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

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Date:	Mon, 04 May 2015 22:25:30 +0200
From:	Noralf Trønnes <noralf@...nnes.org>
To:	Eric Anholt <eric@...olt.net>, linux-arm-kernel@...ts.infradead.org
CC:	linux-kernel@...r.kernel.org, linux-rpi-kernel@...ts.infradead.org
Subject: Re: [PATCH] ARM: bcm2835: Use 0x4 prefix for DMA bus addresses to
 SDRAM.


Den 04.05.2015 21:33, skrev Eric Anholt:
> 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";

This was quite a coincidence. I discovered the need for 'dma-ranges'
yesterday while trying to get the downstream bcm2708_fb driver to
work with ARCH_BCM2835. The driver is using the mailbox to get info
about the framebuffer from the firmware. When it failed I discovered
that the bus address was wrong.

What I don't understand, is that mmc and spi works fine with a "wrong"
bus address. It's only the framebuffer driver and the vchiq driver
when using mailbox that fails.

Tested-by: Noralf Trønnes <noralf@...nnes.org>


Regards,
Noralf Trønnes

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