Date: Mon, 05 Dec 2016 08:38:43 -0800 From: John Baldwin <jhb@freebsd.org> To: Ian Lepore <ian@freebsd.org> Cc: Michal Meloun <mmel@freebsd.org>, src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r309531 - head/sys/arm/include Message-ID: <1919253.AMxT3OP8Q2@ralph.baldwin.cx> In-Reply-To: <1480892114.1889.243.camel@freebsd.org> References: <201612041527.uB4FRduc064051@repo.freebsd.org> <1605461.LLfnXX4YB2@ralph.baldwin.cx> <1480892114.1889.243.camel@freebsd.org>
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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
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