some more generic comments about the data structure: it's main purpose
is to provide a mapping based on (dev,addr). There's little if any
cross-entry interaction - same-address+same-dev DMA is checked.
1)
the hash:
+ return (entry->dev_addr >> HASH_FN_SHIFT) & HASH_FN_MASK;
should mix in entry->dev as well - that way we get not just per
address but per device hash space separation as well.
2)
HASH_FN_SHIFT is 1MB chunks right now - that's probably fine in
practice albeit perhaps a bit too small. There's seldom any coherency
between the physical addresses of DMA - we rarely have any real
(performance-relevant) physical co-location of DMA addresses beyond 4K
granularity. So using 1MB chunking here will discard a good deal of
random low bits we should be hashing on.
3)
And the most scalable locking would be per hash bucket locking - no
global lock is needed. The bucket hash heads should probably be
cacheline sized - so we'd get one lock per bucket.
This way if there's irq+DMA traffic on one CPU from one device into
one range of memory, and irq+DMA traffic on another CPU to another
device, they will map to two different hash buckets.
4)
Plus it might be an option to make hash lookup lockless as well:
depending on the DMA flux we can get a lot of lookups, and taking the
bucket lock can be avoided, if you use RCU-safe list ops and drive the
refilling of the free entries pool from RCU.
Ingo
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