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

[Michal Suchanek]

On 29 August 2014 16:38, Thierry Reding <thierry.reding at gmail.com> 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.
>
>> > 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.

And for simplefb to work it should claim the memory and clock so that
nothing else in the kernel uses them or reclaims them as unused. That
currently the memory is reserved by excluding it from memory usable by
the kernel is a hack, not proper resource management. And so is
keeping all clocks enabled in case something used them.

>
>> > so by any definition of what DT should describe they're useless for
>> > this virtual device.
>> >
>> > Furthermore it's fairly likely that as your kernel support progresses
>> > you'll find that the driver all of a sudden needs to manage some other
>> > type of resource that you just haven't needed until now because it may
>> > default to being always on. Then you'll have a hard time keeping
>> > backwards-compatibility and will have to resort to the kinds of hacks
>> > that you don't want to see in the kernel.
>>
>> Not such a hard time. An older DT wouldn't define the new requirements
>> anyway, so they wouldn't be used, and we would end up in pretty much
>> the current case.
>
> Except that you have firmware in the wild that sets up an incomplete
> simplefb node and if you don't want to break compatibility you need to
> provide fallbacks for the resources that aren't listed in the DT node.
> And given that those fallbacks are all very board specific you'll need
> to find ways to keep them enabled if you want to keep existing setups
> working.
>

Except in this case either the simplefb does not need clocks the user
had to set up the kernel argument to not disable unused clocks so the
user of such firmware is well aware of that limitation.

Thanks

Michal