Re: [linux-sunxi] Re: [PATCH 4/4] simplefb: add clock handling code

[Michal Suchanek]

On 29 September 2014 10:54, Thierry Reding <thierry.reding@xxxxxxxxx> wrote:
> On Mon, Sep 29, 2014 at 10:27:41AM +0200, Geert Uytterhoeven wrote:
>> Hi Thierry,
>>
>> (CC linux-pm, as PM is the real reason behind disabling unused clocks)
>> (CC gregkh and lkml, for driver core)
>>
>> On Mon, Sep 29, 2014 at 10:06 AM, Thierry Reding
>> <thierry.reding@xxxxxxxxx> wrote:
>> > On Sat, Sep 27, 2014 at 04:56:01PM -0700, Mike Turquette wrote:
>> >> Quoting Maxime Ripard (2014-09-02 02:25:08)
>> >> > On Fri, Aug 29, 2014 at 04:38:14PM +0200, Thierry Reding wrote:
>> >> > > On Fri, Aug 29, 2014 at 04:12:44PM +0200, Maxime Ripard wrote:
>> >> > > > On Fri, Aug 29, 2014 at 09:01:17AM +0200, Thierry Reding wrote:
>> >> > > > > I would think the memory should still be reserved anyway to make sure
>> >> > > > > nothing else is writing over it. And it's in the device tree anyway
>> >> > > > > because the driver needs to know where to put framebuffer content. So
>> >> > > > > the point I was trying to make is that we can't treat the memory in the
>> >> > > > > same way as clocks because it needs to be explicitly managed. Whereas
>> >> > > > > clocks don't. The driver is simply too generic to know what to do with
>> >> > > > > the clocks.
>> >> > > >
>> >> > > > You agreed on the fact that the only thing we need to do with the
>> >> > > > clocks is claim them. Really, I don't find what's complicated there
>> >> > > > (or not generic).
>> >> > >
>> >> > > That's not what I agreed on. What I said is that the only thing we need
>> >> > > to do with the clocks is nothing. They are already in the state that
>> >> > > they need to be.
>> >> >
>> >> > Claim was probably a poor choice of words, but still. We have to keep
>> >> > the clock running, and both the solution you've been giving and this
>> >> > patch do so in a generic way.
>> >> >
>> >> > > > > It doesn't know what frequency they should be running at
>> >> > > >
>> >> > > > We don't care about that. Just like we don't care about which
>> >> > > > frequency is the memory bus running at. It will just run at whatever
>> >> > > > frequency is appropriate.
>> >> > >
>> >> > > Exactly. And you shouldn't have to care about them at all. Firmware has
>> >> > > already configured the clocks to run at the correct frequencies, and it
>> >> > > has made sure that they are enabled.
>> >> > >
>> >> > > > > or what they're used for
>> >> > > >
>> >> > > > And we don't care about that either. You're not interested in what
>> >> > > > output the framebuffer is setup to use, which is pretty much the same
>> >> > > > here, this is the same thing here.
>> >> > >
>> >> > > That's precisely what I've been saying. The only thing that simplefb
>> >> > > cares about is the memory it should be using and the format of the
>> >> > > pixels (and how many of them) it writes into that memory. Everything
>> >> > > else is assumed to have been set up.
>> >> > >
>> >> > > Including clocks.
>> >> >
>> >> > We're really discussing in circles here.
>> >> >
>> >> > Mike?
>> >> >
>> >>
>> >> -EHIGHLATENCYRESPONSE
>> >>
>> >> I forgot about this thread. Sorry.
>> >>
>> >> In an attempt to provide the least helpful answer possible, I will stay
>> >> clear of all of the stuff relating to "how simple should simplefb be"
>> >> and the related reserved memory discussion.
>> >>
>> >> A few times in this thread it is stated that we can't prevent unused
>> >> clocks from being disabled. That is only partially true.
>> >>
>> >> The clock framework DOES provide a flag to prevent UNUSED clocks from
>> >> being disabled at late_initcall-time by a clock "garbage collector"
>> >> mechanism. Maxime and others familiar with the clock framework are aware
>> >> of this.
>> >>
>> >> What the framework doesn't do is to allow for a magic flag in DT, in
>> >> platform_data or elsewhere that says, "don't let me get turned off until
>> >> the right driver claims me". That would be an external or alternative
>> >> method for preventing a clock from being disabled. We have a method for
>> >> preventing clocks from being disabled. It is as follows:
>> >>
>> >> struct clk *my_clk = clk_get(...);
>> >> clk_prepare_enable(my_clk);
>> >>
>> >> That is how it should be done. Period.
>> >>
>> >> With that said I think that Luc's approach is very sensible. I'm not
>> >> sure what purpose in the universe DT is supposed to serve if not for
>> >> _this_exact_case_. We have a nice abstracted driver, usable by many
>> >> people. The hardware details of how it is hooked up at the board level
>> >> can all be hidden behind stable DT bindings that everyone already uses.
>> >
>> > simplefb doesn't deal at all with hardware details. It simply uses what
>> > firmware has set up, which is the only reason why it will work for many
>>
>> It doesn't deal with "hardware details for hardware components for which
>> no driver is available (yet)". That's why the hardware is still in a usable
>> state, after the firmware has set it up.
>>
>> Clocks, regulators, PM domains typically have system-wide implications,
>> and thus need system-wide drivers (in the absence of such drivers,
>> things would work as-is).
>>
>> Note that the driver still requests resources (ioremap the frame buffer),
>> so it needs to know about that tiny piece of hardware detail.
>
> That's not a hardware detail. Or at least it isn't hardware specific. It
> is needed and the same irrespective of display hardware. Clocks, power
> domains, regulators and all those are not always the same.

