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: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NPalmer Dabbelt <palmer@sifive.com>

arch/riscv/Kconfig

@@ -114,6 +114,7 @@ config ARCH_RV64I
 	select HAVE_FTRACE_MCOUNT_RECORD
 	select HAVE_DYNAMIC_FTRACE
 	select HAVE_DYNAMIC_FTRACE_WITH_REGS
+	select SWIOTLB
 
 endchoice
 

arch/riscv/include/asm/dma-mapping.h

+// SPDX-License-Identifier: GPL-2.0
+#ifndef _RISCV_ASM_DMA_MAPPING_H
+#define _RISCV_ASM_DMA_MAPPING_H 1
+
+#ifdef CONFIG_SWIOTLB
+#include <linux/swiotlb.h>
+static inline const struct dma_map_ops *get_arch_dma_ops(struct bus_type *bus)
+{
+	return &swiotlb_dma_ops;
+}
+#else
+#include <asm-generic/dma-mapping.h>
+#endif /* CONFIG_SWIOTLB */
+
+#endif /* _RISCV_ASM_DMA_MAPPING_H */

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();