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Subject: Re: [PATCH]: [MPC5200] (v2) Add ATA DMA support
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Tim Yamin <plasm@roo.me.uk>
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Date: Fri, 04 Jul 2008 09:47:03 +1000
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On Thu, 2008-07-03 at 16:35 +0100, Tim Yamin wrote:
> >> +static void
> >> +mpc52xx_bmdma_start(struct ata_queued_cmd *qc)
> >> +{
> >> +     struct ata_port *ap = qc->ap;
> >> +     struct mpc52xx_ata_priv *priv = ap->host->private_data;
> >> +
> >> +     /* LocalBus lock */
> >> +     while (test_and_set_bit(0, &pata_mpc52xx_ata_dma_lock) != 0)
> >> +             ;
> >
> > Need to be able to bail on timeout.
> 
> A deadlock can't occur within the PATA driver because you won't have
> two DMA requests happening at once, so there is no point in adding a
> timeout. And even if you do have a timeout, you'd have to drop the I/O
> request somehow, so it's not really a good idea. If anything else
> needs to touch the DMA lock, it should do so in a sensible fashion...

But why a hand-coded lock with bitops ? Why not a real spinlock then ?
The later is more efficient anyway.

Ben.