• Eric Anholt's avatar
    ARM: bcm2835: dt: Use 0x4 prefix for DMA bus addresses to SDRAM. · 1215baa7
    Eric Anholt authored
    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: default avatarEric Anholt <eric@anholt.net>
    Tested-by: default avatarNoralf Trønnes <noralf@tronnes.org>
    Acked-by: default avatarStephen Warren <swarren@wwwdotorg.org>
    Signed-off-by: default avatarLee Jones <lee.jones@linaro.org>
    1215baa7
bcm2835.dtsi 4.04 KB