Re: [PATCH 1/2] riscv: Implement Zicbom-based cache management operations

Anup Patel
2022-03-25T19:44:48Z

On Fri, Mar 25, 2022 at 10:52 PM Philipp Tomsich
<philipp.tomsich@vrull.eu> wrote:
>
> Anup,
>
> On Fri, 25 Mar 2022 at 17:20, Anup Patel <anup@brainfault.org> wrote:
>>
>> On Tue, Mar 8, 2022 at 4:16 AM Heiko Stuebner <heiko@sntech.de> wrote:
>> >
>> > The Zicbom ISA-extension was ratified in november 2021
>> > and introduces instructions for dcache invalidate, clean
>> > and flush operations.
>> >
>> > Implement cache management operations based on them.
>> >
>> > Of course not all cores will support this, so implement an
>> > alternative-based mechanism that replaces empty instructions
>> > with ones done around Zicbom instructions.
>> >
>> > We're using prebuild instructions for the Zicbom instructions
>> > for now, to not require a bleeding-edge compiler (gcc-12)
>> > for these somewhat simple instructions.
>> >
>> > Signed-off-by: Heiko Stuebner <heiko@sntech.de>
>> > Cc: Christoph Hellwig <hch@lst.de>
>> > Cc: Atish Patra <atish.patra@wdc.com>
>> > Cc: Guo Ren <guoren@kernel.org>
>> > ---
>> > arch/riscv/Kconfig | 8 ++++
>> > arch/riscv/include/asm/errata_list.h | 37 ++++++++++++++++-
>> > arch/riscv/include/asm/hwcap.h | 1 +
>> > arch/riscv/kernel/cpu.c | 1 +
>> > arch/riscv/kernel/cpufeature.c | 17 ++++++++
>> > arch/riscv/mm/Makefile | 1 +
>> > arch/riscv/mm/dma-noncoherent.c | 61 ++++++++++++++++++++++++++++
>> > 7 files changed, 125 insertions(+), 1 deletion(-)
>> > create mode 100644 arch/riscv/mm/dma-noncoherent.c
>> >
>> > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
>> > index 5adcbd9b5e88..d3a1cd41c203 100644
>> > --- a/arch/riscv/Kconfig
>> > +++ b/arch/riscv/Kconfig
>> > @@ -208,6 +208,14 @@ config PGTABLE_LEVELS
>> > config LOCKDEP_SUPPORT
>> > def_bool y
>> >
>> > +config RISCV_DMA_NONCOHERENT
>> > + bool "Support non-coherent dma operation"
>> > + select ARCH_HAS_DMA_PREP_COHERENT
>> > + select ARCH_HAS_SYNC_DMA_FOR_DEVICE
>> > + select ARCH_HAS_SYNC_DMA_FOR_CPU
>> > + select ARCH_HAS_SETUP_DMA_OPS
>> > + select DMA_DIRECT_REMAP
>> > +
>> > source "arch/riscv/Kconfig.socs"
>> > source "arch/riscv/Kconfig.erratas"
>> >
>> > diff --git a/arch/riscv/include/asm/errata_list.h b/arch/riscv/include/asm/errata_list.h
>> > index 4fac46b82c16..7a2dd61af24d 100644
>> > --- a/arch/riscv/include/asm/errata_list.h
>> > +++ b/arch/riscv/include/asm/errata_list.h
>> > @@ -20,7 +20,8 @@
>> > #endif
>> >
>> > #define CPUFEATURE_SVPBMT 0
>> > -#define CPUFEATURE_NUMBER 1
>> > +#define CPUFEATURE_CMO 1
>> > +#define CPUFEATURE_NUMBER 2
>> >
>> > #ifdef __ASSEMBLY__
>> >
>> > @@ -86,6 +87,40 @@ asm volatile(ALTERNATIVE( \
>> > #define ALT_THEAD_PMA(_val)
>> > #endif
>> >
>> > +/*
>> > + * cbo.clean rs1
>> > + * | 31 - 20 | 19 - 15 | 14 - 12 | 11 - 7 | 6 - 0 |
>> > + * 0...01 rs1 010 00000 0001111
>> > + *
>> > + * cbo.flush rs1
>> > + * | 31 - 20 | 19 - 15 | 14 - 12 | 11 - 7 | 6 - 0 |
>> > + * 0...10 rs1 010 00000 0001111
>> > + *
>> > + * cbo.inval rs1
>> > + * | 31 - 20 | 19 - 15 | 14 - 12 | 11 - 7 | 6 - 0 |
>> > + * 0...00 rs1 010 00000 0001111
>> > + */
>> > +#define CBO_INVAL_A0 ".long 0x15200F"
>> > +#define CBO_CLEAN_A0 ".long 0x25200F"
>> > +#define CBO_FLUSH_A0 ".long 0x05200F"
>> > +
>> > +#define ALT_CMO_OP(_op, _start, _size) \
>> > +asm volatile(ALTERNATIVE( \
>> > + "nop\n\t" \
>> > + "nop\n\t" \
>> > + "nop\n\t" \
>> > + "nop\n\t" \
>> > + "nop", \
>> > + "mv a0, %1\n\t" \
>> > + "j 2f\n\t" \
>> > + "3:\n\t" \
>> > + CBO_##_op##_A0 "\n\t" \
>> > + "addi a0, a0, %0\n\t" \
>> > + "2:\n\t" \
>> > + "bltu a0, %2, 3b\n\t", 0, CPUFEATURE_CMO, CONFIG_RISCV_DMA_NONCOHERENT) \
>> > + : : "I"(L1_CACHE_BYTES), "r"((_start) & ~(L1_CACHE_BYTES - 1)), \
>> > + "r"(ALIGN((_start) + (_size), L1_CACHE_BYTES)))
>>
>> Why not use a global variable (e.g. riscv_cbom_block_size) representing
>> exact cbom block size instead of L1_CACHE_BYTES ?
>
>
> Didn't the discussions around platforms gravitate towards a fixed cache-block
> operations size (note that this is orthogonal from the cache-line size) a
> requirement for Linux-capable platforms?

