On Wednesday, June 29, 2016 10:10:59 AM CEST Timur Tabi wrote:
> 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.
>
Usually drivers try 64-bit mask and 32-bit masks, and the 32 bit
mask is practically guaranteed to succeed.
Platforms will also allow allow the driver to set a mask that
is larger than what the bus supports, as long as all RAM is
reachable by the bus.
Arnd
