linux-kernel - Re: [PATCH v3 02/11] clk: davinci

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Message-ID: <50B524BD.7060403@ti.com>
Date:	Tue, 27 Nov 2012 15:38:21 -0500
From:	Murali Karicheri <m-karicheri2@...com>
To:	Mike Turquette <mturquette@...aro.org>
CC:	Sekhar Nori <nsekhar@...com>, <arnd@...db.de>,
	<akpm@...ux-foundation.org>, <shawn.guo@...aro.org>,
	<rob.herring@...xeda.com>, <linus.walleij@...aro.org>,
	<viresh.linux@...il.com>, <linux-kernel@...r.kernel.org>,
	<khilman@...com>, <linux@....linux.org.uk>, <sshtylyov@...sta.com>,
	<davinci-linux-open-source@...ux.davincidsp.com>,
	<linux-arm-kernel@...ts.infradead.org>,
	<linux-keystone@...t.ti.com>
Subject: Re: [PATCH v3 02/11] clk: davinci - add PSC clock driver

On 11/27/2012 12:29 PM, Mike Turquette wrote:
> Quoting Sekhar Nori (2012-11-27 07:05:21)
>> Hi Mike,
>>
>> On 11/10/2012 7:52 AM, Mike Turquette wrote:
>>> Quoting Murali Karicheri (2012-11-05 07:10:52)
>>>> On 11/03/2012 08:07 AM, Sekhar Nori wrote:
>>>>> On 10/25/2012 9:41 PM, Murali Karicheri wrote:
>>>>>> This is the driver for the Power Sleep Controller (PSC) hardware
>>>>>> found on DM SoCs as well Keystone SoCs (c6x). This driver borrowed
>>>>>> code from arch/arm/mach-davinci/psc.c and implemented the driver
>>>>>> as per common clock provider API. The PSC module is responsible for
>>>>>> enabling/disabling the Power Domain and Clock domain for different IPs
>>>>>> present in the SoC. The driver is configured through the clock data
>>>>>> passed to the driver through struct clk_psc_data.
>>>>>>
>>>>>> Signed-off-by: Murali Karicheri <m-karicheri2@...com>
>>>>>> ---
>>>>>> +/**
>>>>>> + * struct clk_psc - DaVinci PSC clock driver data
>>>>>> + *
>>>>>> + * @hw: clk_hw for the psc
>>>>>> + * @psc_data: Driver specific data
>>>>>> + */
>>>>>> +struct clk_psc {
>>>>>> +    struct clk_hw hw;
>>>>>> +    struct clk_psc_data *psc_data;
>>>>>> +    spinlock_t *lock;
>>>>> Unused member? I don't see this being used.
>>>> OK. Will remove.
>>> Those locks are only used for the case where a register might contain
>>> bits for several clocks.  Thus RMW operations are protected.  On OMAP
>>> this isn't necessary due to a very generous register layout (typically
>>> one 32-bit reg per module) controlling clocks.  Seems tha tmaybe this is
>>> not needed for PSC module either?
>> Sorry about the late reply. The above is not totally true for PSC. There
>> are some registers (like PTCMD) which are common for all clocks.
>>
>> There is an enable_lock used in drivers/clk/clk.c which serializes all
>> enable/disable calls across the clock tree. Since that is done, further
>> locking at clk-psc level is not really needed, no?
>>
> I haven't finished looking through the PSC design document yet, but my
> answer to your question is this:
>
> If a register is only used for clk_enable/disable calls (not touched by
> anything held under the prepare_lock mutex) and if that register isn't
> used anywhere else in the code (outside of the clk framework) then yes,
> the enable_lock spinlock is enough for you.
The psc clocks share registers such as PTCMD in addition to the clock 
specific register. So if there are multiple concurrent paths to 
clk_enable()/clk_disable() possible, then PTCMD write is not protected 
through the main enable()/disable() lock. Now I am not sure if there are 
multiple concurrent paths possible such as invoking the API in the 
context of a user process, kernel thread etc. If this is a possibility
then IMO, a lock is needed.

Murali
> Also have you looked into regmap?  Since you are defining your own clock
> type that might be something nice for you.
>
> Regards,
> Mike
>
>> Thanks,
>> Sekhar
>

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