On 11/15/2010 10:57 PM, Grant Likely wrote:
...
I'm not sure I completely follow you. Any memory described in a
device_type="memory" node will by default be used as system RAM by
Linux. If some of that region needs to be used for DMA buffers, then
you'll need to make sure that when a driver allocates a buffer, that
the buffer is made valid for DMA operations. How you do this is
architecture specific.
Okay, I think I've got this down
...
what does 'stbus' mean? If it is the hardware name for the bus, then
it is probably a good choice. 'soc' is often used, but I'm not a big
fan of that convention because it doesn't reflect the actual internal
architecture of the chip.
Yes, this is what STMicroelectronics call their bus, specifically
"STBus". Which looks like what the device tree calls a simple- bus.
...
compatible = "simple-bus";
#address-cells =<1>;
#size-cells =<1>;
dma-ranges =<0x10000000 0x1000000 0x0fc00000
0x20000000 0x10000000 0x0fc00000
0x2fc00000 0x2fc00000 0x10400000
0x60000000 0x60000000 0x20000000>;
...
<digress into details of hardware design>
Is a device legally able to DMA into the address range
0x20000000..0x2fbfffff instead of 0x10000000..0x1fbfffff? What is the
reason for dma'ing to base 0x10000000 for the first 0xfc00000, and to
base 0x20000000 for the second if the two ranges are simple aliases of
each other?
From your earlier description it sounded like the 0x10000000 alias is
only to provide Linux with RAM to run out of, but the real ram base
address is 0x20000000. As such, it still sounds like the dma-ranges
should cover the entire physical memory region from
0x20000000..0x3fffffff, and not reference the 0x10000000 alias at all.
</digress>
Think I've got it--the 0x10000000-0x1fc00000 is not visible to DMA
devices, so it doesn't belong in the dma-ranges, only those address that
can actually be given to DMA devices. So, I'll drop the first triplet
and keep the others.
...
I'd just go with the explicit ranges simply because it is simpler. If
it was a more common use-case, then I might have a different opinion,
but since it is an oddball case I'd stick with familiar patterns for
specifying dma constraints.
Sounds good.
g.
Thanks!
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