Re: [PATCH] take 48-bit lba a bit further

[John Bradford]

> > Then, don't we want to be using 48-bit lba all the time on compatible devices
> > instead of falling back to 28-bit when possible to save a small amount of
> > instruction overhead?  (Or is that what we're doing already?  I haven't really
> > had the time to follow this thread).
> 
> The overhead of the double load of the command registers is microseconds so it
> is actually quite a lot, especially since IDE lacks TCQ so neither end of the
> link is doing *anything* useful. SCSI has similar problems on older SCSI with 
> command sending being slow, but the drive is at least doing other commands during
> this.

So, say you have a choice of either a 256Kb request to a low block number,
which can use the faster 28-bit mode, or a 512Kb request to the same low block
number, which can only be made using 48-bit LBA, which is the best to use?

I originally thought that we might only be honouring 512Kb requests for blocks
over the 28-bit limit, which Jens corrected me on, but maybe we *should* only
do 512Kb requests on high block number, where we have to use 48-bit anyway.

John.