Re: [PATCH v10 3/6] riscv: mm: dma-noncoherent: nonstandard cache operations support

[Emil Renner Berthing]

On Sun, 30 Jul 2023 at 17:11, Jisheng Zhang <jszhang@xxxxxxxxxx> wrote:
>
> On Sun, Jul 02, 2023 at 09:34:26PM +0100, Prabhakar wrote:
> > From: Lad Prabhakar <prabhakar.mahadev-lad.rj@xxxxxxxxxxxxxx>
> >
> > Introduce support for nonstandard noncoherent systems in the RISC-V
> > architecture. It enables function pointer support to handle cache
> > management in such systems.
> >
> > This patch adds a new configuration option called
> > "RISCV_NONSTANDARD_CACHE_OPS." This option is a boolean flag that
> > depends on "RISCV_DMA_NONCOHERENT" and enables the function pointer
> > support for cache management in nonstandard noncoherent systems.
> >
> > Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@xxxxxxxxxxxxxx>
> > Reviewed-by: Conor Dooley <conor.dooley@xxxxxxxxxxxxx>
> > Tested-by: Conor Dooley <conor.dooley@xxxxxxxxxxxxx> # tyre-kicking on a d1
> > ---
> > v9 -> v10
> > * Added __ro_after_init compiler attribute for noncoherent_cache_ops
> > * Renamed clean -> wback
> > * Renamed inval -> inv
> > * Renamed flush -> wback_inv
> >
> > v8 -> v9
> > * New patch
> > ---
> >  arch/riscv/Kconfig                       |  7 ++++
> >  arch/riscv/include/asm/dma-noncoherent.h | 28 +++++++++++++++
> >  arch/riscv/mm/dma-noncoherent.c          | 43 ++++++++++++++++++++++++
> >  arch/riscv/mm/pmem.c                     | 13 +++++++
> >  4 files changed, 91 insertions(+)
> >  create mode 100644 arch/riscv/include/asm/dma-noncoherent.h
> >
> > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> > index d9e451ac862a..42c86b13c5e1 100644
> > --- a/arch/riscv/Kconfig
> > +++ b/arch/riscv/Kconfig
> > @@ -265,6 +265,13 @@ config RISCV_DMA_NONCOHERENT
> >       select ARCH_HAS_SYNC_DMA_FOR_DEVICE
> >       select DMA_DIRECT_REMAP
> >
> > +config RISCV_NONSTANDARD_CACHE_OPS
> > +     bool
> > +     depends on RISCV_DMA_NONCOHERENT
> > +     help
> > +       This enables function pointer support for non-standard noncoherent
> > +       systems to handle cache management.
>
> Per Documentation/riscv/patch-acceptance.rst:
>
> "we'll only consider patches for extensions that either:
>
> - Have been officially frozen or ratified by the RISC-V Foundation, or
> - Have been implemented in hardware that is widely available, per standard
>   Linux practice."
>
> I'm not sure which item this patch series belongs to.

Prabhakar can probably answer better, but my understanding is that
this needed on the Renesas RZ/Five SoC on the Asus Tinker V board:
https://tinker-board.asus.com/product/tinker-v.html

> > +
> >  config AS_HAS_INSN
> >       def_bool $(as-instr,.insn r 51$(comma) 0$(comma) 0$(comma) t0$(comma) t0$(comma) zero)
> >
> > diff --git a/arch/riscv/include/asm/dma-noncoherent.h b/arch/riscv/include/asm/dma-noncoherent.h
> > new file mode 100644
> > index 000000000000..969cf1f1363a
> > --- /dev/null
> > +++ b/arch/riscv/include/asm/dma-noncoherent.h
> > @@ -0,0 +1,28 @@
> > +/* SPDX-License-Identifier: GPL-2.0-only */
> > +/*
> > + * Copyright (C) 2023 Renesas Electronics Corp.
> > + */
> > +
> > +#ifndef __ASM_DMA_NONCOHERENT_H
> > +#define __ASM_DMA_NONCOHERENT_H
> > +
> > +#include <linux/dma-direct.h>
> > +
> > +/*
> > + * struct riscv_cache_ops - Structure for CMO function pointers
>
> can we reword this line as
> "struct riscv_nonstd_cache_ops - Structure for non-standard CMO function
> pointers" to explictly note this is only for non-standard CMO.
>
> > + *
> > + * @wback: Function pointer for cache writeback
> > + * @inv: Function pointer for invalidating cache
> > + * @wback_inv: Function pointer for flushing the cache (writeback + invalidating)
> > + */
> > +struct riscv_cache_ops {
> > +     void (*wback)(phys_addr_t paddr, unsigned long size);
> > +     void (*inv)(phys_addr_t paddr, unsigned long size);
> > +     void (*wback_inv)(phys_addr_t paddr, unsigned long size);
> > +};
> > +
> > +extern struct riscv_cache_ops noncoherent_cache_ops;
> > +
> > +void riscv_noncoherent_register_cache_ops(const struct riscv_cache_ops *ops);
> > +
> > +#endif       /* __ASM_DMA_NONCOHERENT_H */
> > diff --git a/arch/riscv/mm/dma-noncoherent.c b/arch/riscv/mm/dma-noncoherent.c
> > index b9a9f57e02be..4c2e3f1cdfe6 100644
> > --- a/arch/riscv/mm/dma-noncoherent.c
> > +++ b/arch/riscv/mm/dma-noncoherent.c
> > @@ -9,13 +9,26 @@
> >  #include <linux/dma-map-ops.h>
> >  #include <linux/mm.h>
> >  #include <asm/cacheflush.h>
> > +#include <asm/dma-noncoherent.h>
> >
> >  static bool noncoherent_supported;
> >
> > +struct riscv_cache_ops noncoherent_cache_ops __ro_after_init = {
> > +     .wback = NULL,
> > +     .inv = NULL,
> > +     .wback_inv = NULL,
> > +};
> > +
> >  static inline void arch_dma_cache_wback(phys_addr_t paddr, size_t size)
> >  {
> >       void *vaddr = phys_to_virt(paddr);
> >
> > +#ifdef CONFIG_RISCV_NONSTANDARD_CACHE_OPS
> > +     if (unlikely(noncoherent_cache_ops.wback)) {
>
> I'm worried about the performance impact here.
> For unified kernel Image reason, RISCV_NONSTANDARD_CACHE_OPS will be
> enabled by default, so standard CMO and T-HEAD's CMO platform's
> performance will be impacted, because even an unlikely is put
> here, the check action still needs to be done.

