[U-Boot] [RFC PATCH 1/2] arch: armv8: Provide a way to disable cache maintenance ops

[Tero Kristo]

On 25/03/2019 19:21, Vignesh Raghavendra wrote:
> On AM654 SoC(arm64) which is IO coherent and has L3 Cache, cache
> maintenance operations being done to support non-coherent platforms
> causes issues.
> 
> For example, here is how U-Boot prepares/handles a buffer to receive
> data from a device (DMA Write). This may vary slightly depending on the
> driver framework:
> 
> 	Start DMA to write to destination buffer
> 	Wait for DMA to be done (dma_receive()/dma_memcpy())
> 	Invalidate destination buffer (invalidate_dcache_range())
> 	Read from destination buffer
> 
> The invalidate after the DMA is needed in order to read latest data from
> memory that’s updated by DMA write. Also, in case random prefetch has
> pulled in buffer data during the “wait for DMA” before the DMA has
> written to it. This works well for non-coherent architectures.
> 
> In case of coherent architecture with L3 cache, DMA write would directly
> update L3 cache contents (assuming cacheline is present in L3) without
> updating the DDR memory. So invalidate after “wait for DMA” in above
> sequence would discard latest data and read will cause stale data to be
> fetched from DDR. Therefore invalidate after “wait for DMA” is not
> always correct on coherent architecture.
> 
> Therefore, provide a Kconfig option to disable cache maintenance ops on
> coherent architectures. This has added benefit of improving the
> performance of DMA transfers as we no longer need to invalidate/flush
> individual cache lines(especially for buffer thats several KBs in size).
> 
> In order to facilitate use of same Kconfig across different
> architecture, I have added the symbol to top level arch/Kconfig file.
> Patch currently disables cache maintenance ops for arm64 only.
> 
> Signed-off-by: Vignesh Raghavendra <vigneshr at ti.com>
> ---
>   arch/Kconfig                  |  7 +++++++
>   arch/arm/cpu/armv8/cache_v8.c | 18 ++++++++++++++++++
>   2 files changed, 25 insertions(+)
> 
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 2f3d07c13a18..d32a458deedc 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -227,6 +227,13 @@ config SYS_CONFIG_NAME
>   	  The header file include/configs/<CONFIG_SYS_CONFIG_NAME>.h
>   	  should be included from include/config.h.
>   
> +config SYS_ARCH_IS_COHERENT
> +	bool
> +	help
> +	Select this option, if your architecture is coherent and needs
> +	to avoid cache maintenance operations. Note that, its up to the
> +	individual architectures to implement this functionality

I think you should use something more generic here as the name / 
description of the Kconfig. SYS_ARCH_IS_COHERENT is a valid term for 
K2/K3, but there might be reasons why someone would want to disable 
cache ops on other architectures also. Also, SYS_ARCH_IS_COHERENT leads 
the user to believe that some real magic is done, instead of simple 
disabling of cache ops completely. If we are talking about the different 
evolutions of the coherency support in uboot, are you planning to 
re-write this Kconfig option to the mixed coherency support at some 
point, or just introduce a new Kconfig, or drop this completely?

Other than that, series seems fine to me.

-Tero

> +
>   source "arch/arc/Kconfig"
>   source "arch/arm/Kconfig"
>   source "arch/m68k/Kconfig"
> diff --git a/arch/arm/cpu/armv8/cache_v8.c b/arch/arm/cpu/armv8/cache_v8.c
> index 038405173eb1..b39d6a65cf9c 100644
> --- a/arch/arm/cpu/armv8/cache_v8.c
> +++ b/arch/arm/cpu/armv8/cache_v8.c
> @@ -417,6 +417,7 @@ __weak void mmu_setup(void)
>   	set_sctlr(get_sctlr() | CR_M);
>   }
>   
> +#ifndef CONFIG_SYS_ARCH_IS_COHERENT
>   /*
>    * Performs a invalidation of the entire data cache at all levels
>    */
> @@ -458,6 +459,23 @@ void flush_dcache_range(unsigned long start, unsigned long stop)
>   {
>   	__asm_flush_dcache_range(start, stop);
>   }
> +#else
> +void invalidate_dcache_all(void)
> +{
> +}
> +
> +void flush_dcache_all(void)
> +{
> +}
> +
> +void invalidate_dcache_range(unsigned long start, unsigned long stop)
> +{
> +}
> +
> +void flush_dcache_range(unsigned long start, unsigned long stop)
> +{
> +}
> +#endif /* CONFIG_SYS_ARCH_IS_COHERENT */
>   
>   void dcache_enable(void)
>   {
> 

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