Re: [PATCH 0/5] clk: rockchip: add full support for HDMI clock on rk3288

From: Tomeu Vizoso
Date: Fri Jan 22 2016 - 09:01:06 EST

On 21 January 2016 at 21:11, Doug Anderson <dianders@xxxxxxxxxxxx> wrote:
> Hi,
> On Thu, Jan 21, 2016 at 1:03 AM, Tomeu Vizoso <tomeu@xxxxxxxxxxxxxxx> wrote:
>> So we have a mechanism for detecting a conflict in the clock
>> hierarchy, and a mechanism to solve it, but we are missing a way for
>> userspace to communicate policy regarding which clocks should be given
>> priority when solving such a conflict?
> Hrmmm, I guess it could be userspace that makes the decision. It does
> seem a little odd to force it to userspace in all cases, though. For
> a particular laptop that is designed with a specific panel connected
> up eDP it seems less than ideal to push this into userspace. If the
> kernel could just work in the expected sane way (or at least work that
> way by default) it would be ideal.

Ah, I was wrongly assuming that the kernel didn't have enough
information to make an informed decision in this case, sorry.

Guess the per-user rate limits don't help here because the consumer
with higher priority could work with frequencies other than the ideal.

And we cannot have a consumer listening for PRE_RATE_CHANGE and
aborting unwanted changes or rerouting the ancestors of the clocks of
other consumers because that would be a massive violation of
separation of concerns.

If we were to rearrange the clock topology from within the CCF, then
consumers need to have a way to communicate to the core that they are
more important than other consumers. clk_set_important(clk, true)
could be enough in this case, but would be insufficient in more
complex cases where more than two clocks could use the same PLL.

> If the kernel doesn't try to do anything sane by default then you're
> creating a requirement for everyone's userspace to somehow figure this
> out. Do you expect there to be UI here, or that this would be
> something that would be figured out by the Linux distribution?
> Certainly exposing UI on something like a laptop with a builtin panel
> wouldn't make any sense to me, but it might make sense if you had an
> eval board with different display connectors on it. If there's no UI,
> would the Linux distribution need to somehow identify which board we
> were on and then have a big lookup table about how to configure
> things?

If we don't actually need input from userspace for this use case, I
wouldn't go this way right now, because it seems to me like it could
be a really big timesink for little gain.

Once someone comes with a situation in which feedback from userspace
is really needed, that person can propose such an interface ;)