linux-kernel - Re: [PATCH v2] MIPS: Use CPHYSADDR to implement mips32 _

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Date:	Mon, 1 Aug 2016 17:51:10 +0100
From:	Paul Burton <paul.burton@...tec.com>
To:	Ralf Baechle <ralf@...ux-mips.org>
CC:	<linux-mips@...ux-mips.org>,
	Matt Redfearn <matt.redfearn@...tec.com>,
	Guenter Roeck <linux@...ck-us.net>,
	<linux-kernel@...r.kernel.org>,
	Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH v2] MIPS: Use CPHYSADDR to implement mips32 __pa

On 16/02/16 17:41, Paul Burton wrote:
> Use CPHYSADDR to implement the __pa macro converting from a virtual to a
> physical address for MIPS32, much as is already done for MIPS64 (though
> without the complication of having both compatibility & XKPHYS
> segments).
>
> This allows for __pa to work regardless of whether the address being
> translated is in kseg0 or kseg1, unlike the previous subtraction based
> approach which only worked for addresses in kseg0. Working for kseg1
> addresses is important if __pa is used on addresses allocated by
> dma_alloc_coherent, where on systems with non-coherent I/O we provide
> addresses in kseg1. If this address is then used with
> dma_map_single_attrs then it is provided to virt_to_page, which in turn
> calls virt_to_phys which is a wrapper around __pa. The result is that we
> end up with a physical address 0x20000000 bytes (ie. the size of kseg0)
> too high.
>
> In addition to providing consistency with MIPS64 & fixing the kseg1 case
> above this has the added bonus of generating smaller code for systems
> implementing MIPS32r2 & beyond, where a single ext instruction can
> extract the physical address rather than needing to load an immediate
> into a temp register & subtract it. This results in ~1.3KB savings for a
> boston_defconfig kernel adjusted to set CONFIG_32BIT=y.
>
> This patch does not change the EVA case, which may or may not have
> similar issues around handling both cached & uncached addresses but is
> beyond the scope of this patch.
>
> Signed-off-by: Paul Burton <paul.burton@...tec.com>
>

Hi Ralf,

Any thoughts on this one? It matters for Boston (where it affects the 
pch_gbe ethernet driver) which I'll be submitting again for the 4.9 cycle.

Thanks,
     Paul

> ---
>
> Changes in v2:
> - Leave the EVA case as-is.
>
>  arch/mips/include/asm/page.h | 20 ++++++++++++++++++--
>  1 file changed, 18 insertions(+), 2 deletions(-)
>
> diff --git a/arch/mips/include/asm/page.h b/arch/mips/include/asm/page.h
> index 21ed715..ac0c1b7 100644
> --- a/arch/mips/include/asm/page.h
> +++ b/arch/mips/include/asm/page.h
> @@ -169,8 +169,24 @@ typedef struct { unsigned long pgprot; } pgprot_t;
>      __x < CKSEG0 ? XPHYSADDR(__x) : CPHYSADDR(__x);			\
>  })
>  #else
> -#define __pa(x)								\
> -    ((unsigned long)(x) - PAGE_OFFSET + PHYS_OFFSET)
> +static inline unsigned long __pa(unsigned long x)
> +{
> +	if (!config_enabled(CONFIG_EVA)) {
> +		/*
> +		 * We're using the standard MIPS32 legacy memory map, ie.
> +		 * the address x is going to be in kseg0 or kseg1. We can
> +		 * handle either case by masking out the desired bits using
> +		 * CPHYSADDR.
> +		 */
> +		return CPHYSADDR(x);
> +	}
> +
> +	/*
> +	 * EVA is in use so the memory map could be anything, making it not
> +	 * safe to just mask out bits.
> +	 */
> +	return x - PAGE_OFFSET + PHYS_OFFSET;
> +}
>  #endif
>  #define __va(x)		((void *)((unsigned long)(x) + PAGE_OFFSET - PHYS_OFFSET))
>  #include <asm/io.h>
>