On Wednesday 16 March 2016 11:26:48 Adrian Hunter wrote:
> On 16/03/16 11:07, Arnd Bergmann wrote:
> > On Wednesday 16 March 2016 10:43:33 Adrian Hunter wrote:
> >>> +
> >>> + /* 32-bit mask as default & fallback */
> >>> + if (ret) {
> >>> + ret = dma_set_mask_and_coherent(dev, DMA_BIT_MASK(32));
> >>
> >> What happens if device enumeration (e.g. of_dma_configure) has already set
> >> a
> >> more restrictive DMA mask?
> >>
> >>
> >
> > In this case, dma_set_mask_and_coherent() is supposed to check the
> > bus properties settings again and fail dma_set_mask_and_coherent().
>
> So the logic this patch introduces will disable DMA in that case. Would it
> be better just to leave the DMA mask alone (as it does now for most sdhci
> drivers) in the 32-bit case?
It depends to some degree on the specific capabilities of the system.
Basically when the driver asks for a 32-bit mask, we have to check if that
is actually possible, and there are a couple of possible outcomes:
- If the bus is less than 32-bit wide but the RAM is small enough to
to fit within the addressable range of the bus, the
dma_set_mask_and_coherent() should succeed
- If the RAM is larger than what the bus can address, but swiotlb
is configured and the swiotlb bounce buffer is addressable by
the bus, dma_set_mask_and_coherent() should also succeed
- If there is no swiotlb and there is RAM that fits into the 32-bit
mask but that is not addressable by the bus, the
dma_set_mask_and_coherent() should fail, and the driver should not
use DMA.
- Similarly, if swiotlb is enabled, but its bounce buffer is not
reachable by the bus, the call needs to fail and the driver must
not use DMA.
Arnd
