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

[Stephen Warren <swarren@wwwdotorg.org>]

On 05/04/2015 01:33 PM, 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).

> diff --git a/arch/arm/boot/dts/bcm2835.dtsi b/arch/arm/boot/dts/bcm2835.dtsi

>   		#address-cells = <1>;
>   		#size-cells = <1>;
>   		ranges = <0x7e000000 0x20000000 0x02000000>;
> +		dma-ranges = <0x40000000 0x00000000 0x1f000000>;

Oh well that's a nice and simple patch; I had been avoiding looking into 
fixing the kernel for this since I was worried it'd be rather complex!

I'm puzzled why the length cell of ranges and dma-ranges differs though? 
Assuming there's a good explanation for that,

Acked-by: Stephen Warren <swarren@wwwdotorg.org>