linux-kernel - Re: Proposed changes for libata speed handling

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Date:	Mon, 15 Jan 2007 12:09:20 +0900
From:	Tejun Heo <htejun@...il.com>
To:	Alan <alan@...rguk.ukuu.org.uk>
CC:	linux-ide@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Proposed changes for libata speed handling

Alan wrote:
> O> Wouldn't it be better to have ->determine_xfer_mask() and
>> ->set_specific_mode() than having two somewhat overlapping callbacks?
>> Or is there some problem that can't be handled that way?
> 
> I'm not sure I follow what you are suggesting - can you explain further.
> 
> Right now ->set_mode does all the policy management with regards to
> picking the right modes which is sometimes done by the usual ATA rules
> and sometimes by card specific code.
> 
> ->set_specific_mode does no policy work but merely sets up a mode.
> 
> The default behaviour of ->set_mode() is the ATA mode selection by best
> mode available, and this function is normally not provided by a driver.
> 
> The default behaviour of ->set_specific_mode() is to verify the mode is
> valid then issue ->set_pio|dma_mode() (for both devices in case a timing
> change on both is triggered). This function is overridable because of
> things like IT821x where the IDE mode is imaginary.

What I was thinking about was something like the following.

* ops

	unsigned int (*determine_xfer_mask)(struct ata_device *dev);
	int (*set_specific_mode)(struct ata_device *dev,
				unsigned int xfer_mode);

* during init and EH

	if (init) {
		ap->xfer_mask &= ops->determine_xfer_mask(dev);
		DETERMINE best_mode;
	}

	if (ap->ehi.target_mode && valid)
		mode = ap->ehi.target_mode;
	else
		mode = best_mode;

	rc = ops->set_specific_mode(dev, mode);

* when the user issues SET_XFERMODE, in the issue path

	if (command is SET_XFERMODE) {
		if (mode is invalid)
			fail;
		ap->ehi.target_mode = user_specified_mode;
		ata_port_schedule_eh(ap);
		done;
	}

To sum up,

1. separate supported mode detection and mode programming such that we
can use the same programming path for both init and handling user-issued
SET_XFERMODE.

2. group all mode programming callbacks (->set_piomode, ->set_dmamode
and ->post_set_mode into ->set_specific_mode) into ->set_specific_mode
to allow more flexibility and replace ->set_mode.

3. make sure all xfer mode programming is done in EH.  this will ease
support for weird controllers (e.g. cross-port synchronization).

Thanks.

-- 
tejun
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