[linux-sunxi] Re: [PATCH v2 5/9] drm/sun4i: tcon_top: Register clock gates in probe

[Maxime Ripard]

On Mon, Jun 17, 2019 at 09:01:33PM +0800, Chen-Yu Tsai wrote:
> On Mon, Jun 17, 2019 at 7:45 PM Maxime Ripard <maxime.ripard at bootlin.com> wrote:
> >
> > On Fri, Jun 14, 2019 at 10:13:20PM +0530, Jagan Teki wrote:
> > > TCON TOP have clock gates for TV0, TV1, dsi and right
> > > now these are register during bind call.
> > >
> > > Of which, dsi clock gate would required during DPHY probe
> > > but same can miss to get since tcon top is not bound at
> > > that time.
> > >
> > > To solve, this circular dependency move the clock gate
> > > registration from bind to probe so-that DPHY can get the
> > > dsi gate clock on time.
> >
> > It's not really clear to me what the circular dependency is?
> >
> > if you have a chain that is:
> >
> > tcon-top +-> DSI
> >          +-> D-PHY
> >
> > There's no loop, right?
>
> Looking at how the DTSI patch structures things (without going into
> whether it is correct or accurate):
>
> The D-PHY is not part of the component graph. However it requests
> the DSI gate clock from the TCON-TOP.
>
> The TCON-TOP driver, in its current form, only registers the clocks
> it provides at component bind time. Thus the D-PHY can't successfully
> probe until the TCON-TOP has been bound.
>
> The DSI interface requires the D-PHY to bind. It will return -EPROBE_DEFER
> if it cannot request it. This in turn goes into the error path of
> component_bind_all, which unbinds all previous components.
>
> So it's actually
>
>     D-PHY -> TCON-TOP -> DSI
>       ^                   |
>       |--------------------
>
> I've not checked, but I suspect there's no possibility of having other
> drivers probe (to deal with deferred probing) within component_bind_all.
> Otherwise we shouldn't run into this weird circular dependency issue.
>

Ah, yes, that makes sense. It should be cleraer in the commit log then.

Thanks!
Maxime

--
Maxime Ripard, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com
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