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

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Date:	Mon,  4 May 2015 12:33:54 -0700
From:	Eric Anholt <eric@...olt.net>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	linux-rpi-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Stephen Warren <swarren@...dotorg.org>,
	Lee Jones <lee@...nel.org>, Eric Anholt <eric@...olt.net>,
	popcornmix@...il.com
Subject: [PATCH] ARM: bcm2835: Use 0x4 prefix for DMA bus addresses to SDRAM.

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>
Cc: popcornmix@...il.com
---
 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 5734650..2df1b5c 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 0x1f000000>;

 		timer@...03000 {
 			compatible = "brcm,bcm2835-system-timer";
-- 
2.1.4

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