Hi,
On Tue, 2009-06-02 at 15:22 +0000, James Bottomley wrote:
> So what we're looking for is a proposal to discuss the issues
> most affecting embedded architectures, or preview any features affecting
> the main kernel which embedded architectures might need ... or any other
> topics from embedded architectures which might need discussion or
> debate.
Some issues that come up on embedded systems (and not only):
* Multiple coherency domains for devices - the system may have
multiple bus levels, coherency ports, cache levels etc. Some
devices in the system (but not all) may be able to "see" various
cache levels but the DMA API (at least on ARM) cannot handle
this. It may be useful to discuss how other embedded
architectures handle this and come up with a unified solution
* Better support for coherent DMA mask - currently ZONE_DMA is
assumed to be in the bottom part of the memory which isn't
always the case. Enabling NUMA may help but it is overkill for
some systems. As above, a more unified solution across
architectures would help
* PIO block devices and non-coherent hardware - code like mpage.c
assumes that the either the hardware is coherent or the device
driver performs the cache flushing. The latter is true for
DMA-capable device but not for PIO. The issue becomes visible
with write-allocate caches and the device driver may not have
the struct page information to call flush_dcache_page(). A
proposed solution on the ARM lists was to differentiate (via
some flags) between PIO and DMA block devices and use this
information in mpage.c
* Mixed endianness devices in the same system - this may only need
dedicated readl_be/writel_be etc. macros but it could also be
done by having bus-aware readl/writel-like macros
* Asymmetric MP:
* Different CPU frequencies
* Different CPU features (e.g. floating point only one
some CPUs): scheduler awareness, per-CPU hwcap bits (in
case user space wants to set the affinity)
* Asymmetric workload balancing for power consumption (may
be better to load 1 CPU at 60% than 4 at 15%)
Thanks.
--
Catalin
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