linux-kernel - Re: should struct device.dma

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Message-Id: <20101102215741T.fujita.tomonori@lab.ntt.co.jp>
Date:	Tue, 2 Nov 2010 22:03:32 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	u.kleine-koenig@...gutronix.de
Cc:	fujita.tomonori@....ntt.co.jp, 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?

On Tue, 2 Nov 2010 11:41:04 +0100
Uwe Kleine-K.$(D+S.(Bnig <u.kleine-koenig@...gutronix.de> wrote:

> > > 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;
> Does this work in general?  Or are there any corner cases that make this
> trick fail?

It doesn't work if the coherent dma mask of a device is not same as
the dma mask of the device.


> > > 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.
> That applies to dma_mask and coherent_dma_mask, too, I assume?

Yes.


> Instead of converting all at a time, what about adding an
> u64 dma_mask_real to struct device (assuming coherent_dma_mask cannot be
> used for it) and use this if dma_mask is NULL.  For me it would make
> live a bit easier, because for some time I could just use
> device.dma_mask = &device.dma_mask_real instead of allocating an u64
> dynamically.  Together with some accessor functions and deprecating
> direct access to the dma related members of struct device the drivers
> and architectures could be converted one after another.  The final step
> to get rid of the pointers would be small then.

But after we finish the above, after all, we still have dma_mask in
device strcuture. As I said before, we should move dma stuff to
dma_params.

I'm not sure why this really troubles you. Can you give me a pointer
to what you have been working on? You have been working on non pci
device, right? Why can't you do like pci_dev, embedding
device_dma_parameters to your own device structure.
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