Date: Wed, 14 May 2003 09:54:07 +0200
From: Jens Axboe <axboe@suse.de>
To: "Justin T. Gibbs" <gibbs@scsiguy.com>,
       William Lee Irwin III <wli@holomorphy.com>,
       linux-kernel@vger.kernel.org, axel@pearbough.net
Subject: Re: drivers/scsi/aic7xxx/aic7xxx_osm.c: warning is error
Message-ID: <20030514075407.GA10685@suse.de>
References: <20030514004009.GA20914@neon.pearbough.net> <20030514031826.GB29926@holomorphy.com> <493702704.1052892304@aslan.scsiguy.com> <20030514073700.GA3151@beaverton.ibm.com>
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On Wed, May 14 2003, Mike Anderson wrote:
> Justin T. Gibbs [gibbs@scsiguy.com] wrote:
> > Comments have indicated since the 2.4.X days that Linux will never allocate
> > segments that cross a 4GB boundary.  If this is truely enforced, then this
> > code can just be removed.  It was only added out of paranoia (hence the
> > printf) while adding high address support to the driver.
> 
> Jens can give the more complete answer on enforcement, and also correct
> any mis-statements I made.

This property can be toggled with blk_queue_segment_boundary, and we do
default to setting a 4GB boundary mask. So you can be sure that a
request will never straddle a 4GB boundary.

> Base on the queue values below the aic7xxx driver should see the
> following characteristics on IO. The IO should be for no more than 8k
> made up of no more than 128 sg entries with no segment crossing the
> seg_boundary_mask.

I suppose you mean for no more than 8k sectors, ie 4MiB of data.

> Adaptec AIC7xxx driver version: 6.2.33
> scsi_alloc_queue: queue for aic7xxx
> bounce_pfn: 0xfffff
> bounce_gfp: 0x10 (GFP_NOIO)
> queue_flags: 0x1 (QUEUE_FLAG_QUEUED)
> max_sectors: 0x2000 (8192)
> max_phys_segments: 0x80 (128)
> max_hw_segments: 0x80 (128)
> hardsect_size: 0x200 (512)
> max_segment_size: 0x10000 (65536)
> seg_boundary_mask: 0xffffffff

that is the key here.

> dma_alignment: 0x1ff (511)

So to recap, aic7xxx will never see a request that exceeds one of the
above values. Total request size will always be equal to or below 4MiB,
less than or equal to 128 segments, and will never cross a 4GB memory
boundary. Memory above pfn 0xfffff (4GB) will be bounced, but this could
be because that's just the amount of memory the box has you dumped this
info from.

-- 
Jens Axboe

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