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SubjectRe: [RFC PATCH] m68k: set dma and coherent masks for Macintosh SONIC based ethernet
On Mon, 28 May 2018, Geert Uytterhoeven wrote:

> Do we have a consensus on the way forward?

My prefered solution remains the two driver patches that I originally
submitted, which you objected to:

So there is no consensus yet.

To my mind, the existing code is pretty clear: drivers should set the
(default) mask. See also,

$ egrep -r -A1 "coherent_dma_mask.*expected" */
drivers/of/device.c: * Set default coherent_dma_mask to 32 bit. Drivers are expected to
drivers/of/device.c- * setup the correct supported mask.
drivers/acpi/arm64/iort.c: * Set default coherent_dma_mask to 32 bit. Drivers are expected to
drivers/acpi/arm64/iort.c- * setup the correct supported mask.

And drivers/usb/dwc2/platform.c:

* Use reasonable defaults so platforms don't have to provide these.
if (!dev->dev.dma_mask)
dev->dev.dma_mask = &dev->dev.coherent_dma_mask;
retval = dma_set_coherent_mask(&dev->dev, DMA_BIT_MASK(32));


$ egrep -wlr "dma_set_mask_and_coherent|dma_set_coherent_mask|dma_coerce_mask_and_coherent" drivers/ | wc -l

I suppose that a driver should avoid lengthening an existing device mask.

Since an arch gets to apply limits in the dma ops it implements, why would
arch code also have to set a limit in the form of default platform device
masks? Powerpc seems to be the only arch that does this.

The same line of reasoning suggests that the problematic WARN_ON should
not appear in include/linux/ in the first place. If it is needed by
certain architectures, it should be in arch/x.

I would send a patch to revert commit 205e1b7f51e4 ("dma-mapping: warn
when there is no coherent_dma_mask") if I thought that arch code was not
somehow relying on it. But I'll leave that up to Chrisoph.


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