Re: [PATCH] cpufreq: exynos5440: Protect opp search calls with rcu lock

[amit daniel kachhap]

Hi Nishanth,

Thanks for reviewing this patch.

On Mon, Apr 15, 2013 at 7:40 PM, Nishanth Menon <nm@xxxxxx> wrote:
> Daniel,
> On Mon, Apr 15, 2013 at 1:24 AM, Amit Daniel Kachhap
> <amit.daniel@xxxxxxxxxxx> wrote:
>> As per the OPP library documentation(Documentation/power/opp.txt) all
>> opp find/get calls should be protected by rcu locks.
>>
>> Signed-off-by: Amit Daniel Kachhap <amit.daniel@xxxxxxxxxxx>
>> ---
>>
>> This patch is created against linux-next tree and is suggested by
>> Nishanth Menon. (https://lkml.org/lkml/2013/4/12/119)
>>
>>  drivers/cpufreq/exynos5440-cpufreq.c |    3 +++
>>  1 files changed, 3 insertions(+), 0 deletions(-)
>>
>> diff --git a/drivers/cpufreq/exynos5440-cpufreq.c b/drivers/cpufreq/exynos5440-cpufreq.c
>> index ead7ed4..0c74018 100644
>> --- a/drivers/cpufreq/exynos5440-cpufreq.c
>> +++ b/drivers/cpufreq/exynos5440-cpufreq.c
>> @@ -120,11 +120,13 @@ static int init_div_table(void)
>>         int i = 0;
>>         struct opp *opp;
>>
>> +       rcu_read_lock();
>>         for (i = 0; freq_tbl[i].frequency != CPUFREQ_TABLE_END; i++) {
>>
>>                 opp = opp_find_freq_exact(dvfs_info->dev,
>>                                         freq_tbl[i].frequency * 1000, true);
>>                 if (IS_ERR(opp)) {
>> +                       rcu_read_unlock();
>>                         dev_err(dvfs_info->dev,
>>                                 "failed to find valid OPP for %u KHZ\n",
>>                                 freq_tbl[i].frequency);
>> @@ -159,6 +161,7 @@ static int init_div_table(void)
>>                 __raw_writel(tmp, dvfs_info->base + XMU_PMU_P0_7 + 4 * i);
>>         }
>>
>> +       rcu_read_unlock();
>
> Is it not possible to reduce the amount of code protected by RCU lock?
> something like this:
Yes it is possible but then this part of code is a loop of max 8
iteration so rcu_read_lock will be called those many times.
Also this code blocks are just register writes and not doing any time
consuming operations.

Thanks,
Amit Daniel
> +               rcu_read_lock();
>                 opp = opp_find_freq_exact(dvfs_info->dev,
>                                         freq_tbl[i].frequency * 1000, true);
>                 if (IS_ERR(opp)) {
> +                       rcu_read_unlock();
>                         dev_err(dvfs_info->dev,
>                                 "failed to find valid OPP for %u KHZ\n",
>                                 freq_tbl[i].frequency);
>                         return PTR_ERR(opp);
>                 }
> +               volt_id = opp_get_voltage(opp);
> +               rcu_unlock();
>
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