On Sunday, December 04, 2016 03:55:14 PM Ian Lepore wrote: > On Sun, 2016-12-04 at 13:43 -0800, John Baldwin wrote: > > On Sunday, December 04, 2016 03:27:39 PM Michal Meloun wrote: > > > > > > Author: mmel > > > Date: Sun Dec 4 15:27:39 2016 > > > New Revision: 309531 > > > URL: https://svnweb.freebsd.org/changeset/base/309531 > > > > > > Log: > > > Implement fake pmap_mapdev_attr() for ARMv6. > > > This function is referenced, but never called from DRM2 code. > > > Also, > > > real behavior of pmap_mapdev_attr() in ARM world is unclear as we > > > don't > > > have any additional attribute for a device memory type. > > You have VM_MEMATTR constants on ARM though. Some devices might want > > a > > WB mapping (e.g. for a prefetchable memory BAR) rather than UC. > > > > On ARM, 'DEVICE' memory is its own type/attribute and it can't usefully > be changed (if you were to change it, it would no longer be device > memory, it would be some other type). If this function is being used > to map main memory just because that memory happens to be used in some > device driver, then that's a usage model that just doesn't map usefully > to arm memory semantics.
Ah, no. It is used for things like BARs, etc. It is fine if devices are always mapped uncacheable. The requests for write-combining are performance hints, but devices should still function correctly. For example, on i386 without PAT we will map WC requests to UC. -- John Baldwin _______________________________________________ svn-src-head@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/svn-src-head To unsubscribe, send any mail to "svn-src-head-unsubscr...@freebsd.org"
