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

[Atish Patra]

On Fri, Aug 12, 2022 at 9:06 AM Palmer Dabbelt <palmer at 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 at intel.com>
> Signed-off-by: Palmer Dabbelt <palmer at 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.

>  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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-- 
Regards,
Atish