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    SubjectRe: [PATCH 1/3] mpt2sas: remove the use of writeq, since writeq is not atomic
    On Thu, 2011-05-19 at 08:46 +0400, James Bottomley wrote:
    > This can't really be done generically. There are several considerations
    > to do with hardware requirements. I can see some hw requiring a
    > specific write order (I think this applies more to read order, though).

    Right. Or there can be a need for a completely different access pattern
    to do 32-bit, or maybe write only one half because both might have a
    side effect etc etc etc ...

    Also a global lock would be suboptimal vs. a per device lock burried in
    the driver.

    > The specific mpt2sas problem is that if we write a 64 bit register non
    > atomically, we can't allow any interleaving writes for any other region
    > on the chip, otherwise the HW will take the write as complete in the 64
    > bit register and latch the wrong value. The only way to achieve that
    > given the semantics of writeq is a global static spinlock.
    > > How do you think about them? If you cannot agree with the above two
    > > solutions, I'll agree with reverting them.
    > Having x86 roll its own never made any sense, so I think they need
    > reverting anyway.


    > This is a driver/platform bus problem not an
    > architecture problem. The assumption we can make is that the platform
    > CPU can write atomically at its chip width. We *may* be able to make
    > the assumption that the bus controller can translate an atomic chip
    > width transaction to a single atomic bus transaction; I think that
    > assumption holds true for at least PCI and on the parisc legacy busses,
    > so if we can agree on semantics, this should be a global define
    > somewhere. If there are problems with the bus assumption, we'll likely
    > need some type of opt-in (or just not bother).

    And we want a well defined #ifdef drivers test to know whether there's a
    writeq/readq (just #define writeq/readq itself is fine even if it's an
    inline function, we do that elsewhere) so they can have a fallback

    This is important as these can be used in very performance critical code


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