Kumar Gala wrote:
On Aug 16, 2010, at 3:17 PM, Mitch Bradley wrote:
Kumar Gala wrote:
Do we or should we have a standard property to convey that address width a
device is capable of?
...
If I had to describe a partial address, I'd consider a property name like
"dma-address-mask", whose value is a bitmap of implemented bits, corresponding
to the bits in a unit address for the parent bus. Low-order bits might be zero if the
DMA addressing hardware had alignment restrictions.
As I said to Stuart. On the Freescale SOCs we have different device blocks
w/varying dma address capabilities. Some are limited to 32-bits some are
capable of 36-bits on the same SOC.
- k
The closest existing property that I know of is "dma-ranges" - see
http://playground.sun.com/1275/proposals/Closed/Accepted/410-it.txt
That's not directly applicable, as "dma-ranges" in its current form
applies to bus bridges, describing the translation between DMA addresses
on a child bus to DMA addresses on a parent bus.
The case in question has some general similarities, in that the limited
devices have an implicit 32-bit "child bus" that is translated to the
36-bit parent bus, presumably by concatenating zeros in bits 35:32. Is
that correct, i.e. are the high bits implicitly 0?
The dma-ranges representation doesn't assume the "zero-extend" property,
but rather explicitly lists child-base-address,parent-base-address,size
triples. I wonder if that generality is justifiable in this case?
I see several obvious representations with varying degrees of generality:
a) boolean property saying "this is one of those 32-bit only devices,
with all the implications"
b) width in bits - assumes an implicit translation rule (e.g. zero-extend)
c) bitmask - assumes implicit translation rule, capable of representing
alignment restrictions with low-order zeros
d) something like dma-ranges - explicitly represents translation rule,
no representation for alignment restriction
My 2 cents: Generality is often only justified when you have a good
collection of problem instances, so you can amortize the generality over
several specific known examples. Otherwise you often end up
implementing something elaborate that never gets used.
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