Re: [PATCH 2/2] riscv: implement cache-management errata for T-Head SoCs

Palmer Dabbelt
2022-04-20T00:18:41Z

On Thu, 31 Mar 2022 01:22:29 PDT (-0700), heiko@sntech.de wrote:
> Hi Palmer,
>
> Am Donnerstag, 31. März 2022, 04:30:36 CEST schrieb Palmer Dabbelt:
>> On Mon, 07 Mar 2022 14:46:20 PST (-0800), heiko@sntech.de wrote:
>> > The T-Head C906 and C910 implement a scheme for handling
>> > cache operations different from the generic Zicbom extension.
>> >
>> > Add an errata for it next to the generic dma coherency ops.
>> >
>> > Signed-off-by: Heiko Stuebner <heiko@sntech.de>
>> > ---
>> > arch/riscv/Kconfig.erratas | 10 +++++++
>> > arch/riscv/errata/thead/errata.c | 5 ++++
>> > arch/riscv/include/asm/errata_list.h | 45 ++++++++++++++++++++++++++--
>> > 3 files changed, 57 insertions(+), 3 deletions(-)
>> >
>> > diff --git a/arch/riscv/Kconfig.erratas b/arch/riscv/Kconfig.erratas
>> > index de4002baa1d0..89a6dcb8ac2a 100644
>> > --- a/arch/riscv/Kconfig.erratas
>> > +++ b/arch/riscv/Kconfig.erratas
>> > @@ -50,4 +50,14 @@ config ERRATA_THEAD_PBMT
>> >
>> > If you don't know what to do here, say "Y".
>> >
>> > +config ERRATA_THEAD_CMO
>> > + bool "Apply T-Head cache management errata"
>> > + depends on ERRATA_THEAD && RISCV_DMA_NONCOHERENT
>> > + default y
>> > + help
>> > + This will apply the cache management errata to handle the
>> > + non-standard handling on non-coherent operations on T-Head SoCs.
>> > +
>> > + If you don't know what to do here, say "Y".
>> > +
>> > endmenu
>> > diff --git a/arch/riscv/errata/thead/errata.c b/arch/riscv/errata/thead/errata.c
>> > index fd8e0538a3f0..11c26c37425f 100644
>> > --- a/arch/riscv/errata/thead/errata.c
>> > +++ b/arch/riscv/errata/thead/errata.c
>> > @@ -33,6 +33,11 @@ static const struct errata_info errata_list[ERRATA_THEAD_NUMBER] = {
>> > .stage = RISCV_ALTERNATIVES_EARLY_BOOT,
>> > .check_func = errata_mt_check_func
>> > },
>> > + {
>> > + .name = "cache-management",
>> > + .stage = RISCV_ALTERNATIVES_BOOT,
>> > + .check_func = errata_mt_check_func
>> > + },
>> > };
>> >
>> > static u32 thead_errata_probe(unsigned int stage, unsigned long archid, unsigned long impid)
>> > diff --git a/arch/riscv/include/asm/errata_list.h b/arch/riscv/include/asm/errata_list.h
>> > index 7a2dd61af24d..f7c6805daeab 100644
>> > --- a/arch/riscv/include/asm/errata_list.h
>> > +++ b/arch/riscv/include/asm/errata_list.h
>> > @@ -16,7 +16,8 @@
>> >
>> > #ifdef CONFIG_ERRATA_THEAD
>> > #define ERRATA_THEAD_PBMT 0
>> > -#define ERRATA_THEAD_NUMBER 1
>> > +#define ERRATA_THEAD_CMO 1
>> > +#define ERRATA_THEAD_NUMBER 2
>> > #endif
>> >
>> > #define CPUFEATURE_SVPBMT 0
>> > @@ -104,8 +105,37 @@ asm volatile(ALTERNATIVE( \
>> > #define CBO_CLEAN_A0 ".long 0x25200F"
>> > #define CBO_FLUSH_A0 ".long 0x05200F"
>> >
>> > +/*
>> > + * dcache.ipa rs1 (invalidate, physical address)
>> > + * | 31 - 25 | 24 - 20 | 19 - 15 | 14 - 12 | 11 - 7 | 6 - 0 |
>> > + * 0000001 01010 rs1 000 00000 0001011
>> > + * dache.iva rs1 (invalida, virtual address)
>> > + * 0000001 00110 rs1 000 00000 0001011
>> > + *
>> > + * dcache.cpa rs1 (clean, physical address)
>> > + * | 31 - 25 | 24 - 20 | 19 - 15 | 14 - 12 | 11 - 7 | 6 - 0 |
>> > + * 0000001 01001 rs1 000 00000 0001011
>> > + * dcache.cva rs1 (clean, virtual address)
>> > + * 0000001 00100 rs1 000 00000 0001011
>> > + *
>> > + * dcache.cipa rs1 (clean then invalidate, physical address)
>> > + * | 31 - 25 | 24 - 20 | 19 - 15 | 14 - 12 | 11 - 7 | 6 - 0 |
>> > + * 0000001 01011 rs1 000 00000 0001011
>> > + * dcache.civa rs1 (... virtual address)
>> > + * 0000001 00111 rs1 000 00000 0001011
>> > + *
>> > + * sync.s (make sure all cache operations finished)
>> > + * | 31 - 25 | 24 - 20 | 19 - 15 | 14 - 12 | 11 - 7 | 6 - 0 |
>> > + * 0000000 11001 00000 000 00000 0001011
>> > + */
>> > +#define THEAD_INVAL_A0 ".long 0x0265000b"
>> > +#define THEAD_CLEAN_A0 ".long 0x0245000b"
>> > +#define THEAD_FLUSH_A0 ".long 0x0275000b"
>> > +#define THEAD_SYNC_S ".long 0x0190000b"
>>
>> IIRC this came up before, but these really need to get into the
>> assembler as actual instructions.
>
> okay :-) .
>
> But just for my understanding which of the two ways going forward:
> - keep this in the waiting area _until_ a suitable binutils is released
> - use the coded instructions now and convert later once binutils is released
>
> The reason I ask is, that any chip with a t-head core like the Allwinner-D1
> will need this for things like basic networking, so with the binutils
> release schedule, I guess we'd be looking at autumn 2022 at the earliest.

