linux-kernel - Re: should struct device.dma

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Message-Id: <20100701103544A.fujita.tomonori@lab.ntt.co.jp>
Date:	Thu, 1 Jul 2010 10:35:58 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	u.kleine-koenig@...gutronix.de
Cc:	linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
	James.Bottomley@...senPartnership.com, akpm@...ux-foundation.org,
	julia@...u.dk
Subject: Re: should struct device.dma_mask still be a pointer?

> 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.

Yeah, that's a strange design. As the commit log said, it's due to the
historical reason. We invented the pci dma model first then moved to
the generic dma model.


> 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.

The popular trick to avoid allocating the extra memory for that is:

device.dma_mask = &device.coherent_dma_mask;


> 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
> )

That's exactly the perturbation that the commit log refers to.

We need to modify all the struct device at a time. We could, however,
I don't think that it's worth doing. Little gain.


> Additionally this could be done for struct device.dma_parms.

Yeah, we should have all the dma parameters in dma_parms.
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