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>

1 files changed, 16 insertions, 2 deletions

diff --git a/lib/swiotlb.c b/lib/swiotlb.c
index a32dce6d5101..9def738af4f4 100644
--- a/lib/swiotlb.c
+++ b/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.