On Tue, Mar 9, 2021 at 1:41 PM Arnd Bergmann [off-list ref] wrote:

- A driver writer may want to choose between posted and
  nonposted mmio based on performance considerations:
  if writes are never serialized, posted writes should always
  be faster. However, if the driver uses a spinlock to serialize
  writes, then a nonposted write is likely faster than a posted
  write followed by a read that serializes the spin_unlock.
  In this case we want the driver to explicitly pick one over
  the other, and not have rely on bus specific magic.

OK then I am all for having drivers explicitly choose access
method. Openness to speed optimization is a well established
Linux kernel design principle.

Yours,
Linus Walleij