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Message-ID: <58cb370e050302020950da588a@mail.gmail.com>
Date: Wed, 2 Mar 2005 11:09:02 +0100
From: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Reply-To: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
To: Jeff Garzik <jgarzik@pobox.com>
Subject: Re: [patch ide-dev 8/9] make ide_task_ioctl() use REQ_DRIVE_TASKFILE
Cc: Tejun Heo <htejun@gmail.com>, linux-ide@vger.kernel.org,
       linux-kernel@vger.kernel.org
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On Wed, 02 Mar 2005 01:08:56 -0500, Jeff Garzik <jgarzik@pobox.com> wrote:
> 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

certainly it is not _zero_ ;-)

I agree that it is negligible compared to the cost of I/O

> 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.

agreed

Tejun, one-path approach for IDE driver is fine with me
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