On Tue, Apr 28, 2015 at 09:58:48PM -0700, Kevin Cernekee wrote: > On Sat, Apr 25, 2015 at 4:32 AM, Mark Brown <broonie@xxxxxxxxxx> wrote: > > What we should be doing here is providing a way for users to tell regmap > > if they've reset the register map and actually we already have that > > interface, it's just not got the best name - regcache_mark_dirty() is > > effectively it since there's really not a lot of other reasons why a > > driver would need to mark the cache as dirty. We're just not handling > 1) How do we tell the difference between "regcache contains a > non-default value that correctly reflects the hardware register > contents" versus "regcache contains a non-default value that is > waiting to be written when we exit cache_only mode"? Like I said above we can tell if the hardware was reset because mark_dirty() is called. > 2) Does that also mean that we should store default values in the > rbtree if they are part of a deferred cache_only write, but not store > them if the write went through to the hardware? Well, remember that it's very expensive to remove a value from the cache so actively trying to prune the cache would be bad. > 3) If we're caching the default values lazily, does that mean that > every regcache read would incur both an rbtree lookup and a bsearch of > the reg_defaults array? That'd happen on first read, yes. > 4) If "the only things in the cache will be things that have been > explicitly changed," that could impact the semantics of > regcache_drop_region(). Which fortunately has no users. Could you articulate what changes you believe would be seen? > Seems like it would be more straightforward just to add an > rbnode->dirty bitmask alongside rbnode->cache_present, rather than > trying to infer the hardware state from the presence/absence of the > cache entry. Knowing whether each individual register is out of sync > with the hardware lets us avoid unnecessary writes in both situations: > full reset, and temporary loss of register access. I'm not suggesting that we do anything based on the presence of a cache entry, I'm suggesting that we could avoid having to ever cache values that never get referenced on a system (which can be a lot of them for common use cases) saving us memory. Maintaining a dirty bitmask would work too, but it does push the memory consumption up further which might be a concern.
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