Message-ID: <42255878.7080908@pobox.com>
Date: Wed, 02 Mar 2005 01:08:56 -0500
From: Jeff Garzik <jgarzik@pobox.com>
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To: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
CC: Tejun Heo <htejun@gmail.com>, linux-ide@vger.kernel.org,
       linux-kernel@vger.kernel.org
Subject: Re: [patch ide-dev 8/9] make ide_task_ioctl() use REQ_DRIVE_TASKFILE
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Bartlomiej Zolnierkiewicz wrote:
> Yes but it seems that you've assumed that ioctl == flagged taskfile
> and fs/internal == normal taskfile which is _not_ what I aim for.
> 
> I want fully-flagged taskfile handling like flagged_taskfile() and "hot path"
> simpler taskfile handling like do_rw_taskfile() (at least for now - we can
> remove "hot path" later) where both can be used for fs/internal/ioctl requests
> (depending on the flags).


There is no effective difference in performance between

	writeb()
	writeb()
	writeb()
	writeb()

and

	if (bit 1)
		writeb()
	if (bit 2)
		writeb()
	if (bit 3)
		writeb()
	if (bit 4)
		writeb()

The cost of a repeated bit test on the same unsigned long is _zero_. 
It's already in L1 cache.  The I/Os are slow, and adding bit tests will 
not measurably decrease performance.  (this is the reason why I do not 
object to using ioread32() and iowrite32()...  it just adds a simple test)

Plus, it is better to have a single path for all taskfiles, to ensure 
that the path is well-tested.

libata's ->tf_load() and ->tf_read() hooks should be updated to use the 
more fine-grained flags that Tejun is proposing.

Note that on SATA, this is largely irrelevant.  The functions 
ata_tf_read() and ata_tf_load() should be updated for flagged taskfiles, 
because these will be used with PATA drivers.

The hooks implemented in individual SATA drivers will not be updated. 
The reason is that SATA transmits an entire copy of the taskfile to/from 
the device all at once, in the form of a Frame Information Structure 
(FIS) -- essentially a SATA packet.

	Jeff


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