Re: [PATCH 5/5] dma-direct: always allow dma mask <= physiscal memory size

On Thu, Sep 27, 2018 at 11:50:14AM +1000, Benjamin Herrenschmidt wrote:

quoted

-	 * to be able to satisfy them - either by not supporting more physical
-	 * memory, or by providing a ZONE_DMA32.  If neither is the case, the
-	 * architecture needs to use an IOMMU instead of the direct mapping.
-	 */
-	if (mask < phys_to_dma(dev, DMA_BIT_MASK(32)))
+	u64 min_mask;
+
+	if (IS_ENABLED(CONFIG_ZONE_DMA))
+		min_mask = DMA_BIT_MASK(ARCH_ZONE_DMA_BITS);
+	else
+		min_mask = min_t(u64, DMA_BIT_MASK(32),
+				 (max_pfn - 1) << PAGE_SHIFT);
+
+	if (mask >= phys_to_dma(dev, min_mask))
 		return 0;

nitpick ... to be completely "correct", I would have written

	if (IS_ENABLED(CONFIG_ZONE_DMA))
		min_mask = DMA_BIT_MASK(ARCH_ZONE_DMA_BITS);
	else
		min_mask = DMA_BIT_MASK(32);

	min_mask = min_t(u64, min_mask,	(max_pfn - 1) << PAGE_SHIFT);

In "theory" it's also ok to have a mask < ZONE_DMA_BITS as long as it's
big enough to fit all memory :-)

Yeah, we could do that.