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>
This commit is contained in:
Christoph Hellwig 2018-04-27 08:43:14 +02:00
parent f1306f0423
commit 10314e09d0
3 changed files with 18 additions and 0 deletions

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@ -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

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@ -0,0 +1,15 @@
// 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 */

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@ -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();