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    SubjectRe: [PATCH 06/11] persistent_ram: Make it possible to use memory outside of bootmem
    On Wed, Jun 06, 2012 at 02:10:34PM -0700, Colin Cross wrote:
    > On Fri, May 11, 2012 at 5:17 PM, Anton Vorontsov
    > <> wrote:
    > > This includes devices' memory (e.g. framebuffers or memory mapped
    > > EEPROMs on a local bus), as well as the normal RAM that we don't use
    > > for the main memory.
    > >
    > > For the normal (but unused) ram we could use kmaps, but this assumes
    > > highmem support, so we don't bother and just use the memory via
    > > ioremap.
    > >
    > > As a side effect, the following hack is possible: when used together
    > > with pstore_ram (new ramoops) module, we can limit the normal RAM region
    > > with mem= and then point ramoops to use the rest of the memory, e.g.
    > >
    > >        mem=128M ramoops.mem_address=0x8000000
    > >
    > > Sure, we could just reserve the region with memblock_reserve() early in
    > > the arch/ code, and then register a pstore_ram platform device pointing
    > > to the reserved region. It's still a viable option if platform wants
    > > to do so.
    > >
    > > Also, we might want to use IO accessors in case of a real device,
    > > but for now we don't bother (the old ramoops wasn't using it either, so
    > > at least we don't make things worse).
    > This is long merged, but I remembered why I moved away from using
    > ioremap. The current code uses atomics to track the ringbuffer
    > positions, which results in ldrex and strex instructions on ARM.
    > ldrex and strex on memory that is mapped as Device memory (which is
    > what ioremap maps as) is implementation defined, and is unpredictable
    > at the architecture level.

    Makes sense, thanks for sharing! Fortunately, we still map things
    w/ vmap if pfn appears to be valid. :-)


    Anton Vorontsov
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