I'm not the binutils release maintainer, so I can't really sign off on a
release date, but give the history that sounds about right to me. I get
it's a headache to have to have a toolchain that supports the ISA, but
if it was really that important it would have made one of the last two
releases -- I very specifically remember talking to the folks at the
RISC-V foundation about this the better part of a year ago, but they
decided to play at politics instead of being constructive so now we have
two messes to clean up.

I volunteered Patrick to send binutils patches for the T-Head cache
control stuff (as I didn't have time to write it myself this weekend),
it's only a dozen or so instructions and thus shouldn't take that long.
At least that way we can get a rough consensus on how we're going to
move forward with the toolchain support, which we really need before
we're going to start depending on anything.

Sorry you got pulled into all this.

> Thanks
> Heiko
>
>> > +
>> > #define ALT_CMO_OP(_op, _start, _size) \
>> > -asm volatile(ALTERNATIVE( \
>> > +asm volatile(ALTERNATIVE_2( \
>> > + "nop\n\t" \
>> > "nop\n\t" \
>> > "nop\n\t" \
>> > "nop\n\t" \
>> > @@ -117,7 +147,16 @@ asm volatile(ALTERNATIVE( \
>> > 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) \
>> > + "bltu a0, %2, 3b\n\t" \
>> > + "nop", 0, CPUFEATURE_CMO, CONFIG_RISCV_DMA_NONCOHERENT, \
>> > + "mv a0, %1\n\t" \
>> > + "j 2f\n\t" \
>> > + "3:\n\t" \
>> > + THEAD_##_op##_A0 "\n\t" \
>> > + "addi a0, a0, %0\n\t" \
>> > + "2:\n\t" \
>> > + "bltu a0, %2, 3b\n\t" \
>> > + THEAD_SYNC_S, THEAD_VENDOR_ID, ERRATA_THEAD_CMO, CONFIG_ERRATA_THEAD_CMO) \
>> > : : "I"(L1_CACHE_BYTES), "r"((_start) & ~(L1_CACHE_BYTES - 1)), \
>> > "r"(ALIGN((_start) + (_size), L1_CACHE_BYTES)))
>>