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

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Date:	Thu, 10 Oct 2013 07:21:40 +0200
From:	Michal Simek <monstr@...str.eu>
To:	Sören Brinkmann <soren.brinkmann@...inx.com>
CC:	Mark Rutland <mark.rutland@....com>,
	Michal Simek <michal.simek@...inx.com>,
	"rob.herring@...xeda.com" <rob.herring@...xeda.com>,
	Pawel Moll <Pawel.Moll@....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>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>
Subject: Re: [PATCH 1/2] clk/zynq/clkc: Add 'fclk-enable' feature

On 10/09/2013 05:25 PM, Sören Brinkmann wrote:
> On Tue, Oct 08, 2013 at 04:38:17PM +0100, Mark Rutland wrote:
>> On Tue, Oct 08, 2013 at 03:36:11PM +0100, 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.
>>>
>>> The default value for 'fclk-enable' is '0xf' (all FCLK's enabled by the
>>> bootloader are enabled through the clkc.
>>
>> Why? Juding by the diff that's not what the code currently does, so why
>> not leave it as 0, and only set it where required as a work-around?
> The default is set as described here:
> [...]
>>> +	ret = of_property_read_u32(np, "fclk-enable", &fclk_enable);
>>> +	if (ret)
>>> +		fclk_enable = 0xf;
>>> +
> 
> The default has been chosen this way, because it avoids trouble with
> customers. Customers expect the FPGA clocks to be running, once they
> configured them in the FPGA tools. If Linux gates them off for some
> reason, it creates confusion and often ends up in customer support.
> But in general I agree. It is kind of a work around and having zero as
> default would be desirable. I'd leave this to Michal.

No problem to have default value setup to 0. And use this workaround
when it is set in DTS.

Thanks,
Michal

-- 
Michal Simek, Ing. (M.Eng), OpenPGP -> KeyID: FE3D1F91
w: www.monstr.eu p: +42-0-721842854
Maintainer of Linux kernel - Microblaze cpu - http://www.monstr.eu/fdt/
Maintainer of Linux kernel - Xilinx Zynq ARM architecture
Microblaze U-BOOT custodian and responsible for u-boot arm zynq platform



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