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. Agreed. > 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 scenario. This is important as these can be used in very performance critical code path. Cheers, Ben. -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
