swiotlb: Add swiotlb=noforce debug option

commit fff5d99225107f5f13fe4a9805adc2a1c4b5fb00 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 2 insertions, 1 deletions

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 37babf91f2cb..922dec8fa07e 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 | 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]