The framebuffer address, format, etc. is as hardware specific as the
clocks needed to run the crtc, regulators to power the backlight, etc.

You see this from the point of view of a platform that has not clock
driver. Then the platform has no clock information whatsoever, for any
driver. That's platform specific too.

Why do we have to go back to this *again*?

>
>> > There may be also resets involved. Fortunately the reset framework is
>> > minimalistic enough not to care about asserting all unused resets at
>> > late_initcall. And other things like power domains may also need to be
>> > kept on.
>>
>> Fortunately, unlike clocks, PM domains are first class citizens in the
>> device framework, as they're handled by the driver core.
>> So just adding a power-domains property to DTS will work, without any
>> driver change.
>
> Well, the device driver would also need to call into the PM runtime
> framework to enable all the first class citizen magic. But even if it
> were to do that, you'd still need to add all the domains to the DTB.
>
> Note that I'm saying DT*B* here, because the firmware needs to fill in
> those properties after the DTS has been compiled. And since most of
> these resources are linked via phandle you actually need to resolve
> these first before you can fill in the new properties of this
> dynamically created node.
>
> So firmware needs to know exactly what device tree node to look for,
> find a corresponding phandle and then put the phandle value in the
> simplefb device tree node. And it needs to know intimate details about
> the clock provider binding because it needs to add an appropriate
> specifier, too.
>
> And then all of a sudden something that was supposed to be simple and
> generic needs to know the specifics of some hardware device.

Note however that all the specific is in the firmware driver. The
linux simplefb driver only needs to read DT entries which is generic
hardware-neutral thing. It needs to handle clocks on platform that do
have a clock driver, yes. There is no way around that. See below.

>
>> > Passing in clock information via the device tree already requires a non-
>> > trivial amount of code in the firmware. A similar amount of code would
>> > be necessary for each type of resource that needs to be kept enabled. In
>> > addition to the above some devices may also require resources that have
>> > no generic bindings. That just doesn't scale.
>>
>> [*] The firmware does need to make sure the clocks, regulators, PM domains,
>> ... are up and running for the initial video mode, too. So it already needs
>> to have this knowledge (unless enabled by SoC reset-state).
>
> Certainly. But not all firmware will use a DTB (or in fact the same DTB
> as the kernel) to derive this information from, so it still needs to
> inspect the DTB that will be passed to the kernel and painfully extract
> information from various nodes and put it into a new node.
>
> But that's not even the issue. You say yourself that the firmware set
> everything up itself already. My point is: why should the kernel have to
> do everything again, only to come to the conclusion that it doesn't have
> to touch hardware at all because it's already in the state that it
> should be?

