linux-kernel - Re: [PATCH RFC 3/4] clk: samsung: Add driver to control CLKOUT line on Exynos SoCs

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Message-id: <5376279C.4070006@samsung.com>
Date:	Fri, 16 May 2014 16:58:36 +0200
From:	Tomasz Figa <t.figa@...sung.com>
To:	Rahul Sharma <rahul.sharma@...sung.com>
Cc:	Mark Rutland <mark.rutland@....com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
	Mike Turquette <mturquette@...aro.org>,
	Pankaj Dubey <pankaj.dubey@...sung.com>,
	Mark Brown <broonie@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Rob Herring <robh+dt@...nel.org>,
	Tomasz Figa <tomasz.figa@...il.com>,
	Kukjin Kim <kgene.kim@...sung.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Tushar Behera <tushar.behera@...aro.org>
Subject: Re: [PATCH RFC 3/4] clk: samsung: Add driver to control CLKOUT line on
 Exynos SoCs

On 16.05.2014 16:35, Rahul Sharma wrote:
> On 16 May 2014 16:22, Tomasz Figa <t.figa@...sung.com> wrote:
>> Hi Rahul,
>>
>> On 16.05.2014 12:39, Rahul Sharma wrote:
>>> [snip]
>>>> +       gate->lock = &clkout_lock;
>>>> +
>>>> +       mux->reg = reg + EXYNOS_PMU_DEBUG_REG;
>>>> +       mux->mask = EXYNOS_CLKOUT_MUX_MASK;
>>>> +       mux->shift = EXYNOS_CLKOUT_MUX_SHIFT;
>>>> +       mux->lock = &clkout_lock;
>>>> +
>>>> +       clk = clk_register_composite(NULL, "clkout", parent_names,
>>>> +                                       parent_count, &mux->hw,
>>>> +                                       &clk_mux_ops, NULL, NULL, &gate->hw,
>>>> +                                       &clk_gate_ops, 0);
>>>> +       if (IS_ERR(clk))
>>>> +               goto err_unmap;
>>>> +
>>>
>>> Hi Tomasz,
>>>
>>> Do we really need a composite clock here? How about registering
>>> a mux and a gate separately?
>>
>> What's wrong with a composite clock? It simplifies the code as just a
>> single clock needs to be registered. I don't see any drawbacks compared
>> to registering two clocks separately.
>>
> 
> I always took it as a thumb rule to not to use composite clocks if you
> can easily represent the block using basic clocks structures.
> 
> There can be a problem when drivers using such clocks assume that such
> clock continue to offer composite functionality for all futures SoCs and
> write code around it. This is what we faced when fixing drivers during
> CCF migration.

The drivers using CLKOUT need to be designed this way, because they are
not Exynos-specific, such as drivers for HSIC hubs or audio codecs. They
can't have any Exynos-specific knowledge about clock hierarchy.

So regardless of whether this is implemented using the composite clock
or not, consumer device drivers need to be able to use just a single
clock to do whatever they need, e.g. gating or rate configuration.

However I can see one problem here with my implementation - it is
missing the CLK_SET_RATE_PARENT flag, so dividers of particular CLK_OUTs
from CMU blocks could be reconfigured.

Also the driver is missing save and restore of PMU_DEBUG register.

I will fix both in next version.

Best regards,
Tomasz
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