linux-kernel - Re: sata_nv and dynamically changing DMA mask?

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Message-id: <4727C578.80900@shaw.ca>
Date:	Tue, 30 Oct 2007 17:59:52 -0600
From:	Robert Hancock <hancockr@...w.ca>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	ide <linux-ide@...r.kernel.org>
Subject: Re: sata_nv and dynamically changing DMA mask?

Alan Cox wrote:
> On Mon, 29 Oct 2007 22:17:40 -0600
> Robert Hancock <hancockr@...w.ca> wrote:
> 
>> In the sata_nv driver, when running in ADMA mode, we can do 64-bit DMA. 
>> However, when an ATAPI device like a DVD drive is connected, we can't 
>> use ADMA mode, and so we have to abide by the restrictions of a normal 
>> SFF ATA controller and can only do 32-bit DMA. We detect this and try to 
>> set the blk_queue_bounce_limit, blk_queue_segment_boundary and 
>> blk_queue_max_hw_segments to the values corresponding to a normal SFF 
>> controller.
> 
> What about the DMA padding buffer from nv_adma_port_start and internal
> buffers for commands like request sense that don't come via the request
> queue directly.

Indeed we do call ata_port_start from nv_adma_port_start, which calls 
dmam_alloc_coherent to allocate the SFF PRD table. Since the DMA mask is 
64-bit, this could indeed be allocated above 4GB which would be bad.

I suppose what we could do is just not call ata_port_start there, but 
move it into nv_adma_slave_config and call it when going into non-ADMA 
mode. We'd have to drop the DMA mask down to 32-bit first as well as 
setting blk_queue_bounce_limit though, which is one of my questions, is 
this OK to do?

> Also it seems nv_adma_use_reg_mode() can decide to send other commands
> via the non ADMA interface even for ATA devices. Are we 100% certain it
> never decides to let through a command with DMA via the register
> interface in this case - what do you see if you instrument the function ?

The only cases where that could happen are for polling DMA commands 
(which I presume we never do) or where result taskfile is requested. The 
latter could be a problem for ATA passthrough commands using DMA, I 
suppose.. Question is what we can do about it.. We have to switch out of 
ADMA mode to read a result taskfile. I guess that's not really a problem 
unless somebody starts issuing NCQ commands via ATA pass-through. Do we 
allow that?
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