Re: [PATCH] NVMe: Fix compilation on architecturs without readq/writeq

[H. Peter Anvin]

On 02/01/2012 05:05 PM, James Bottomley wrote:
> Incidentally, the last time this came up was with mpt fusion: for a
> write to a 64 bit register, it didn't care about order, but it did care
> about interleaving as in if you write one half of a 64 bit register and
> then write to another register, the 64 bit register effectively gets
> written with zeros in the part you didn't write to, so we had to put a
> spin lock in the open coded writeb/w/l/q() to make sure the card didn't
> get interleaved writes.

There are always going to be hardware which have specific needs, and for 
those open-coding makes sense, but the littleendian/bigendian pair is 
going to cover ~90% of users and make sense to can.

I worked myself on a driver (which sadly never shipped) which had an WC 
window and a UC window... the final write in a series had a completion 
bit in it and would go to the UC window after setting up a whole chunk 
of operations in the WC window (writing UC memory flushes WC memory 
ahead of it.)

Thus, the two-part breakdown of writeq() to the UC window had to write 
the low half to the WC window instead.  This is clearly not generic.

	-hpa


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