Re: [PATCH 1/1] libata: use polling pio for identify device

[Jeff Garzik <jgarzik@pobox.com>]

Albert Lee wrote:
> Use polling pio for identify device.
> This can save some time to detect the phantom device 1 during boot.
> 
> Signed-off-by: Albert Lee <albertcc@tw.ibm.com>
> ---
> For the phantom slave device (found with some IBM DVD-RAM drive and
> Intel ICHx chip), irq-pio has to wait 30 seconds timeout to detect them.
> 
> Use polling pio can detect the phantom slave quicker. (Verified ok by Tejun.)
> 
> Patch again the current upstream branch (b74ba22f030eb7ab88f7d8954ad18ecc0ac5ce3c).
> For your review, thanks.
> 
> --- upstream/drivers/scsi/libata-core.c	2006-05-22 10:04:12.000000000 +0800
> +++ polling_identify/drivers/scsi/libata-core.c	2006-05-22 11:07:02.000000000 +0800
> @@ -1196,6 +1196,8 @@ static int ata_dev_read_id(struct ata_de
>  	}
>  
>  	tf.protocol = ATA_PROT_PIO;
> +	/* Use polling for early detection of phantom dev 1 */
> +	tf.flags |= ATA_TFLAG_POLLING;

Sorry I didn't jump into the thread earlier.  I'm trying out this new 
no-work-on-weekends thing, which is a change from my usual 
working-7-days-a-week thing.  :)

NAK.  The above change is going backwards from the direction we took by 
merging irq-pio, and it reintroduces problems that irq-pio solved. 
Additionally, for irq problems unrelated to libata (we will continue to 
see such problems FOREVER), this simply shifts the I-notice-a-problem 
moment from IDENTIFY DEVICE to SET FEATURES, or perhaps the first READ 
or WRITE.

SATA is inherently event driven (read: irq-driven), and PATA has 
traditionally been irq-driven in the Linux drivers/ide code.  Using 
polling for solving this specific problem just isn't enough 
justification.  We should be able to fix this elsewhere.  Take a look at 
the drivers/ide "bang at the door w/ IDENTIFY" probing method.  We took 
a different approach (device signature, etc.) but its still a useful 
working case to examine.

As an aside, phantom devices make me think that somewhere we might be 
getting device-select wrong, or accidentally skipping it entirely.

	Jeff