[PATCH 2/2] swiotlb: Add swiotlb=nobounce debug option

[Geert Uytterhoeven <geert+renesas@glider.be>]

On architectures like arm64, swiotlb is tied intimately to the core
architecture DMA support. In addition, ZONE_DMA cannot be disabled.

To aid debugging and catch devices not supporting DMA to memory outside
the 32-bit address space, add a kernel command line option
"swiotlb=nobounce", which disables the use of bounce buffers.
If specified, trying to map memory that cannot be used with DMA will
fail, and a warning will be printed (rate-limited).

Note that io_tlb_nslabs is set to 1, which is the minimal supported
value.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
---
 Documentation/kernel-parameters.txt |  3 ++-
 lib/swiotlb.c                       | 19 +++++++++++++++++--
 2 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 37babf91f2cb6de2..38556cdceabaf087 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -3998,10 +3998,11 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			it if 0 is given (See Documentation/cgroup-v1/memory.txt)
 
 	swiotlb=	[ARM,IA-64,PPC,MIPS,X86]
-			Format: { <int> | force }
+			Format: { <int> | force | nobounce }
 			<int> -- Number of I/O TLB slabs
 			force -- force using of bounce buffers even if they
 			         wouldn't be automatically used by the kernel
+			nobounce -- Never use bounce buffers (for debugging)
 
 	switches=	[HW,M68k]
 
diff --git a/lib/swiotlb.c b/lib/swiotlb.c
index 6ce764410ae475cc..4550e6b516c2a4c0 100644
--- a/lib/swiotlb.c
+++ b/lib/swiotlb.c
@@ -54,6 +54,7 @@
 #define IO_TLB_MIN_SLABS ((1<<20) >> IO_TLB_SHIFT)
 
 int swiotlb_force;
+static int swiotlb_nobounce;
 
 /*
  * Used to do a quick range check in swiotlb_tbl_unmap_single and
@@ -106,8 +107,12 @@
 	}
 	if (*str == ',')
 		++str;
-	if (!strcmp(str, "force"))
+	if (!strcmp(str, "force")) {
 		swiotlb_force = 1;
+	} else if (!strcmp(str, "nobounce")) {
+		swiotlb_nobounce = 1;
+		io_tlb_nslabs = 1;
+	}
 
 	return 0;
 }
@@ -541,8 +546,15 @@ phys_addr_t swiotlb_tbl_map_single(struct device *hwdev,
 map_single(struct device *hwdev, phys_addr_t phys, size_t size,
 	   enum dma_data_direction dir)
 {
-	dma_addr_t start_dma_addr = phys_to_dma(hwdev, io_tlb_start);
+	dma_addr_t start_dma_addr;
+
+	if (swiotlb_nobounce) {
+		dev_warn_ratelimited(hwdev, "Cannot do DMA to address %pa\n",
+				     &phys);
+		return SWIOTLB_MAP_ERROR;
+	}
 
+	start_dma_addr = phys_to_dma(hwdev, io_tlb_start);
 	return swiotlb_tbl_map_single(hwdev, start_dma_addr, phys, size, dir);
 }
 
@@ -707,6 +719,9 @@ void swiotlb_tbl_sync_single(struct device *hwdev, phys_addr_t tlb_addr,
 swiotlb_full(struct device *dev, size_t size, enum dma_data_direction dir,
 	     int do_panic)
 {
+	if (swiotlb_nobounce)
+		return;
+
 	/*
 	 * Ran out of IOMMU space for this operation. This is very bad.
 	 * Unfortunately the drivers cannot handle this operation properly.
-- 
1.9.1