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    SubjectRe: [PATCH 08/10] mm: cma: Contiguous Memory Allocator added
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    On Tuesday 14 June 2011, Michal Nazarewicz wrote:
    > On Tue, 14 Jun 2011 15:49:29 +0200, Arnd Bergmann <arnd@arndb.de> wrote:
    > > Please explain the exact requirements that lead you to defining multiple
    > > contexts.
    >
    > Some devices may have access only to some banks of memory. Some devices
    > may use different banks of memory for different purposes.

    For all I know, that is something that is only true for a few very special
    Samsung devices, and is completely unrelated of the need for contiguous
    allocations, so this approach becomes pointless as soon as the next
    generation of that chip grows an IOMMU, where we don't handle the special
    bank attributes. Also, the way I understood the situation for the Samsung
    SoC during the Budapest discussion, it's only a performance hack, not a
    functional requirement, unless you count '1080p playback' as a functional
    requirement.

    Supporting contiguous allocation is a very useful goal and many people want
    this, but supporting a crazy one-off hardware design with lots of generic
    infrastructure is going a bit too far. If you can't be more specific than
    'some devices may need this', I would suggest going forward without having
    multiple regions:

    * Remove the registration of specific addresses from the initial patch
    set (but keep the patch).
    * Add a heuristic plus command-line override to automatically come up
    with a reasonable location+size for *one* CMA area in the system.
    * Ship the patch to add support for multiple CMA areas with the BSP
    for the boards that need it (if any).
    * Wait for someone on a non-Samsung SoC to run into the same problem,
    then have /them/ get the final patch in.

    Even if you think you can convince enough people that having support
    for distinct predefined regions is a good idea, I would recommend
    splitting that out of the initial merge so we can have that discussion
    separately from the other issues.

    Arnd


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