It will do nothing. It will just read from DTB what firmware has set
up and inform all the relevant generic frameworks that these resources
are in use. The device-specific driver (if any) will then keep the
resource in question enabled. So all that is this patch doing is to
correct the kernel's resource bookkeeping.

>
> In fact, Linux will at some point set things up from scratch anyway with
> a fully-fledged driver. But doing it in simplefb too would be doubly
> wasteful.

It may or may not. And by having the bookkeeping straight we give that
choice to the user.

>
>> > The only reasonable thing for simplefb to do is not deal with any kind
>> > of resource at all (except perhaps area that contains the framebuffer
>> > memory).
>> >
>> > So how about instead of requiring resources to be explicitly claimed we
>> > introduce something like the below patch? The intention being to give
>> > "firmware device" drivers a way of signalling to the clock framework
>> > that they need rely on clocks set up by firmware and when they no longer
>> > need them. This implements essentially what Mark (CC'ing again on this
>> > subthread) suggested earlier in this thread. Basically, it will allow
>> > drivers to determine the time when unused clocks are really unused. It
>> > will of course only work when used correctly by drivers. For the case of
>> > simplefb I'd expect its .probe() implementation to call the new
>> > clk_ignore_unused() function and once it has handed over control of the
>> > display hardware to the real driver it can call clk_unignore_unused() to
>> > signal that all unused clocks that it cares about have now been claimed.
>> > This is "reference counted" and can therefore be used by more than a
>> > single driver if necessary. Similar functionality could be added for
>> > other resource subsystems as needed.
>>
>> This still won't work for modules, right? Or am I missing something?
>> With modules you will never know in advance what will be used and what
>> won't be used, so you need to keep all clocks, regulators, PM domains, ...
>> up and running?

Simplefb cannot work as module on any platform. Once kernel reclaims
the resources used by simplefb it cannot be started because kernel has
no way to reenable the resources (memory, clocks, etc). Reclaiming the
simplefb memory is currently not supported but once it is added
loading simplefb as module is no-go on any platform.

>
> No. The way this works is that your firmware shim driver, simplefb in
> this case, will call clk_ignore_unused() to tell the clock framework
> that it uses clocks set up by the firmware, and therefore requests that
> no clocks should be considered unused (for now). Later on when the
> proper driver has successfully taken over from the shim driver, the shim
> driver can unregister itself and call clk_unignore_unused(), which will
> drop its "reference" on the unused clocks. When all references have been
> dropped the clock framework will then disable all remaining unused
> clocks.

> In practice this will mean that all unused clocks will remain in their
> current state until the shim driver relinquishes control to the proper
> OS driver. And if that never happens then we're at least still left with
> a working framebuffer.

This does not work. Firstly if you do not have a full driver (because
there is none or because kernel did not find the module, ...) then you
could run with the shim driver and have all clocks managed properly.
This is not possible with clk_ignore_unused.

Secondly clk_ignore_unused does not really mark clock as used. It only
skips disabling them at certain point at kernel startup so that kernel
survives that point and clock problems can be debugged. It is not
meant as an option to be used for running the kernel indefinitely. It
is indeed flawed: if at later point enabling or disabling a clock
results in a parent clock becoming unused it is still disabled. This
can affect many clocks if enabling a clock results in clock
reparenting.

Sure, clk_ignore_unused could be fixed to actually mark all clocks
enabled at boot time as used so that they are never disabled but
that's orthogonal to fixing simplefb so that when it is in use the
clock framework can work normally and can be tested, debugged, and
used for saving power by disabling unused clocks.

So please stop repeating that managing system resources is
system-specific and not fit for simplefb driver. It's been said enough
times, and enough times pointed out that simplefb driver must as any
other driver manage its resources (or more specifically tell the
kernel to manage them) to work properly. And that reading a list of
resources from DT and telling kernel to manage them is not system
specific, only specific to resource frameworks enabled on the platform
in question. That is you may need support for as many resource
frameworks as the platform has if they are ever used for framebuffer.

Thanks

Michal
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