linux-kernel - Re: [PATCH] cpufreq: set value of CPUFREQ_BOOST

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Message-ID: <20140324085239.GA30828@in.ibm.com>
Date:	Mon, 24 Mar 2014 14:22:39 +0530
From:	Gautham R Shenoy <ego@...ux.vnet.ibm.com>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	rjw@...ysocki.net, linaro-kernel@...ts.linaro.org,
	cpufreq@...r.kernel.org, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org, srivatsa.bhat@...ux.vnet.ibm.com,
	ego@...ux.vnet.ibm.com, svaidy@...ux.vnet.ibm.com,
	l.majewski@...sung.com
Subject: Re: [PATCH] cpufreq: set value of CPUFREQ_BOOST_FREQ to 0xABABABAB

On Mon, Mar 24, 2014 at 12:18:14PM +0530, Viresh Kumar wrote:
> Ideally, .driver_data field of struct cpufreq_frequency_table must not be used
> by core at all. But during a recent change if its value is same as
> CPUFREQ_BOOST_FREQ macro, then it is treated specially by core.
> 
> The value of this macro was set to ~2 earlier, i.e. 0xFFFFFFFD. In case some
> driver is using this field for its own data and sets this field to -3, then with
> two's complement that value will also become 0xFFFFFFFD.
> 
> To fix this issue, lets change value of this flag to a very uncommon value which
> shouldn't be used by any driver unless they want to use BOOST
> feature
> 
> Along with this update comments to make this more clear.
> 
> Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
> ---
> 
> Gautham/Vaidy: I hope this fixes the problem we discussed for your
> patchset.

Since the type of .driver_data is "unsigned int", the cpufreq core
seems to be assuming that the value cannot be -ve. So, drivers should
be storing -ve values in these fields at their own risk. Because apart
from determining whether the corresponding frequency is a boost
frequency or not, the cpufreq core seems to be using the .driver_data
field in pr_debugs(). The value of .driver_data formatted as "%u" is
not useful in these pr_debugs, if the driver stores -ve value in this
field.

On the other hand, if we change the type of .driver_data to "int" then
restricting the driver to not use specific values is unreasonable
since .driver_data field is supposed to be private to the driver and
the core is not supposed to intepret it. In which case we should
be having a separate field for determining if the frequency is a BOOST
frequency or not.

So while it fixes the problem for us, I don't think this patch fixes
the problem in general for the reasons mentioned above.

--
Thanks and Regards
gautham.

> 
>  include/linux/cpufreq.h | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h
> index c48e595..9f25d9d 100644
> --- a/include/linux/cpufreq.h
> +++ b/include/linux/cpufreq.h
> @@ -455,12 +455,18 @@ extern struct cpufreq_governor cpufreq_gov_conservative;
>   *                     FREQUENCY TABLE HELPERS                       *
>   *********************************************************************/
> 
> +/* Special Values of .frequency field */
>  #define CPUFREQ_ENTRY_INVALID ~0
>  #define CPUFREQ_TABLE_END     ~1
> -#define CPUFREQ_BOOST_FREQ    ~2
> +/* Special Values of .driver_data field */
> +#define CPUFREQ_BOOST_FREQ    0xABABABAB
> 
>  struct cpufreq_frequency_table {
> -	unsigned int	driver_data; /* driver specific data, not used by core */
> +	/*
> +	 * driver specific data, not used by core unless it is set to
> +	 * CPUFREQ_BOOST_FREQ.
> +	 */
> +	unsigned int	driver_data;
>  	unsigned int	frequency; /* kHz - doesn't need to be in ascending
>  				    * order */
>  };
> -- 
> 1.7.12.rc2.18.g61b472e
> 

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