref))
kref_get(&chan->refcount);
else {
local_inc(&(per_cpu_ptr(chan->local, get_cpu())->refcount));
put_cpu();
}
}
static inline void dma_chan_put(struct dma_chan *chan)
{
if (unlikely(chan->slow_ref))
kref_put(&chan->refcount, dma_chan_cleanup);
else {
local_dec(&(per_cpu_ptr(chan->local, get_cpu())->refcount));
put_cpu();
}
}
/*
* typedef dma_event_callback - function pointer to a DMA event callback
* For each channel added to the system this routine is called for each client.