[linux-sunxi] Re: [PATCH 4/4] simplefb: add clock handling code
julian.calaby at gmail.com
Mon Sep 29 07:46:11 PDT 2014
On Mon, Sep 29, 2014 at 11:21 PM, Thierry Reding
<thierry.reding at gmail.com> wrote:
> On Mon, Sep 29, 2014 at 09:00:01PM +1000, Julian Calaby wrote:
>> Hi Thierry,
> If you address people directly please make sure they are in the To:
> line. Or at least Cc.
Sorry about that, the mailing list I received this through (Google
Groups based) generally strips to: and CC: lines, so my mail client
(Gmail) doesn't do it automatically. I'm still getting used to it.
>> On Mon, Sep 29, 2014 at 8:18 PM, Thierry Reding
>> <thierry.reding at gmail.com> wrote:
>> > On Mon, Sep 29, 2014 at 11:23:01AM +0200, Maxime Ripard wrote:
>> >> On Mon, Sep 29, 2014 at 10:06:39AM +0200, Thierry Reding wrote:
>> > [...]
>> >> > 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
>> >> > people. What is passed in via its device tree node is the minimum amount
>> >> > of information needed to draw something into the framebuffer. Also note
>> >> > that the simplefb device tree node is not statically added to a DTS file
>> >> > but needs to be dynamically generated by firmware at runtime.
>> >> Which makes the whole even simpler, since the firmware already knows
>> >> all about which clocks it had to enable.
>> > It makes things very complicated in the firmware because it now needs to
>> > be able to generate DTB content that we would otherwise be able to do
>> > much easier with a text editor.
>> As far as the kernel is concerned, this is a solved problem.
> It's not completely solved. There's still the issue of no generic way to
> specify regulators like you can do for clocks, resets or power domains.
> But the kernel isn't the real issue here. The issue is the firmware that
> now has to go out of its way not only to initialize display hardware but
> also create device tree content just to make Linux not turn everything
My point is that the firmware is going to be doing complicated stuff
already, adding and using some helpers to configure a device tree node
is relatively simple in comparison to dealing with the actual
hardware. It wouldn't surprise me if u-boot, for example, ended up
with a set of functions to handle this exact case as more graphics
hardware gets brought up.
>> Firmware is going to be doing some dark magic to set up the hardware
>> to be a dumb frame buffer and some other stuff to add the simplefb
>> device node - so by this point, adding the clocks (or whatever)
>> required by the hardware should be fairly uncomplicated - the firmware
>> already knows the hardware intimately. As for the actual device tree
>> manipulations, U-boot (or whatever) will probably just grow some
>> helper functions to make this simple.
> Have you looked at the code needed to do this? It's not at all trivial.
> And the point is really that all this information is there already, so
> we're completely duplicating this into a dynamically created device tree
> node and for what reason? Only to have one driver request all these
> resources and have them forcefully released a few seconds later.
>> Alternatively, it could simply add the relevant data to an existing
>> device node and munge it's compatible property so simplefb picks it
> Yes, I think that'd be a much better solution. Of course it's going to
> be very difficult to make that work with a generic driver because now
> that generic driver needs to parse the DT binding for any number of
> "compatible" devices.
The patch that started this discussion can work with any number of
clocks specified in a "clocks" property. Therefore all that needs to
happen is that the final hardware binding specifies it's clocks that
way. This is how, for example, the ahci_platform driver's clock code
I'm sure that as hardware diversifies, the other subsystems will grow
in similar directions and eventually be dealt with using similarly
Email: julian.calaby at gmail.com
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