[PATCH v3 14/15] nvme: Support atomic writes

[Nilay Shroff]

>Support reading atomic write registers to fill in request_queue
>properties.

>Use following method to calculate limits:
>atomic_write_max_bytes = flp2(NAWUPF ?: AWUPF)
>atomic_write_unit_min = logical_block_size
>atomic_write_unit_max = flp2(NAWUPF ?: AWUPF)
>atomic_write_boundary = NABSPF

In case the device doesn't support namespace atomic boundary size (i.e. NABSPF 
is zero) then while merging atomic block-IO we should allow merge.
 
For example, while front/back merging the atomic block IO, we check whether 
boundary is defined or not. In case if boundary is not-defined (i.e. it's zero) 
then we simply reject merging ateempt (as implemented in 
rq_straddles_atomic_write_boundary()).  

I am quoting this from NVMe spec (Command Set Specification, revision 1.0a, 
Section 2.1.4.3) : "To ensure backwards compatibility, the values reported for 
AWUN, AWUPF, and ACWU shall be set such that they  are  supported  even  if  a  
write  crosses  an  atomic  boundary.  If  a  controller  does  not  guarantee 
atomicity across atomic boundaries, the controller shall set AWUN, AWUPF, and 
ACWU to 0h (1 LBA)." 

Thanks,
--Nilay