Re: [PATCH v2 1/3] clk: Introduce a clock request API

[Stephen Boyd <sboyd@kernel.org>]

Quoting Maxime Ripard (2022-01-12 03:46:52)
> Hi Stephen,
> 
> Thanks for your answer
> 
> On Tue, Jan 11, 2022 at 07:37:15PM -0800, Stephen Boyd wrote:
> > Sorry for being super delayed on response here. I'm buried in other
> > work. +Jerome for exclusive clk API.
> > 
> > Quoting Maxime Ripard (2021-09-14 02:35:13)
> > > It's not unusual to find clocks being shared across multiple devices
> > > that need to change the rate depending on what the device is doing at a
> > > given time.
> > > 
> > > The SoC found on the RaspberryPi4 (BCM2711) is in such a situation
> > > between its two HDMI controllers that share a clock that needs to be
> > > raised depending on the output resolution of each controller.
> > > 
> > > The current clk_set_rate API doesn't really allow to support that case
> > > since there's really no synchronisation between multiple users, it's
> > > essentially a fire-and-forget solution.
> > 
> > I'd also say a "last caller wins"
> > 
> > > 
> > > clk_set_min_rate does allow for such a synchronisation, but has another
> > > drawback: it doesn't allow to reduce the clock rate once the work is
> > > over.
> > 
> > What does "work over" mean specifically? Does it mean one of the clk
> > consumers has decided to stop using the clk?
> 
> That, or it doesn't need to enforce that minimum anymore. We have
> several cases like this on the RPi. For example, during a change of
> display mode a (shared) clock needs to be raised to a minimum, but
> another shared one needs to raise its minimum based on the resolution.
> 
> In the former case, we only need the minimum to be enforced during the
> resolution change, so it's fairly quick, but the latter requires its
> minimum for as long as the display is on.
> 
> > Why doesn't clk_set_rate_range() work? Or clk_set_rate_range() combined
> > with clk_set_rate_exclusive()?
> 
> clk_set_rate_range could work (it's what we have right now in mainline
> after all), but it's suboptimal since the clock is never scaled down.

Alright, I didn't see any mention of clk_set_rate_range() in the commit
text so did I miss it? Maybe it's used interchangeably with
clk_set_min_rate()?

> 
> It's especially showing in my first example where we need to raise the
> clock only for the duration of the resolution change. Using
> clk_set_min_rate works but we end up with that fairly high clock (at
> least 500MHz) for the rest of the system life even though we usually can
> get away with using a clock around 200MHz outside of that (short) window.
> 
> This is fairly inefficient, and is mostly what I'm trying to address.

Got it!

> 
> > > In our previous example, this means that if we were to raise the
> > > resolution of one HDMI controller to the largest resolution and then
> > > changing for a smaller one, we would still have the clock running at the
> > > largest resolution rate resulting in a poor power-efficiency.
> > 
> > Does this example have two HDMI controllers where they share one clk and
> > want to use the most efficient frequency for both of the HDMI devices? I
> > think I'm following along but it's hard. It would be clearer if there
> > was some psuedo-code explaining how it is both non-workable with current
> > APIs and workable with the new APIs.
> 
> The fact that we have two HDMI controllers that share one clock is why
> we use clk_set_min_rate in the first place, but you can have that
> behavior with clk_set_min_rate only with a single user.
> 
> With pseudo-code, if you do something like
> 
> clk = clk_get(NULL);
> clk_set_min_rate(600 * 1000 * 1000);
> clk_set_min_rate(1000);
> 
> The clock will still remain at 600MHz, even though you would be totally
> fine with the clock running at 1kHz.

That looks like a bug. While we could happily ignore the rate floor
being lowered because we're still within constraints, it looks like we
should always re-evaluate the constraints when they change.

> 
> If you really wanted to make the clock run at 1kHz, you'd need to have:
> 
> clk = clk_get(NULL);
> clk_set_min_rate(600 * 1000 * 1000);
> clk_set_min_rate(1000);
> clk_set_rate(1000);
> 
> And that works fine for a single user.
> 
> If you have a clock shared by multiple drivers though, things get
> tricky. Indeed, you can't really find out what the minimum for that
> clock is, so figuring out the rate to pass to the clk_set_rate call
> would be difficult already. And it wouldn't be atomic anyway.

Right.

> 
> It's made even more difficult since in clk_calc_new_rates the core
> checks that the rate is within the boundaries and will error out if it
> isn't, so even using clk_set_rate(0) wouldn't work.

clk_set_rate(0) is pretty gross!

> 
> It could work if the clock driver makes sure in round/determine_rate
> that the rate passed in within the boundaries of the clock, but then you
> start working around the core and relying on the behavior of clock
> drivers, which is a fairly significant abstraction violation.
> 
> > > In order to address both issues, let's create an API that allows user to
> > > create temporary requests to increase the rate to a minimum, before
> > > going back to the initial rate once the request is done.
> > > 
> > > This introduces mainly two side-effects:
> > > 
> > >   * There's an interaction between clk_set_rate and requests. This has
> > >     been addressed by having clk_set_rate increasing the rate if it's
> > >     greater than what the requests asked for, and in any case changing
> > >     the rate the clock will return to once all the requests are done.
> > > 
> > >   * Similarly, clk_round_rate has been adjusted to take the requests
> > >     into account and return a rate that will be greater or equal to the
> > >     requested rates.
> > > 
> > 
> > I believe clk_set_rate_range() is broken but it can be fixed. I'm
> > forgetting the details though. If the intended user of this new API
> > can't use that range API then it would be good to understand why it
> > can't be used. I imagine it would be something like
> > 
> >       struct clk *clk_hdmi1, *clk_hdmi2;
> > 
> >       clk_set_rate_range(&clk_hdmi1, HDMI1_MIN, HDMI1_MAX);
> >       clk_set_rate_range(&clk_hdmi2, HDMI2_MIN, HDMI2_MAX);
> >       clk_set_rate_range(&clk_hdmi2, 0, UINT_MAX);
> > 
> > and then the goal would be for HDMI1_MIN to be used, or at the least for
> > the last call to clk_set_rate_range() to drop the rate constraint and
> > re-evaluate the frequency of the clk again based on hdmi1's rate range.
> 
> This is pretty much what this series was doing. I was being conservative
> and didn't really want to modify the behavior of existing functions, but
> that will work fine.
> 

I don't see a problem with re-evaluating the rate every time we call
clk_set_rate_range(). That's probably the bug that I can't recall. Can
you fix the API so it works that way?