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    SubjectRe: [PATCH] OMAP: iommu flush page table entries from L1 and L2 cache
    On Thu, Aug 11, 2011 at 02:28:39PM -0500, Gupta, Ramesh wrote:
    > Hi Russel,


    > On Thu, Apr 28, 2011 at 11:48 AM, Gupta, Ramesh <> wrote:
    > > Hi Russel,
    > >
    > > On Thu, Apr 28, 2011 at 8:40 AM, Russell King - ARM Linux
    > > <> wrote:
    > >> We _could_ invent a new API to deal with this, which is probably going
    > >> to be far better in the longer term for page table based iommus.  That's
    > >> going to need some thought - eg, do we need to pass a struct device
    > >> argument for the iommu cache flushing so we know whether we need to flush
    > >> or not (eg, if we have cache coherent iommus)...
    > my apologies for a late mail on this topic.
    > do you think of any other requirements for this new API?
    > Could we use the existing dmac_flush_range(), outer_flush_range()
    > for this purpose instead of a new API?
    > I see a comment in the arch/arm/include/asm/cacheflush.h
    > for _not_ to use these APIs directly, but I am not really understand
    > the reason for that.

    When I create APIs, I create them to solve a _purpose_ to code which wants
    to do something. They're not created to provide some facility which can
    be re-used for unrelated stuff.

    This has been proven many times to be the correct approach. Over time,
    things change.

    Let's say for arguments sake that you decide to use the DMA API stuff
    to achieve your goals. Then, lets say that ARM DMA becomes fully
    cache coherent, but your IOMMU tables still need to be flushed from
    the L1 caches.

    Suddenly, dmac_flush_range() starts doing absolutely nothing. Your
    driver breaks. I get whinged at because a previously working driver
    stops working. In effect, that usage _prevents_ me making the changes
    necessary to keep the core architecture support moving forward as
    things develop. Or, alternatively I just ignore your driver, make the
    changes anyway and leave it to rot.

    So, APIs get created to provide a purpose. Like - handling the DMA
    issues when mapping a buffer to DMA. Like handling the DMA issues
    when unmapping a buffer from DMA. If you start using those _because_
    they happen to clean or invalidate the cache for you, you're really
    asking for your driver to be broken at some point in the future.

    What is far better to do is to ensure that we have the right APIs in
    place for the purposes for which they are to be used. So, if we need
    an API to flush out the IOMMU page table entries, then that's what
    we need, not some bodged lets-reuse-the-dma-flushing-functions thing.
    Inside the processors implementation, yes, it may well be the same
    thing, but that's a decision for the _processor_ support code to make,
    not the IOMMU writer.

    As to what shape a new API should be - as I said above, maybe it should
    take a struct device argument, virtual base address and size. Or maybe
    if we don't have any coherent IOMMUs then just ignore the device
    argument for the time being, and just pass the virtual base address and

    The next issue is whether it should require the virtual base address to
    be in the kernel direct mapped region. If you're touching L2, then
    that's a yes, because we need to use virt_to_phys on it to get at the
    phys address for the L2 operations.

    So, I think: extend the cpu cache operations structure to have a method
    for dealing with IOMMUs. Add an inline function to deal with calling
    that, and the L2 ops if that's what's required.
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