Date: Fri, 13 Oct 2006 10:04:35 +0200
From: Jens Axboe <jens.axboe@oracle.com>
To: Robert Hancock <hancockr@shaw.ca>
Cc: Allen Martin <AMartin@nvidia.com>, Jeff Garzik <jeff@garzik.org>,
       linux-kernel <linux-kernel@vger.kernel.org>, linux-ide@vger.kernel.org,
       prakash@punnoor.de
Subject: Re: [RFC PATCH] nForce4 ADMA with NCQ: It's aliiiive..
Message-ID: <20061013080434.GE6515@kernel.dk>
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On Thu, Oct 12 2006, Robert Hancock wrote:
> Jens Axboe wrote:
> >On Tue, Oct 10 2006, Robert Hancock wrote:
> >>Allen Martin wrote:
> >>>>But I really don't think that is necessary.  I will take a 
> >>>>look at docs and see how things match up, when I am much more 
> >>>>awake.  Most likely you need to be using another set of 
> >>>>registers, and be all MMIO, all the time.
> >>>You shouldn't be touching BM registers when ADMA is enabled, it can
> >>>cause bad things to happen.
> >>>
> >>>You should be using BM registers when doing ATAPI protocol though, as it
> >>>doesn't work through ADMA.  So I wouldn't say you should be using MMIO
> >>>all the time.
> >>>
> >>>-Allen
> >>OK, I've updated the code to take this into account, an updated patch is 
> >>attached. However, this does raise an issue. If we have to fall back to 
> >>legacy mode to do ATAPI DMA, this means that we can't do 64-bit DMA for 
> >>such transfers. Since by the time the driver gets a request the SGs have 
> >>already been created based on the set DMA mask, the only way I can see 
> >>to handle this is to either allow ATAPI DMA or 64-bit DMA, not both. 
> >>I've chosen to default to 64-bit DMA in this version, but there is a 
> >>module parameter which allows overriding this if you care more about 
> >>using ATAPI devices than efficiency with over 4GB of RAM. I'm open to 
> >>suggestions on a better way to handle this..
> >
> >Should be easily fixable - in general, set 64-bit dma mask. Then when
> >you detect an atapi device, lower the dma mask settings to 32-bit dma
> >for that device only. So the pci device in question gets a full 64-bit
> >dma mask, the attached scsi devices can have lower masks if necessary.
> >I'd suggest doing this off slave config.
> >
> 
> I think that should be feasible.. However, one problem is that 
> slave_config only has access to the struct scsi_device and the 
> ata_scsi_find_dev function to turn that into a struct ata_device isn't 
> exported, which it would need to be in order to do anything useful 
> inside the driver for slave_config. We could export it, or I suppose the 
> other place we could do this handling would be postreset, as at that 
> point we should know what kind of device is attached.. any comments?

What else do you need? From the scsi device, call the
blk_queue_bounce_limit() on the queue tored in there. That shold be it.

> Also, how is the driver supposed to be setting the DMA mask for the SCSI 
> device? I suppose blk_queue_bounce_limit would work, but it seems a bit 
> odd to use block layer calls at the libata driver level.

That is the right thing to do.

> I also noticed that I'm still using the default 64KB libata dma_boundary 
> value, this should be 4GB for ADMA mode (but fixed up back to the 
> default if an ATAPI device is connected, same as with the DMA mask).

You can set that from the same location as the bounce limit.

-- 
Jens Axboe

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