Re: [RFC PATCH 3/3] clk: tegra: Implement Tegra124 shared/cbus clks

Andrew Bresticker
2014-05-14T17:58:31Z

On Wed, May 14, 2014 at 7:27 AM, Thierry Reding
<thierry.reding@gmail.com> wrote:
> As for shared clocks I'm only aware of one use-case, namely EMC scaling.
> Using clocks for that doesn't seem like the best option to me. While it
> can probably fix the immediate issue of choosing an appropriate
> frequency for the EMC clock it isn't a complete solution for the problem
> that we're trying to solve. From what I understand EMC scaling is one
> part of ensuring quality of service. The current implementations of that
> seems to abuse clocks (essentially one X.emc clock per X clock) to
> signal the amount of memory bandwidth required by any given device. But
> there are other parts to the puzzle. Latency allowance is one. The value
> programmed to the latency allowance registers for example depends on the
> EMC frequency.
>
> Has anyone ever looked into using a different framework to model all of
> these requirements? PM QoS looks like it might fit, but if none of the
> existing frameworks have what we need, perhaps something new can be
> created.

On Exynos we use devfreq, though in that case we monitor performance
counters to determine how internal buses should be scaled - not sure
if Tegra SoCs have similar counters that could be used for this
purpose. It seems like EMC scaling would fit nicely within the PM QoS
framework, perhaps with a new PM_QOS_MEMORY_THROUGHPUT class.

-Andrew