I recall past platform discussions. Implementations compliant with platforms
spec (whenever that is ratified) will converge to a fixed cache-block side but
this does not mean Linux CMO support should not work for implementations
with a different cache-block size.

For e.g. the ARM64 cache operations (arch/arm64/mm/cache.S) use
determine cache line size using ctr_el0 MSR.

Regards,
Anup

>
> Philipp.
>
>> The default value of riscv_cbom_block_size can be L1_CACHE_BYTES
>> which can be overridden at boot-time using optional "riscv,cbom-block-size"
>> DT property.
>>
>> The rationale here is that if underlying RISC-V implementation has cbom
>> block size smaller than L1_CACHE_BYTES then it will result in incomplete
>> cbom range operation. The riscv_cbom_block_size global variable ensures
>> that the right block size is used at least for cbom operations.
>>
>> Regards,
>> Anup
>>
>> > +
>> > #endif /* __ASSEMBLY__ */
>> >
>> > #endif
>> > diff --git a/arch/riscv/include/asm/hwcap.h b/arch/riscv/include/asm/hwcap.h
>> > index 656cd626eb1a..5943d5125a51 100644
>> > --- a/arch/riscv/include/asm/hwcap.h
>> > +++ b/arch/riscv/include/asm/hwcap.h
>> > @@ -52,6 +52,7 @@ extern unsigned long elf_hwcap;
>> > */
>> > enum riscv_isa_ext_id {
>> > RISCV_ISA_EXT_SVPBMT = RISCV_ISA_EXT_BASE,
>> > + RISCV_ISA_EXT_ZICBOM,
>> > RISCV_ISA_EXT_ID_MAX = RISCV_ISA_EXT_MAX,
>> > };
>> >
>> > diff --git a/arch/riscv/kernel/cpu.c b/arch/riscv/kernel/cpu.c
>> > index c582d557e555..dfcf592273a7 100644
>> > --- a/arch/riscv/kernel/cpu.c
>> > +++ b/arch/riscv/kernel/cpu.c
>> > @@ -72,6 +72,7 @@ int riscv_of_parent_hartid(struct device_node *node)
>> >
>> > static struct riscv_isa_ext_data isa_ext_arr[] = {
>> > __RISCV_ISA_EXT_DATA("svpbmt", RISCV_ISA_EXT_SVPBMT),
>> > + __RISCV_ISA_EXT_DATA("zicbom", RISCV_ISA_EXT_ZICBOM),
>>
>> Drop the quotes around zicbom because __RISCV_ISA_EXT_DATA() will
>> stringify the first parameter.
>>
>> > __RISCV_ISA_EXT_DATA("", RISCV_ISA_EXT_MAX),
>> > };
>> >
>> > diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
>> > index 5c5e6e7488ce..0e997fa5524a 100644
>> > --- a/arch/riscv/kernel/cpufeature.c
>> > +++ b/arch/riscv/kernel/cpufeature.c
>> > @@ -200,6 +200,7 @@ void __init riscv_fill_hwcap(void)
>> > set_bit(*ext - 'a', this_isa);
>> > } else {
>> > SET_ISA_EXT_MAP("svpbmt", RISCV_ISA_EXT_SVPBMT);
>> > + SET_ISA_EXT_MAP("zicbom", RISCV_ISA_EXT_ZICBOM);
>> > }
>> > #undef SET_ISA_EXT_MAP
>> > }
>> > @@ -267,11 +268,27 @@ static bool __init_or_module cpufeature_svpbmt_check_func(unsigned int stage)
>> > return ret;
>> > }
>> >
>> > +static bool cpufeature_cmo_check_func(unsigned int stage)
>> > +{
>> > + switch (stage) {
>> > + case RISCV_ALTERNATIVES_EARLY_BOOT:
>> > + return false;
>> > + default:
>> > + return riscv_isa_extension_available(NULL, ZICBOM);
>> > + }
>> > +
