Re: [PATCH 4/5] dma-direct: implement complete bus_dma_mask handling

On Thu, Sep 27, 2018 at 03:58:04PM +0100, Robin Murphy wrote:

quoted

  }
  #endif /* !CONFIG_ARCH_HAS_PHYS_TO_DMA */
  diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
index 3c404e33d946..64466b7ef67b 100644

--- a/kernel/dma/direct.c
+++ b/kernel/dma/direct.c

@@ -43,10 +43,11 @@ check_addr(struct device *dev, dma_addr_t dma_addr, size_t size,
  			return false;
  		}
  -		if (*dev->dma_mask >= DMA_BIT_MASK(32)) {
+		if (*dev->dma_mask >= DMA_BIT_MASK(32) || dev->bus_dma_mask) {

Hmm... say *dev->dma_mask is 31 bits and dev->bus_dma_mask is 40 bits due 
to a global DT property, we'll now scream where we didn't before even 
though the bus mask is almost certainly irrelevant - is that desirable?

This is just the reporting in the error case - we'll only hit this
IFF dma_capable already returned false.  But if you don't want a message
here we can probably drop this hunk.

quoted

@@ -65,12 +66,18 @@ u64 dma_direct_get_required_mask(struct device *dev)
  {
  	u64 max_dma = phys_to_dma_direct(dev, (max_pfn - 1) << PAGE_SHIFT);
  +	if (dev->bus_dma_mask && dev->bus_dma_mask < max_dma)
+		max_dma = dev->bus_dma_mask;

Again, I think we could just do another min_not_zero() here. A device wired 
to address only one single page of RAM isn't a realistic prospect (and we 
could just flip the -1 and the shift in the max_dma calculation if we 
*really* wanted to support such things).

This just seemed more readable to me than min_not_zero, but if others
prefer min_not_zero I can switch.