linux-kernel - Re: [RFC v1 PATCH 1/2] of/pci: add of_pci_dma

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Message-ID: <21520995.3ITgVmVTeV@wuerfel>
Date:	Thu, 18 Dec 2014 23:29:25 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Murali Karicheri <m-karicheri2@...com>
Cc:	bhelgaas@...gle.com, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org, grant.likely@...aro.org,
	robh+dt@...nel.org, devicetree@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, will.deacon@....com
Subject: Re: [RFC v1 PATCH 1/2] of/pci: add of_pci_dma_configure() update dma configuration

On Thursday 18 December 2014 17:07:04 Murali Karicheri wrote:
> Add of_pci_dma_configure() to allow updating the dma configuration
> of the pci device using the configuration from the parent of
> the root bridge device.
> 
> Signed-off-by: Murali Karicheri <m-karicheri2@...com>

Much better!

There is one detail that we should get right from the start here,
which is currently wrong in the platform device code, but I have so
far not dared change it:

> +	/*
> +	 * Set default dma-mask to 32 bit. Drivers are expected to setup
> +	 * the correct supported dma_mask.
> +	 */
> +	dev->coherent_dma_mask = DMA_BIT_MASK(32);

The 32-bit mask is indeed correct as the default for almost all cases,
and we must not set it larger than DMA_BIT_MASK(32). There is one
corner case though, which happens on some shmobile machines that have
a 31-bit mask on the PCI host, and I think we should use that as the
default.

> +	/*
> +	 * Set it to coherent_dma_mask by default if the architecture
> +	 * code has not set it.
> +	 */
> +	if (!dev->dma_mask)
> +		dev->dma_mask = &dev->coherent_dma_mask;
> +
> +	ret = of_dma_get_range(parent_np, &dma_addr, &paddr, &size);
> +	if (ret < 0) {
> +		dma_addr = offset = 0;
> +		size = dev->coherent_dma_mask;

Can you check one thing here? I believe the size argument as returned
from of_dma_get_range is inclusive (e.g. 0x100000000), while the coherent
mask by definition is exlusive (e.g. 0xffffffff), so the size needs to
be adapted here. I haven't checked all the code here though, so I may
be wrong.

> +	} else {
> +		offset = PFN_DOWN(paddr - dma_addr);
> +		dev_dbg(dev, "dma_pfn_offset(%#08lx)\n", dev->dma_pfn_offset);
> +	}
> +	dev->dma_pfn_offset = offset;
> +
> +	coherent = of_dma_is_coherent(parent_np);
> +	dev_dbg(dev, "device is%sdma coherent\n",
> +		coherent ? " " : " not ");
> +
> +	arch_setup_dma_ops(dev, dma_addr, size, NULL, coherent);

Basically, I would use limit the size argument we pass into
arch_setup_dma_ops to the minimum of 'size' and 'dma_mask' here,
after converting into the same format. We should make sure we do the
same thing for platform_device as well, so it might be better to do
it inside of arch_setup_dma_ops instead.

	Arnd
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