swiotlb: Add swiotlb=noforce debug option

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=noforce", which disables the use of bounce buffers.
If specified, trying to map memory that cannot be used with DMA will
fail, and a rate-limited warning will be printed.

Note that io_tlb_nslabs is set to 1, which is the minimal supported
value.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

Documentation/admin-guide/kernel-parameters.txt

@@ -3807,10 +3807,11 @@
 			it if 0 is given (See Documentation/cgroup-v1/memory.txt)
 
 	swiotlb=	[ARM,IA-64,PPC,MIPS,X86]
-			Format: { <int> | force }
+			Format: { <int> | force | noforce }
 			<int> -- Number of I/O TLB slabs
 			force -- force using of bounce buffers even if they
 			         wouldn't be automatically used by the kernel
+			noforce -- Never use bounce buffers (for debugging)
 
 	switches=	[HW,M68k]
 

include/linux/swiotlb.h

@@ -12,6 +12,7 @@ struct scatterlist;
 enum swiotlb_force {
 	SWIOTLB_NORMAL,		/* Default - depending on HW DMA mask etc. */
 	SWIOTLB_FORCE,		/* swiotlb=force */
+	SWIOTLB_NO_FORCE,	/* swiotlb=noforce */
 };
 
 extern enum swiotlb_force swiotlb_force;

include/trace/events/swiotlb.h

@@ -39,7 +39,8 @@ TRACE_EVENT(swiotlb_bounced,
 		__entry->size,
 		__print_symbolic(__entry->swiotlb_force,
 			{ SWIOTLB_NORMAL,	"NORMAL" },
-			{ SWIOTLB_FORCE,	"FORCE" }))
+			{ SWIOTLB_FORCE,	"FORCE" },
+			{ SWIOTLB_NO_FORCE,	"NO_FORCE" }))
 );
 
 #endif /*  _TRACE_SWIOTLB_H */

lib/swiotlb.c

@@ -106,8 +106,12 @@ setup_io_tlb_npages(char *str)
 	}
 	if (*str == ',')
 		++str;
-	if (!strcmp(str, "force"))
+	if (!strcmp(str, "force")) {
 		swiotlb_force = SWIOTLB_FORCE;
+	} else if (!strcmp(str, "noforce")) {
+		swiotlb_force = SWIOTLB_NO_FORCE;
+		io_tlb_nslabs = 1;
+	}
 
 	return 0;
 }
@@ -543,8 +547,15 @@ static phys_addr_t
 map_single(struct device *hwdev, phys_addr_t phys, size_t size,
 	   enum dma_data_direction dir, unsigned long attrs)
 {
-	dma_addr_t start_dma_addr = phys_to_dma(hwdev, io_tlb_start);
+	dma_addr_t start_dma_addr;
+
+	if (swiotlb_force == SWIOTLB_NO_FORCE) {
+		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, attrs);
 }
@@ -721,6 +732,9 @@ static void
 swiotlb_full(struct device *dev, size_t size, enum dma_data_direction dir,
 	     int do_panic)
 {
+	if (swiotlb_force == SWIOTLB_NO_FORCE)
+		return;
+
 	/*
 	 * Ran out of IOMMU space for this operation. This is very bad.
 	 * Unfortunately the drivers cannot handle this operation properly.