linux-kernel - Re: should struct device.dma

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Date:	Tue, 2 Nov 2010 14:45:11 +0100
From:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
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?

Hello,

On Tue, Nov 02, 2010 at 10:03:32PM +0900, FUJITA Tomonori wrote:
> On Tue, 2 Nov 2010 11:41:04 +0100
> Uwe Kleine-König <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.
After we finished the above it's quite easy to move everything into
dma_parms.  (At least if it's not a pointer, that then again needs an
additional allocation.)
 
> 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.
I'm changing the way imx (ARCH=arm) registers its devices.  Currently
we have in arch/arm/mach-imx/devices.c:

	static u64 imx1_camera_dmamask = DMA_BIT_MASK(32);

	struct platform_device imx1_camera_device = {
		...
		.dev = {
			.dma_mask = &imx1_camera_dmamask,
			.coherent_dma_mask = DMA_BIT_MASK(32),
		},
		...
	}

and I want to make registration dynamic (e.g. using
platform_device_register_resndata, see
arch/arm/plat-mxc/devices/platform-imx-i2c.c for an example).

Currently I have a function imx_add_platform_device (that does the same
as platform_device_register_resndata[1]) and now I want to register a
device that needs .dma_mask set, so I added something like:

	if (dmamask) {
                /*
                 * This memory isn't freed when the device is put,
                 * I don't have a nice idea for that though.  Conceptually
                 * dma_mask in struct device should not be a pointer.
                 * See http://thread.gmane.org/gmane.linux.kernel.pci/9081
                 */
                pdev->dev.dma_mask =
                        kmalloc(sizeof(*pdev->dev.dma_mask), GFP_KERNEL);
                if (!pdev->dev.dma_mask)
                        goto err;

                *pdev->dev.dma_mask = dmamask;
                pdev->dev.coherent_dma_mask = dmamask;
	}

So there is no private struct I could extend easily.  And I prefer
cleaning up global oddities instead of being the x-th person to work
around it.

Best regards
Uwe

[1] platform_device_register_resndata is newer and the result of making
imx_add_platform_device global :-)

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