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

[Jisheng Zhang]

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.

> +
>  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.

> +		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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