linux-kernel - should struct device.dma

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Message-ID: <20100622105233.GA4755@pengutronix.de>
Date:	Tue, 22 Jun 2010 12:52:33 +0200
From:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
To:	linux-kernel@...r.kernel.org
Cc:	linux-pci@...r.kernel.org,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Julia Lawall <julia@...u.dk>
Subject: should struct device.dma_mask still be a pointer?

Hello,

IMHO it's strange that struct device.dma_mask is a pointer instead of a
plain u64.  The reason this was done back then is described in
8ab1bc19e974fdebe76c065fe444979c84ba2f74[1]:

	Attached is a patch which moves dma_mask into struct device and cleans up the
	scsi mid-layer to use it (instead of using struct pci_dev).  The advantage to
	doing this is probably most apparent on non-pci bus architectures where
	currently you have to construct a fake pci_dev just so you can get the bounce
	buffers to work correctly.

	The patch tries to perturb the minimum amount of code, so dma_mask in struct
	device is simply a pointer to the one in pci_dev.  However, it will make it
	easy for me now to add generic device to MCA without having to go the fake pci
	route.

As I work on such a non-pci bus architecture it's still ugly that this
is a pointer because I have to allocate extra memory for that.

Is there a reason not to get rid of struct pci_dev.dma_mask and use
struct pci_dev.dev.dma_mask instead?  (Well apart from the needed
effort of course.)

If not, the following would be needed:

	- remove struct pci.dma_mask
	- make struct device.dma_mask an u64 (instead of u64*)
	- substitue var.dma_mask by var.dev.dma_mask for all
	  struct pci_dev var
	- substitue var.dma_mask by &(var.dma_mask) for all
	  struct device var

and note that there are statically initialized struct device (and maybe
struct pci_dev?) that need fixing, too.  (e.g.
http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob;f=arch/arm/mach-mx2/devices.c;h=a0aeb8a4adc19ef419a0a045ad3b882131597106;hb=HEAD#l265
)

Additionally this could be done for struct device.dma_parms.

Julia: help!

Best regards
Uwe

[1] as this is pre-2.6.12 it's only available in tglx' histroy tree,
e.g.  http://git.kernel.org/?p=linux/kernel/git/tglx/history.git;a=commitdiff;h=8ab1bc19e974fdebe76c065fe444979c84ba2f74

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
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