riscv: add swiotlb support

All RISC-V platforms today lack an IOMMU. However, legacy PCI devices
sometimes require DMA-memory to be in the low 32 bits.  To make this work,
we enable the software-based bounce buffers from swiotlb.  They only impose
overhead when the device in question cannot address the full 64-bit address
space, so a perfect fit.

This patch assumes that DMA is coherent with the processor and the PCI
bus.  It also assumes that the processor and devices share a common
address space. This is true for all RISC-V platforms so far.

[changelog stolen from an earlier patch by Palmer Dabbelt that did the
 more complicated swiotlb wireup before the recent consolidation]

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
index c11f40c1b2a8..ee44a48faf79 100644
--- a/arch/riscv/kernel/setup.c
+++ b/arch/riscv/kernel/setup.c
@@ -29,6 +29,7 @@
 #include <linux/of_fdt.h>
 #include <linux/of_platform.h>
 #include <linux/sched/task.h>
+#include <linux/swiotlb.h>
 
 #include <asm/setup.h>
 #include <asm/sections.h>
@@ -206,6 +207,7 @@ void __init setup_arch(char **cmdline_p)
 	setup_bootmem();
 	paging_init();
 	unflatten_device_tree();
+	swiotlb_init(1);
 
 #ifdef CONFIG_SMP
 	setup_smp();