linux-kernel - Re: DMA masks

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Date:	Fri, 9 Aug 2013 14:56:14 +0000
From:	James Bottomley <jbottomley@...allels.com>
To:	Krzysztof Hałasa <khalasa@...p.pl>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"David S.Miller" <davem@...emloft.net>,
	Russell King <rmk+kernel@....linux.org.uk>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: DMA masks

On Fri, 2013-08-09 at 11:12 +0200, Krzysztof Hałasa wrote:
> Hi,
> 
> I'm trying to understand why the struct device contains a pointer to
> dma_mask and not the actual dma_mask:

It's an anachronism.  The original reason was when this was introduced,
struct pci_device was the only device that actually had one.  On PA-RISC
we needed to walk the device tree by generic device and set the DMA mask
on generic devices, which meant we either had to special case the PCI
device, rearchitect the whole of the PCI device mask setting to use the
generic device, or cheat and make the generic device dma mask a pointer
to the PCI one.  I took the latter option because it's simpler.

James