linux-kernel - Re: DMA mapping on SCSI device?

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Date:	Tue, 29 Jan 2008 14:09:52 -0800 (PST)
From:	Luben Tuikov <ltuikov@...oo.com>
To:	linux-arch@...r.kernel.org, ide <linux-ide@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-scsi@...r.kernel.org, Robert Hancock <hancockr@...w.ca>
Subject: Re: DMA mapping on SCSI device?

--- On Mon, 1/28/08, Robert Hancock <hancockr@...w.ca> wrote:
> The trick is that if an ATAPI device is connected, we (as
> far as I'm 
> aware) can't use ADMA mode, so we have to switch that
> port into legacy 
> mode.

Can you double check this with the HW architect of the
HW DMA engine of the ASIC?

> This means it's only capable of 32-bit DMA.
> However the other port 
> on the controller may be connected to a hard drive and
> therefore still 
> capable of 64-bit DMA.

If this is indeed the case as you've presented it here,
it sounds like a HW shortcoming.  I cannot see how the device
type (or protocol) dictate how the DMA engine operates.
They live in two different domains.

> The ideal solution would be to do mapping against a
> different struct 
> device for each port, so that we could maintain the proper
> DMA mask for 
> each of them at all times. However I'm not sure if
> that's possible. The 
> thought of using the SCSI struct device for DMA mapping was
> brought up 
> at one point.. any thoughts on that?

The reason for this is that the object that a struct scsi_dev
represents has nothing to do with HW DMA engines.

It looks like your current solution is correct and
x86_64's blk_queue_bounce_limit needs work.

    Luben

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