From mboxrd@z Thu Jan  1 00:00:00 1970
From: Mark Lord <mlord@pobox.com>
Subject: Re: [PATCH/RFC 4/4] irq-pio: add read/write multiple support
Date: Mon, 03 Oct 2005 09:35:02 -0400
Message-ID: <43413386.6000203@pobox.com>
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To: Albert Lee <albertcc@tw.ibm.com>
Cc: Jeff Garzik <jgarzik@pobox.com>, Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>, Linux IDE <linux-ide@vger.kernel.org>, Doug Maxey <dwm@maxeymade.com>, Tejun Heo <htejun@gmail.com>, Brett Russ <russb@emc.com>

Albert Lee wrote:
> Patch 4/4: add read/write multiple support
> 

One "fine point" when programming the R/W MULT commands
is to remember that each register access is very costly
in terms of processor time.

So whenever an access can be done with 28-bit LBA,
it should not use the EXT versions of the instructions,
even though the device may be LBA48 capable.

The various drivers I have written for Linux and other OSs
that deal with this always logic equivalen to this somewhere:

if (lba_sector < (1<<28) && sector_count <= 256) {
     use LBA28
} else if (device is lba48 capable) {
     use LBA48
} else {
     error
}

For SATA drives, this is unlikely to make any difference,
but for PATA it can reduce the CPU overhead of issuing
a command by 40% or so --> saves three PCI bus transactions,
which amounts to about 1-4us real-time, depending on the
exact hardware in question.

I'm really looking forward to having PIO in libata,
so that we can support compactflash cards and the like.
My Delkin 32-bit Cardbus Adapter driver (currently in 2.4.x IDE)
will be one of the first new pure PIO drivers in libata..

Cheers
-- 
Mark Lord
Real-Time Remedies Inc.
mlord@pobox.com