linux-kernel - DMA/PCIe driver: pci_map_sg not merging chunks in the scatterlist?

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Message-ID: <56EB2B1A.8050006@cyconix.com>
Date:	Thu, 17 Mar 2016 22:09:30 +0000
From:	Evan Lavelle <sa212+lkml@...onix.com>
To:	LKML <linux-kernel@...r.kernel.org>
Subject: DMA/PCIe driver: pci_map_sg not merging chunks in the scatterlist?

Everything I've read makes me think that pci_map_sg will merge 
consecutive physical pages into one scatterlist entry, if possible.

However, this isn't happening on a driver I'm currently writing. The 
returned scatterlist contains one entry per page, with the physical 
addresses incrementing by 4K per page.

Is this expected behaviour, or do I have to do something to enable 
merging? This wouldn't be a major problem, except that the PCIe card I'm 
working on only has enough local storage for about 300 scatterlist 
entries, which means that DMA ops are limited to a little over 1Mbyte at 
a time, which isn't much use.

I'm on x86_64, RHEL6.7, IOMMU not enabled, and I don't need to support 
anything prior to 2.6.32. The PCIe card has 32-bit addressing. uname shows:

2.6.32-431.el6.x86_64 #1 SMP x86_64 x86_64 x86_64

Thanks.