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

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Message-ID: <87zj584boh.fsf@eliezer.anholt.net>
Date:	Wed, 13 May 2015 10:41:50 -0700
From:	Eric Anholt <eric@...olt.net>
To:	Lee Jones <lee@...nel.org>
Cc:	Stephen Warren <swarren@...dotorg.org>,
	linux-arm-kernel@...ts.infradead.org,
	linux-rpi-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	popcornmix@...il.com
Subject: Re: [PATCH] ARM: bcm2835: Use 0x4 prefix for DMA bus addresses to SDRAM.

Lee Jones <lee@...nel.org> writes:

> On Tue, 05 May 2015, Eric Anholt wrote:
>
>> Stephen Warren <swarren@...dotorg.org> writes:
>> 
>> > 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,
>> 
>> Nope, you're right, it should be 0x20000000.  '0x1f' came from going
>> back from the '0x3f' on the pi2, but pi2 just has a chunk lost to the
>> bus mapping.
>
> So are you going to fix this and send another patch?

I see it having hit the list:

http://lists.infradead.org/pipermail/linux-rpi-kernel/2015-May/001699.html

but I'm missing both versions in my inbox, so I'm not sure what
happened.

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