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regmap: rbtree: Make cache_present bitmap per node
With devices which have a dense and small register map but placed at a large offset the global cache_present bitmap imposes a huge memory overhead. Making the cache_present per rbtree node avoids the issue and easily reduces the memory footprint by a factor of ten. For devices with a more sparse map or without a large base register offset the memory usage might increase slightly by a few bytes, but not significantly. E.g. for a device which has ~50 registers at offset 0x4000 the memory footprint of the register cache goes down form 2496 bytes to 175 bytes. Moving the bitmap to a per node basis means that the handling of the bitmap is now cache implementation specific and can no longer be managed by the core. The regcache_sync_block() function is extended by a additional parameter so that the cache implementation can tell the core which registers in the block are set and which are not. The parameter is optional and if NULL the core assumes that all registers are set. The rbtree cache also needs to implement its own drop callback instead of relying on the core to handle this. Signed-off-by:Lars-Peter Clausen <lars@metafoo.de> Signed-off-by: