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

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Date:	Thu, 16 May 2013 10:44:16 -0400
From:	Philip Balister <philip@...ister.org>
To:	Saravana Kannan <skannan@...eaurora.org>
CC:	Mark Brown <broonie@...nel.org>,
	linux-arm-kernel@...ts.infradead.org,
	Mike Turquette <mturquette@...aro.org>,
	linux-kernel@...r.kernel.org,
	Sören Brinkmann <soren.brinkmann@...inx.com>,
	Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>
Subject: Re: [PATCH RFC] clk: Introduce userspace clock driver

On 05/16/2013 12:28 AM, Saravana Kannan wrote:
> On 05/14/2013 09:46 PM, Mark Brown wrote:
>> On Tue, May 14, 2013 at 02:09:47PM -0400, Philip Balister wrote:
>>
>>> First of all, the driver that loads the bitstream into the fpga
>>> fabric does not know ANYTHING about what the bitstream does. So it
>>> cannot do any setup based on the contents of the file that is
>>> loaded. (And this can also be loaded during the SoC bootup,
>>> bypassing this driver completely)
>>
>> This is a problem that is going to need to be fixed - there will be some
>> things going on the FPGAs that do need drivers and so there needs to be
>> some way to instantiate a driver for a FPGA image.  Things like adding
>> extra DT blobs along with the FPGA image have been talked about
>>
>>> Second, there are four clocks that feed the FPGA fabric. We will
>>> want to set these clocks from user space somehow. It is perfectly
>>> valid to use a uio driver to interface with logic in the fpga. If we
>>> take the approach of using such general purpose techniques to
>>> interface with fpga logic, we must have ways for the user to control
>>> the fpga clocks.
>>
>> Right, and if the specific device is being controlled by UIO then having
>> UIO create some clocks makes sense but then that should be integrated
>> into the UIO instantiation rather than done as a separate thing.
> 
> Agreed. I was about to reply with exactly the same point. I haven't done
> any UIO coding, but that device file will eventually have to be opened.
> Turn on the clocks in the open and turn them off at close.
> 
> Rate to request can be a DT property.

But you are assuming each device implemented in the fpga uses one clock.
This assumption is clearly not valid since devices will ahve to share
clocks.

Philip
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