linux-kernel - Re: [PATCH v2 1/2] clk/zynq/clkc: Add 'fclk-enable' feature

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Date:	Mon, 28 Oct 2013 23:17:42 +0100
From:	Tomasz Figa <tomasz.figa@...il.com>
To:	Sören Brinkmann <soren.brinkmann@...inx.com>
Cc:	Rob Herring <rob.herring@...xeda.com>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Stephen Warren <swarren@...dotorg.org>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Rob Landley <rob@...dley.net>,
	Russell King <linux@....linux.org.uk>,
	Mike Turquette <mturquette@...aro.org>,
	Michal Simek <michal.simek@...inx.com>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-doc@...r.kernel.org, devicetree@...r.kernel.org
Subject: Re: [PATCH v2 1/2] clk/zynq/clkc: Add 'fclk-enable' feature

On Monday 28 of October 2013 14:43:35 Sören Brinkmann wrote:
> On Mon, Oct 28, 2013 at 10:13:28PM +0100, Tomasz Figa wrote:
> > Hi Soren,
> > 
> > On Thursday 10 of October 2013 10:10:17 Soren Brinkmann wrote:
> > > In some use cases Zynq's FPGA clocks are used as static clock
> > > generators for IP in the FPGA part of the SOC for which no Linux
> > > driver
> > > exists and would control those clocks. To avoid automatic
> > > gating of these clocks in such cases a new property - fclk-enable -
> > > is
> > > added to the clock controller's DT description to accomodate such
> > > use
> > > cases. It's value is a bitmask, where a set bit results in enabling
> > > the corresponding FCLK through the clkc.
> > > 
> > > FPGA clocks are handled following the rules below:
> > > 
> > > If an FCLK is not enabled by bootloaders, that FCLK will be disabled
> > > in
> > > Linux. Drivers can enable and control it through the CCF as usual.
> > > 
> > > If an FCLK is enabled by bootloaders AND the corresponding bit in
> > > the
> > > 'fclk-enable' DT property is set, that FCLK will be enabled by the
> > > clkc, resulting in an off by one reference count for that clock.
> > > Ensuring it will always be running.
> > > 
> > > Signed-off-by: Soren Brinkmann <soren.brinkmann@...inx.com>
> > > ---
> > > 
> > > v2:
> > >  - change default value for fclk-enable to '0'
> > > 
> > > ---
> > > 
> > >  Documentation/devicetree/bindings/clock/zynq-7000.txt |  4 ++++
> > >  drivers/clk/zynq/clkc.c                               | 18
> > > 
> > > +++++++++++++++--- 2 files changed, 19 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/Documentation/devicetree/bindings/clock/zynq-7000.txt
> > > b/Documentation/devicetree/bindings/clock/zynq-7000.txt index
> > > d99af878f5d7..11fdd146ec83 100644
> > > --- a/Documentation/devicetree/bindings/clock/zynq-7000.txt
> > > +++ b/Documentation/devicetree/bindings/clock/zynq-7000.txt
> > > 
> > > @@ -22,6 +22,10 @@ Required properties:
> > >  Optional properties:
> > >   - clocks : as described in the clock bindings
> > >   - clock-names : as described in the clock bindings
> > > 
> > > + - fclk-enable : Bit mask to enable FCLKs in cases no proper CCF
> > 
> > Since it's a vendor specific property, it should include vendor
> > prefix.
> 
> The whole driver is vendor specific. Should there really be another
> prefix for that property?

Yes. If a property is introduced just for use by this particular driver 
then it must be prepended by a vendor prefix. That's a general rule.

> > Also CCF is a Linux-specific implementation detail, which DT bindings
> > should not be involved into. If you really need to implement this
> > using
> > this way, then at least property description should say something like
> > this:
> > 
> > xlnx,fclk-enable : Bit mask of bits of fclk enable register that must
> > be statically enabled at boot-up time.
> 
> Fair enough. I'll change the description
> 
> > However, I wonder why you can't simply define an FPGA block using a
> > single node, which would be a consumer to all the fclk clocks you
> > need to enable and then make a driver for it that would simply enable
> > all clocks specified in clocks property.
> 
> Well, then we'd have a dummy driver that wouldn't fit into any subsystem
> and wouldn't do anything but enabling clocks. Seems much easier to
> handle it in this driver. Especially, since I hope that this is just a
> workaround and that the majority of use cases involves drivers for
> their soft-IP that simply uses the CCF.

Hmm, I'm not really convinced, but well, let's say that I'm fine with your 
proposed solution, unless someone else complains.

Best regards,
Tomasz

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