Linus Torvalds writes: (Summary) But ...
Side note: this is obviously a negative feedback circle, where because we know we can't do all that great in general, few platforms really bother to do as well as they perhaps could, since nobody should generally care that deeply due to the generic problem.. None of this "udelay()" crud, you knew that doing a single PIO read took one usec, so you'd just delay that way. Doing things like a status register read is often a good way (sometimes the _only_ way) to make sure preceding writes have been flushed all the way to the device anyway, so it can have those kinds of incidental advantages.
of incidental advantages.
Of course, all of this is relevant only for delays on the scale of individual device accesses.
Side note: this is obviously a negative feedback circle, where because we know we can't do all that great in general, few platforms really bother to do as well as they perhaps could, since nobody should generally care that deeply due to the generic problem.. None of this "udelay()" crud, you knew that doing a single PIO read took one usec, so you'd just delay that way. Doing things like a status register read is often a good way (sometimes the _only_ way) to make sure preceding writes have been flushed all the way to the device anyway, so it can have those kinds of incidental advantages.
of incidental advantages.
Of course, all of this is relevant only for delays on the scale of individual device accesses.