Re: RFC: New bus_space routine: bus_space_sync

[Eduardo Horvath <eeh%NetBSD.org@localhost>]

On Thu, 19 Jan 2012, Matt Thomas wrote:

> For prefetchable regions (like framebuffers) mapped by bus_space_map, there 
> is a need to able force the contents out of the cache back into memory 
> (especially when the cache is a writeback cache).
> 
> There is no MI way to do this with the bus_space framework so I'm proposing 
> we add a:
> 
>       void bus_space_sync(bus_space_tag_t bst,
>               bus_space_handle_t bsh,
>               bus_size_t offset,
>               bus_size_t length,
>               int ops);
> 
> where ops is one of:
> 
> #define       BUS_SPACE_SYNC_WB       1 // defined by MD
> #define       BUS_SPACE_SYNC_WBINV    2 // defined by MD
> 
> One caveat is that though a BUS_SPACE_SYNC_WB was requested, a platform can 
> perform BUS_SPACE_SYNC_WBINV instead.  If the platform can't support just 
> writeback, it is allowed to silently do a writeback-invalidate instead.

Could elaborate a bit more on how you plan to use this and why 
bus_space_barrier() with BUS_SPACE_BARRIER_SYNC is insufficient?

Yes, I know the current implementation ov bus_space_barrier() on the 
architecture you're using doesn't do this, but why can't it be enhanced to 
if you do the initial mapping with BUS_SPACE_MAP_CACHEABLE?  Presumably if 
bus_space_sync() is passd a handle, offset, and length you're using the 
bus_space_{read,write}*() accessors rather than the pointer returned by 
bus_space_vaddr().

Also, how would this work if your machine has an I/O cache (separate from 
the CPU cache)?

Eduardo