linux-kernel - Re: [PATCH v2] ARM: bcm2835: Use 0x4 prefix for DMA bus addresses to SDRAM.

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Date:	Thu, 14 May 2015 09:43:07 +0100
From:	Lee Jones <lee@...nel.org>
To:	Eric Anholt <eric@...olt.net>
Cc:	linux-arm-kernel@...ts.infradead.org,
	linux-rpi-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Stephen Warren <swarren@...dotorg.org>, popcornmix@...il.com
Subject: Re: [PATCH v2] ARM: bcm2835: Use 0x4 prefix for DMA bus addresses to
 SDRAM.

On Tue, 05 May 2015, Eric Anholt wrote:

> There exists a tiny MMU, configurable only by the VC (running the
> closed firmware), which maps from the ARM's physical addresses to bus
> addresses.  These bus addresses determine the caching behavior in the
> VC's L1/L2 (note: separate from the ARM's L1/L2) according to the top
> 2 bits.  The bits in the bus address mean:
> 
> From the VideoCore processor:
> 0x0... L1 and L2 cache allocating and coherent
> 0x4... L1 non-allocating, but coherent. L2 allocating and coherent
> 0x8... L1 non-allocating, but coherent. L2 non-allocating, but coherent
> 0xc... SDRAM alias. Cache is bypassed. Not L1 or L2 allocating or coherent
> 
> From the GPU peripherals (note: all peripherals bypass the L1
> cache. The ARM will see this view once through the VC MMU):
> 0x0... Do not use
> 0x4... L1 non-allocating, and incoherent. L2 allocating and coherent.
> 0x8... L1 non-allocating, and incoherent. L2 non-allocating, but coherent
> 0xc... SDRAM alias. Cache is bypassed. Not L1 or L2 allocating or coherent
> 
> The 2835 firmware always configures the MMU to turn ARM physical
> addresses with 0x0 top bits to 0x4, meaning present in L2 but
> incoherent with L1.  However, any bus addresses we were generating in
> the kernel to be passed to a device had 0x0 bits.  That would be a
> reserved (possibly totally incoherent) value if sent to a GPU
> peripheral like USB, or L1 allocating if sent to the VC (like a
> firmware property request).  By setting dma-ranges, all of the devices
> below it get a dev->dma_pfn_offset, so that dma_alloc_coherent() and
> friends return addresses with 0x4 bits and avoid cache incoherency.
> 
> This matches the behavior in the downstream 2708 kernel (see
> BUS_OFFSET in arch/arm/mach-bcm2708/include/mach/memory.h).
> 
> Signed-off-by: Eric Anholt <eric@...olt.net>
> Tested-by: Noralf Trønnes <noralf@...nnes.org>
> Acked-by: Stephen Warren <swarren@...dotorg.org>
> Cc: popcornmix@...il.com

Applied, thanks.

> ---
> 
> v2: Fix length of the range from 0x1f000000 to 0x20000000, fixing the
>     translation for the last 16MB.
> 
>  arch/arm/boot/dts/bcm2835.dtsi | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/arm/boot/dts/bcm2835.dtsi b/arch/arm/boot/dts/bcm2835.dtsi
> index eb33a8c..3c899b3 100644
> --- a/arch/arm/boot/dts/bcm2835.dtsi
> +++ b/arch/arm/boot/dts/bcm2835.dtsi
> @@ -15,6 +15,7 @@
>  		#address-cells = <1>;
>  		#size-cells = <1>;
>  		ranges = <0x7e000000 0x20000000 0x02000000>;
> +		dma-ranges = <0x40000000 0x00000000 0x20000000>;
>  
>  		timer@...03000 {
>  			compatible = "brcm,bcm2835-system-timer";
--
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