Subject: Re: [PATCH] [v6] net: emac: emac gigabit ethernet controller driver

From: Timur Tabi <[email-masked]>
Date: Wed, 29 Jun 2016 10:10:59 -0500

Arnd Bergmann wrote:
> That's also not how it works: each device starts out with a 32-bit mask,
> because that's what historically all PCI devices can do. If a device
> is 64-bit DMA capable, it can extend the mask by passing DMA_BIT_MASK(64)
> (or whatever it can support), and the platform code checks if that's
> possible.

So if it's not possible, then dma_set_mask returns an error, and the 
driver should try a smaller mask?  Doesn't that mean that every driver 
for a 64-bit device should do this:

	for (i = 64; i >=32; i--) {
		ret = dma_set_mask_and_coherent(dev, DMA_BIT_MASK(i));
		if (!ret)
			break;
	}

	if (ret)
		return ret;

Sure, this is overkill, but it seems to me that the driver does not 
really know what mask is actually valid, so it has to find the largest 
mask that works.

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