From mboxrd@z Thu Jan 1 00:00:00 1970 From: swarren@wwwdotorg.org (Stephen Warren) Date: Tue, 05 May 2015 13:29:22 -0600 Subject: [PATCH] ARM: bcm2835: Use 0x4 prefix for DMA bus addresses to SDRAM. In-Reply-To: <1430768034-12734-1-git-send-email-eric@anholt.net> References: <1430768034-12734-1-git-send-email-eric@anholt.net> Message-ID: <55491A12.3070608@wwwdotorg.org> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.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