Re: [PATCH] Configure IDE probe delays
From: Mark Lord
Date: Wed Sep 01 2004 - 08:28:55 EST
Bartlomiej Zolnierkiewicz wrote:
What determines whether 48 bit addressing will be used then?
Availability of 48-bit addressing feature set and host capabilities
(some don't support LBA48 when DMA is used etc.).
I haven't examined the "released" IDE drivers in some time,
but one optimisation that can save a LOT of CPU usage
is for the driver to only use LBA48 *when necessary*,
and use LBA28 I/O otherwise.
Each access to an IDE register typically chews up 600+ns,
or the equivalent of a couple thousand instruction executions
on a modern core. Avoiding LBA48 when it's not needed will
save four such accesses per I/O, or about 2.5us.
LBA48 is only needed when (1) the sector count is greater than 256,
and/or (2) the ending sector number >= (1<<28).
I regularly include this optimisation in the drivers I have been
working on since LBA48 first appeared.
Cheers
--
Mark Lord
(hdparm keeper & the original "Linux IDE Guy")
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