Re: [PATCH] powerpc/mm: setting mmaped page cache property through device tree

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Tue, 2009-12-01 at 18:30 +0800, Li Yang wrote:
> The patch adds the ability for powerpc architecture to set page cache
> property of mmaped area through device tree.  This is useful for two
> cases.  First, for memory shared with other OS'es to have the same cache
> property to avoid cache paradoxes.  Second, enabling application to map
> memory which is not managed by kernel as cacheable for better performance.

But that doesn't solve the problem of those same pages being mapped
cachable as part of the linear mapping does it ?

Can you tell us more about your precise usage scenario ? What are you
trying to achieve here ? We can find a solution though it might involve
a specific driver to handle that memory.

Cheers,
Ben.

> Signed-off-by: Li Yang <leoli@freescale.com>
> ---
> Although it will be better if we can come up with a generic solution
> not only for powerpc arch.  Changing the behavior of O_SYNC seems to
> draw concerns over compatibility of old applications.  Suggestions
> are welcomed.
> 
>  arch/powerpc/mm/mem.c          |   49 +++++++++++++++++++++++++++++++++++++--
>  arch/powerpc/platforms/Kconfig |    7 +++++
>  2 files changed, 53 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/powerpc/mm/mem.c b/arch/powerpc/mm/mem.c
> index 579382c..02da2c8 100644
> --- a/arch/powerpc/mm/mem.c
> +++ b/arch/powerpc/mm/mem.c
> @@ -95,16 +95,59 @@ int page_is_ram(unsigned long pfn)
>  #endif
>  }
>  
> +#ifdef CONFIG_OF_MMAP_CACHE_PROPERTY
> +pgprot_t pgprot_from_dt(unsigned long pfn, pgprot_t vma_prot)
> +{
> +	struct device_node *np;
> +	struct resource res;
> +	unsigned long paddr = (pfn << PAGE_SHIFT);
> +	int i;
> +	const int *prop;
> +
> +	for_each_node_by_name(np, "mmap-region")
> +		for (i = 0; of_address_to_resource(np, i, &res) == 0; i++)
> +			if ((paddr >= res.start) && (paddr <= res.end)) {
> +				unsigned long _prot;
> +				prop = of_get_property(np, "cache-property",
> +						NULL);
> +
> +				if (prop == NULL)
> +					return vma_prot;
> +
> +				_prot = pgprot_val(vma_prot) & ~_PAGE_CACHE_CTL;
> +
> +				/* bit map of WIMG */
> +				if (*prop & 0x8)
> +					_prot |= _PAGE_WRITETHRU;
> +				if (*prop & 0x4)
> +					_prot |= _PAGE_NO_CACHE;
> +				if (*prop & 0x2)
> +					_prot |= _PAGE_COHERENT;
> +				if (*prop & 0x1)
> +					_prot |= _PAGE_GUARDED;
> +
> +				return __pgprot(_prot);
> +			}
> +
> +	return vma_prot;
> +}
> +#endif
> +
>  pgprot_t phys_mem_access_prot(struct file *file, unsigned long pfn,
>  			      unsigned long size, pgprot_t vma_prot)
>  {
>  	if (ppc_md.phys_mem_access_prot)
>  		return ppc_md.phys_mem_access_prot(file, pfn, size, vma_prot);
>  
> -	if (!page_is_ram(pfn))
> -		vma_prot = pgprot_noncached(vma_prot);
> +	/* kernel managed memory is always mapped as cacheable */
> +	if (page_is_ram(pfn))
> +		return vma_prot;
>  
> -	return vma_prot;
> +#ifdef CONFIG_OF_MMAP_CACHE_PROPERTY
> +	return pgprot_from_dt(pfn, vma_prot);
> +#else
> +	return pgprot_noncached(vma_prot);
> +#endif
>  }
>  EXPORT_SYMBOL(phys_mem_access_prot);
>  
> diff --git a/arch/powerpc/platforms/Kconfig b/arch/powerpc/platforms/Kconfig
> index 12bc2ce..de0f57c 100644
> --- a/arch/powerpc/platforms/Kconfig
> +++ b/arch/powerpc/platforms/Kconfig
> @@ -333,4 +333,11 @@ config MCU_MPC8349EMITX
>  	  also register MCU GPIOs with the generic GPIO API, so you'll able
>  	  to use MCU pins as GPIOs.
>  
> +config OF_MMAP_CACHE_PROPERTY
> +	bool "Support setting cache property of mmap through device tree"
> +	default n
> +	help
> +	  Say Y here to support setting cache property of mmaped region via
> +	  mmap-region device tree node.
> +
>  endmenu