linux-kernel - Re: [PATCH RFC] clk: Introduce userspace clock driver

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Date:	Mon, 13 May 2013 10:24:33 -0700
From:	Sören Brinkmann <soren.brinkmann@...inx.com>
To:	Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>
CC:	Mark Brown <broonie@...nel.org>,
	Mike Turquette <mturquette@...aro.org>,
	<linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH RFC] clk: Introduce userspace clock driver

On Mon, May 13, 2013 at 06:21:13PM +0200, Sebastian Hesselbarth wrote:
> On 05/13/13 18:09, Sören Brinkmann wrote:
> >On Mon, May 13, 2013 at 09:21:35AM +0400, Mark Brown wrote:
> >>On Sun, May 12, 2013 at 12:05:04PM -0700, Sören Brinkmann wrote:
> >>>On Sun, May 12, 2013 at 06:33:44PM +0400, Mark Brown wrote:
> >>>>No, there's no confusion here - the clocks that are being exposed to
> >>>>userspace are the clocks which enter the FPGA.  The driver or whatever
> >>>>that understands the FPGA can do what is needed to control them,
> >>>>including routing them on to subdevices it instantiates or exposing them
> >>>>to userspace.
> >>
> >>>Such a driver does not exist in general.
> >>>For some IP cores, Linux drivers do exist and then
> >>>they are supposed to directly use the CCF, IMHO, no need to expose
> >>>things to userspace in that case.
> >>>I'm trying to cover cases, in which there is no driver available/needed for
> >>>the FPGA design, other than some simple clock controls.
> >>
> >>You're not understanding the point here.  If you've got a
> >>reprogrammmable FPGA you at least need some way to get the FPGA image in
> >>there.  This driver is presumably responsible for instantiating whatever
> >>is needed to control what is on the FPGA, that could include punting the
> >>clocks to userspace if that's sane.
> >Well, that driver actually exists. But that just programs a bitstream
> >you give it to program. It does not know anything about the design it
> >programs and cannot make any kind of decision whether the clocks should
> >be userspace controlled or not.
> 
> Soeren,
> 
> what Mark wants to point out is that you add fabric clocks to the Xilinx
> driver instead. This way, you will have user-space controllable clocks
> but only if you loaded the xilinx driver first.
What "Xilinx driver", are we talking about?

> 
> IIRC the fabric clock controller provided by Zynq _is_ always there and
> accessible from ARM CPUs. You just don't have a new generic driver
> allowing to poke with all clocks, but a xilinx only driver allowing you
> to set the (xilinx only) fabric clocks.
Right.

> 
> I've played with Zynq a while ago, did Xilinx mainline the bitfile
> driver already? If not, why don't you give it a shot?
The device config driver is not in mainline, AFAIK. And I think it's in
rather bad shape and needs a lot of work before it is upstreamable. But
this is probably the right place to put this.
On the other hand, currently this driver is only required for
programming the FPGA from Linux, which is not required. One of the
bootloaders could do this for you earlier.

	Sören


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