Re: Question regarding clocks in the DW-HDMI DT bindings

From: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To: Fabio Estevam <festevam@gmail.com>
Cc: "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
	Mike Turquette <mturquette@baylibre.com>,
	Stephen Boyd <sboyd@codeaurora.org>,
	DRI mailing list <dri-devel@lists.freedesktop.org>,
	Andy Yan <andy.yan@rock-chips.com>,
	Vladimir Zapolskiy <vladimir_zapolskiy@mentor.com>
Subject: Re: Question regarding clocks in the DW-HDMI DT bindings
Date: Fri, 25 Nov 2016 15:25 +0200	[thread overview]
Message-ID: <1543509.CFZUmxu4GH@avalon> (raw)
In-Reply-To: <CAOMZO5CgUm39E1hhLrs5SX9ALMj9BGiRKaY874exNWoQhH2AWQ@mail.gmail.com>

Hi Fabio,

On Friday 25 Nov 2016 10:43:04 Fabio Estevam wrote:
> On Thu, Nov 24, 2016 at 9:26 PM, Laurent Pinchart wrote:
> > Another question I have about the bus clock (CC'ing the devicetree mailing
> > list as well as the clock maintainers) is whether it should be made
> > optional. The clock is obviously mandatory from a hardware point of view
> > (given that APB is a synchronous bus and thus requires a clock), but in
> > some SoCs (specifically for the Renesas SoCs) that clock is always on and
> > can't be controlled. We already omit bus clocks in DT for most IP cores
> > when the clock can never be controlled (and we also omit a bunch of other
> > clocks that we don't even know exist), so it could make sense to make the
> > clock optional. Otherwise there would be runtime overhead trying to handle
> > a clock that can't be controlled.
> 
> What if you register the clock as a "dummy" clock instead?

In that case I can as well specify the correct clock. My point was that, for 
clocks that we know are always on, specifying them in DT will lead to runtime 
overhead (registration of the clock, lookup, runtime handling, ...) that we 
could as well avoid. I think the question has a broader scope than just the 
dw-hdmi driver, hence the widened audience in CC.

-- 
Regards,

Laurent Pinchart

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