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

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Date:	Mon, 13 May 2013 19:37:23 +0200
From:	Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>
To:	Sören Brinkmann <soren.brinkmann@...inx.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 05/13/13 19:24, Sören Brinkmann wrote:
> On Mon, May 13, 2013 at 06:21:13PM +0200, Sebastian Hesselbarth wrote:
>>> 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.
>>
>> 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?

The device config driver you mention below.

>> 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.

It is, as it will only expose clocks on Zynq and that's what Mark and
Mike are worried about. Expose clocks to user space and you will have
people mess with it for sure.

About the shape of it, I didn't expect that to change at all. Just
wondering, if it still requires you to manually end it's endianess
mess with the bitfiles. If you go at it, consider reading the magic
hidden in the bitfile and swap it when it is required. But that will
go OT here.

> 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.

Well, exposing clocks to user space is not required _at all_ on all
other platforms you can think of. So, if you have a Zynq, you can
always load a Zynq driver even if you are not going to use it's
bitfile programming capabilities but only to configure clocks.

If you don't want to merge both drivers, have a Zynq-only clock
fabric driver instead?

Sebastian
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