ARM: dma-api: fix max_pfn off-by-one error in __dma_supported()

From: Chen-Yu Tsai <[email protected]>

max_pfn, as set in arch/arm/mm/init.c:

    static void __init find_limits(unsigned long *min,
				   unsigned long *max_low,
				   unsigned long *max_high)
    {
	    *max_low = PFN_DOWN(memblock_get_current_limit());
	    *min = PFN_UP(memblock_start_of_DRAM());
	    *max_high = PFN_DOWN(memblock_end_of_DRAM());
    }

with memblock_end_of_DRAM() pointing to the next byte after DRAM. As such, max_pfn points to the PFN after the end of DRAM.

Thus when using max_pfn to check DMA masks, we should subtract one when checking DMA ranges against it.

Commit 8bf1268f48ad ("ARM: dma-api: fix off-by-one error in __dma_supported()") fixed the same issue, but missed this spot.

This issue was found while working on the sun4i-csi v4l2 driver on the Allwinner R40 SoC. On Allwinner SoCs, DRAM is offset at 0x40000000, and we are starting to use of_dma_configure() with the "dma-ranges" property in the device tree to have the DMA API handle the offset.

In this particular instance, dma-ranges was set to the same range as the actual available (2 GiB) DRAM. The following error appeared when the driver attempted to allocate a buffer:

    sun4i-csi 1c09000.csi: Coherent DMA mask 0x7fffffff (pfn 0x40000-0xc0000)
    covers a smaller range of system memory than the DMA zone pfn 0x0-0xc0001
    sun4i-csi 1c09000.csi: dma_alloc_coherent of size 307200 failed

Fixing the off-by-one error makes things work.

---

It's been close to two weeks since I sent this. I wasn't sure if I reached the right maintainers, so I'm resending this to more people.

Changes since v1:

- correct max_pfn offset in the correct place.

---
 arch/arm/mm/dma-mapping.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index e822af0d9219..9414d72f664b 100644
@@ -221,7 +221,7 @@ EXPORT_SYMBOL(arm_coherent_dma_ops);
 
 static int __dma_supported(struct device *dev, u64 mask, bool warn)
 {
-	unsigned long max_dma_pfn = min(max_pfn, arm_dma_pfn_limit);
+	unsigned long max_dma_pfn = min(max_pfn - 1, arm_dma_pfn_limit);
 
 	/*
 	 * Translate the device's DMA mask to a PFN limit.  This

2.24.1

Looks good: