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

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Date:	Fri, 22 Jan 2016 15:00:27 +0100
From:	Tomeu Vizoso <tomeu@...euvizoso.net>
To:	Doug Anderson <dianders@...omium.org>
Cc:	Kever Yang <kever.yang@...k-chips.com>,
	Heiko Stübner <heiko@...ech.de>,
	Sonny Rao <sonnyrao@...omium.org>,
	Addy Ke <addy.ke@...k-chips.com>,
	Eddie Cai <cf@...k-chips.com>,
	ZhenFu Fang <fzf@...k-chips.com>,
	Yakir Yang <ykk@...k-chips.com>,
	姚智情 <yzq@...k-chips.com>,
	戴克霖 (Jack) <dkl@...k-chips.com>,
	Tao Huang <huangtao@...k-chips.com>,
	"open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-clk <linux-clk@...r.kernel.org>,
	Michael Turquette <mturquette@...libre.com>,
	Stephen Boyd <sboyd@...eaurora.org>
Subject: Re: [PATCH 0/5] clk: rockchip: add full support for HDMI clock on rk3288

On 21 January 2016 at 21:11, Doug Anderson <dianders@...omium.org> wrote:
> Hi,
>
> On Thu, Jan 21, 2016 at 1:03 AM, Tomeu Vizoso <tomeu@...euvizoso.net> 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 ;)

Regards,

Tomeu