Re: [PATCH v2] RISC-V: Clean up the Zicbom block size probing

Palmer Dabbelt
2022-08-12T23:08:00Z

On Fri, 12 Aug 2022 10:43:02 PDT (-0700), atishp@atishpatra.org wrote:
> On Fri, Aug 12, 2022 at 9:06 AM Palmer Dabbelt <palmer@rivosinc.com> wrote:
>>
>> This fixes two issues: I truncated the warning's hart ID when porting to
>> the 64-bit hart ID code, and the original code's warning handling could
>> fire on an uninitialized hart ID.
>>
>> The biggest change here is that riscv_cbom_block_size is no longer
>> initialized, as IMO the default isn't sane: there's nothing in the ISA
>> that mandates any specific cache block size, so falling back to one will
>> just silently produce the wrong answer on some systems. This also
>> changes the probing order so the cache block size is known before
>> enabling Zicbom support.
>>
>> Fixes: 3aefb2ee5bdd ("riscv: implement Zicbom-based CMO instructions + the t-head variant")
>> Fixes: 1631ba1259d6 ("riscv: Add support for non-coherent devices using zicbom extension")
>> Reported-by: kernel test robot <lkp@intel.com>
>> Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
>>
>> ---
>>
>> Changes since v1 <https://lore.kernel.org/all/20220812142400.7186-1-palmer@rivosinc.com/>:
>>
>> * Everything but the unsigned long cbom_hartid.
>> ---
>> arch/riscv/kernel/setup.c | 2 +-
>> arch/riscv/mm/dma-noncoherent.c | 22 ++++++++++++----------
>> 2 files changed, 13 insertions(+), 11 deletions(-)
>>
>> diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
>> index 95ef6e2bf45c..2dfc463b86bb 100644
>> --- a/arch/riscv/kernel/setup.c
>> +++ b/arch/riscv/kernel/setup.c
>> @@ -296,8 +296,8 @@ void __init setup_arch(char **cmdline_p)
>> setup_smp();
>> #endif
>>
>> - riscv_fill_hwcap();
>> riscv_init_cbom_blocksize();
>> + riscv_fill_hwcap();
>> apply_boot_alternatives();
>> }
>>
>> diff --git a/arch/riscv/mm/dma-noncoherent.c b/arch/riscv/mm/dma-noncoherent.c
>> index cd2225304c82..3aa3572715d6 100644
>> --- a/arch/riscv/mm/dma-noncoherent.c
>> +++ b/arch/riscv/mm/dma-noncoherent.c
>> @@ -12,7 +12,7 @@
>> #include <linux/of_device.h>
>> #include <asm/cacheflush.h>
>>
>> -static unsigned int riscv_cbom_block_size = L1_CACHE_BYTES;
>> +static unsigned int riscv_cbom_block_size;
>
> What is the expected behavior if the block size is zero in CMO
> operations ? As per my understanding, it will be equivalent to a nop.
> Let me know if I am wrong.
>
> If that is the case, this is misleading as well. Maybe we should just
> disable CMO extension altogether if it can't find the DT property.

That seems reasonable to me, even if having a 0 block size is allowed by
the spec it seems way more likely to have been a mistake. I'll send a
v3, after puttting together a toolchain that actually builds this
(assuming that's why I'm not getting the failures/warnings).

>
>> static bool noncoherent_supported;
>>
>> void arch_sync_dma_for_device(phys_addr_t paddr, size_t size,
>> @@ -80,37 +80,39 @@ void riscv_init_cbom_blocksize(void)
>> {
>> struct device_node *node;
>> int ret;
>> - u32 val;
>> + u32 val, probed_block_size;
>>
>> + probed_block_size = 0;
>> for_each_of_cpu_node(node) {
>> - unsigned long hartid;
>> - int cbom_hartid;
>> + unsigned long hartid, cbom_hartid;
>>
>> ret = riscv_of_processor_hartid(node, &hartid);
>> if (ret)
>> continue;
>>
>> - if (hartid < 0)
>> - continue;
>> -
>> /* set block-size for cbom extension if available */
>> ret = of_property_read_u32(node, "riscv,cbom-block-size", &val);
>> if (ret)
>> continue;
>>
>> - if (!riscv_cbom_block_size) {
>> - riscv_cbom_block_size = val;
>> + if (!probed_block_size) {
>> + probed_block_size = val;
>> cbom_hartid = hartid;
>> } else {
>> - if (riscv_cbom_block_size != val)
>> + if (probed_block_size != val)
>> pr_warn("cbom-block-size mismatched between harts %d and %lu\n",
>> cbom_hartid, hartid);
>> }
>> }
>> +
>> + if (probed_block_size)
>> + riscv_cbom_block_size = probed_block_size;
>> }
>> #endif
>>
>> void riscv_noncoherent_supported(void)
>> {
>> + WARN_ON(!riscv_cbom_block_size,
>> + "Non-coherent DMA support enabled without a block size\n");
>> noncoherent_supported = true;
>> }
>> --
>> 2.34.1
>>
>>
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