On IRC I asked why not use a static key so the overhead is just a
single nop when the standard CMO ops are available, but the consensus
seemed to be that the flushing would completely dominate this branch.
And on platforms with the standard CMO ops the branch be correctly
predicted anyway.

But I  agree, in the future it would be great to convert the T-Head
non-standard CMO ops to this, so there is only one way to do
non-standard ops.

/Emil

> > +             noncoherent_cache_ops.wback(paddr, size);
> > +             return;
> > +     }
> > +#endif
> >       ALT_CMO_OP(clean, vaddr, size, riscv_cbom_block_size);
> >  }
> >
> > @@ -23,6 +36,13 @@ static inline void arch_dma_cache_inv(phys_addr_t paddr, size_t size)
> >  {
> >       void *vaddr = phys_to_virt(paddr);
> >
> > +#ifdef CONFIG_RISCV_NONSTANDARD_CACHE_OPS
> > +     if (unlikely(noncoherent_cache_ops.inv)) {
> > +             noncoherent_cache_ops.inv(paddr, size);
> > +             return;
> > +     }
> > +#endif
> > +
> >       ALT_CMO_OP(inval, vaddr, size, riscv_cbom_block_size);
> >  }
> >
> > @@ -30,6 +50,13 @@ static inline void arch_dma_cache_wback_inv(phys_addr_t paddr, size_t size)
> >  {
> >       void *vaddr = phys_to_virt(paddr);
> >
> > +#ifdef CONFIG_RISCV_NONSTANDARD_CACHE_OPS
> > +     if (unlikely(noncoherent_cache_ops.wback_inv)) {
> > +             noncoherent_cache_ops.wback_inv(paddr, size);
> > +             return;
> > +     }
> > +#endif
> > +
> >       ALT_CMO_OP(flush, vaddr, size, riscv_cbom_block_size);
> >  }
> >
> > @@ -50,6 +77,13 @@ void arch_dma_prep_coherent(struct page *page, size_t size)
> >  {
> >       void *flush_addr = page_address(page);
> >
> > +#ifdef CONFIG_RISCV_NONSTANDARD_CACHE_OPS
> > +     if (unlikely(noncoherent_cache_ops.wback_inv)) {
> > +             noncoherent_cache_ops.wback_inv(page_to_phys(page), size);
> > +             return;
> > +     }
> > +#endif
> > +
> >       ALT_CMO_OP(flush, flush_addr, size, riscv_cbom_block_size);
> >  }
> >
> > @@ -75,3 +109,12 @@ void riscv_noncoherent_supported(void)
> >            "Non-coherent DMA support enabled without a block size\n");
> >       noncoherent_supported = true;
> >  }
> > +
> > +void riscv_noncoherent_register_cache_ops(const struct riscv_cache_ops *ops)
> > +{
> > +     if (!ops)
> > +             return;
> > +
> > +     noncoherent_cache_ops = *ops;
> > +}
> > +EXPORT_SYMBOL_GPL(riscv_noncoherent_register_cache_ops);
> > diff --git a/arch/riscv/mm/pmem.c b/arch/riscv/mm/pmem.c
> > index 089df92ae876..c5fc5ec96f6d 100644
> > --- a/arch/riscv/mm/pmem.c
> > +++ b/arch/riscv/mm/pmem.c
> > @@ -7,15 +7,28 @@
> >  #include <linux/libnvdimm.h>
> >
> >  #include <asm/cacheflush.h>
> > +#include <asm/dma-noncoherent.h>
> >
> >  void arch_wb_cache_pmem(void *addr, size_t size)
> >  {
> > +#ifdef CONFIG_RISCV_NONSTANDARD_CACHE_OPS
> > +     if (unlikely(noncoherent_cache_ops.wback)) {
> > +             noncoherent_cache_ops.wback(virt_to_phys(addr), size);
> > +             return;
> > +     }
> > +#endif
> >       ALT_CMO_OP(clean, addr, size, riscv_cbom_block_size);
> >  }
> >  EXPORT_SYMBOL_GPL(arch_wb_cache_pmem);
> >
> >  void arch_invalidate_pmem(void *addr, size_t size)
> >  {
> > +#ifdef CONFIG_RISCV_NONSTANDARD_CACHE_OPS
> > +     if (unlikely(noncoherent_cache_ops.inv)) {
> > +             noncoherent_cache_ops.inv(virt_to_phys(addr), size);
> > +             return;
> > +     }
> > +#endif
> >       ALT_CMO_OP(inval, addr, size, riscv_cbom_block_size);
> >  }
> >  EXPORT_SYMBOL_GPL(arch_invalidate_pmem);
> > --
> > 2.34.1
> >
> >
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>
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