>> > + return false;
>> > +}
>> > +
>> > static const struct cpufeature_info __initdata_or_module cpufeature_list[CPUFEATURE_NUMBER] = {
>> > {
>> > .name = "svpbmt",
>> > .check_func = cpufeature_svpbmt_check_func
>> > },
>> > + {
>> > + .name = "cmo",
>> > + .check_func = cpufeature_cmo_check_func
>> > + },
>> > };
>> >
>> > static u32 __init_or_module cpufeature_probe(unsigned int stage)
>> > diff --git a/arch/riscv/mm/Makefile b/arch/riscv/mm/Makefile
>> > index ac7a25298a04..d76aabf4b94d 100644
>> > --- a/arch/riscv/mm/Makefile
>> > +++ b/arch/riscv/mm/Makefile
>> > @@ -30,3 +30,4 @@ endif
>> > endif
>> >
>> > obj-$(CONFIG_DEBUG_VIRTUAL) += physaddr.o
>> > +obj-$(CONFIG_RISCV_DMA_NONCOHERENT) += dma-noncoherent.o
>> > diff --git a/arch/riscv/mm/dma-noncoherent.c b/arch/riscv/mm/dma-noncoherent.c
>> > new file mode 100644
>> > index 000000000000..2c124bcc1932
>> > --- /dev/null
>> > +++ b/arch/riscv/mm/dma-noncoherent.c
>> > @@ -0,0 +1,61 @@
>> > +// SPDX-License-Identifier: GPL-2.0-only
>> > +/*
>> > + * RISC-V specific functions to support DMA for non-coherent devices
>> > + *
>> > + * Copyright (c) 2021 Western Digital Corporation or its affiliates.
>> > + */
>> > +
>> > +#include <linux/dma-direct.h>
>> > +#include <linux/dma-map-ops.h>
>> > +#include <linux/init.h>
>> > +#include <linux/io.h>
>> > +#include <linux/libfdt.h>
>> > +#include <linux/mm.h>
>> > +#include <linux/of.h>
>> > +#include <linux/of_fdt.h>
>> > +
>> > +void arch_sync_dma_for_device(phys_addr_t paddr, size_t size, enum dma_data_direction dir)
>> > +{
>> > + switch (dir) {
>> > + case DMA_TO_DEVICE:
>> > + ALT_CMO_OP(CLEAN, (unsigned long)phys_to_virt(paddr), size);
>> > + break;
>> > + case DMA_FROM_DEVICE:
>> > + ALT_CMO_OP(INVAL, (unsigned long)phys_to_virt(paddr), size);
>> > + break;
>> > + case DMA_BIDIRECTIONAL:
>> > + ALT_CMO_OP(FLUSH, (unsigned long)phys_to_virt(paddr), size);
>> > + break;
>> > + default:
>> > + break;
>> > + }
>> > +}
>> > +
>> > +void arch_sync_dma_for_cpu(phys_addr_t paddr, size_t size, enum dma_data_direction dir)
>> > +{
>> > + switch (dir) {
>> > + case DMA_TO_DEVICE:
>> > + break;
>> > + case DMA_FROM_DEVICE:
>> > + case DMA_BIDIRECTIONAL:
>> > + ALT_CMO_OP(INVAL, (unsigned long)phys_to_virt(paddr), size);
>> > + break;
>> > + default:
>> > + break;
>> > + }
>> > +}
>> > +
>> > +void arch_dma_prep_coherent(struct page *page, size_t size)
>> > +{
>> > + void *flush_addr = page_address(page);
>> > +
>> > + memset(flush_addr, 0, size);
>> > + ALT_CMO_OP(FLUSH, (unsigned long)flush_addr, size);
>> > +}
>> > +
>> > +void arch_setup_dma_ops(struct device *dev, u64 dma_base, u64 size,
>> > + const struct iommu_ops *iommu, bool coherent)
>> > +{
>> > + /* If a specific device is dma-coherent, set it here */
>> > + dev->dma_coherent = coherent;
>> > +}
>> > --
>> > 2.30.2
>> >
>>
>>