[RFC] memory: exynos5422-dmc: Document mutex scope

[RFC] memory: exynos5422-dmc: Document mutex scope

[Krzysztof Kozlowski]

Document scope of the mutex used by driver.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>

---

It seems mutex was introduced to protect:
1. setting actual frequency/voltage,
2. dmc->curr_rate (in exynos5_dmc_get_cur_freq()).

However dmc->curr_rate in exynos5_dmc_get_status() is not protected. Is
it a bug?
---
 drivers/memory/samsung/exynos5422-dmc.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/memory/samsung/exynos5422-dmc.c b/drivers/memory/samsung/exynos5422-dmc.c
index 93e9c2429c0d..0388066a7d96 100644
--- a/drivers/memory/samsung/exynos5422-dmc.c
+++ b/drivers/memory/samsung/exynos5422-dmc.c
@@ -114,6 +114,7 @@ struct exynos5_dmc {
 	void __iomem *base_drexi0;
 	void __iomem *base_drexi1;
 	struct regmap *clk_regmap;
+	/* Protects curr_rate and frequency/voltage setting section */
 	struct mutex lock;
 	unsigned long curr_rate;
 	unsigned long curr_volt;
-- 
2.17.1

Re: [RFC] memory: exynos5422-dmc: Document mutex scope

[Lukasz Luba]

Hi Krzysztof,

On 7/24/20 7:08 PM, Krzysztof Kozlowski wrote:
> Document scope of the mutex used by driver.
> 
> Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
> 
> ---
> 
> It seems mutex was introduced to protect:
> 1. setting actual frequency/voltage,
> 2. dmc->curr_rate (in exynos5_dmc_get_cur_freq()).
> 
> However dmc->curr_rate in exynos5_dmc_get_status() is not protected. Is
> it a bug?

The callback get_dev_status() from devfreq->profile, which here is the
exynos5_dmc_get_status() should be already called with devfreq->lock
mutex hold, like e.g from simple_ondemand governor or directly
using update_devfreq exported function:
update_devfreq()
   ->get_target_freq()
     devfreq_update_stats()
         df->profile->get_dev_status()

The dmc->curr_rate is also used from sysfs interface from devfreq.
The local dmc lock serializes also this use case (when the HW freq
has changed but not set yet into curr_rate.


> ---
>   drivers/memory/samsung/exynos5422-dmc.c | 1 +
>   1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/memory/samsung/exynos5422-dmc.c b/drivers/memory/samsung/exynos5422-dmc.c
> index 93e9c2429c0d..0388066a7d96 100644
> --- a/drivers/memory/samsung/exynos5422-dmc.c
> +++ b/drivers/memory/samsung/exynos5422-dmc.c
> @@ -114,6 +114,7 @@ struct exynos5_dmc {
>   	void __iomem *base_drexi0;
>   	void __iomem *base_drexi1;
>   	struct regmap *clk_regmap;
> +	/* Protects curr_rate and frequency/voltage setting section */
>   	struct mutex lock;
>   	unsigned long curr_rate;
>   	unsigned long curr_volt;
> 

I assume this missing comment for the lock was required by some scripts.
In this case LGTM:

Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>

Regards,
Lukasz

Re: [RFC] memory: exynos5422-dmc: Document mutex scope

[Krzysztof Kozlowski]

On Tue, Aug 04, 2020 at 11:40:07AM +0100, Lukasz Luba wrote:
> Hi Krzysztof,
>
> On 7/24/20 7:08 PM, Krzysztof Kozlowski wrote:
> > Document scope of the mutex used by driver.
> >
> > Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
> >
> > ---
> >
> > It seems mutex was introduced to protect:
> > 1. setting actual frequency/voltage,
> > 2. dmc->curr_rate (in exynos5_dmc_get_cur_freq()).
> >
> > However dmc->curr_rate in exynos5_dmc_get_status() is not protected. Is
> > it a bug?
>
> The callback get_dev_status() from devfreq->profile, which here is the
> exynos5_dmc_get_status() should be already called with devfreq->lock
> mutex hold, like e.g from simple_ondemand governor or directly
> using update_devfreq exported function:
> update_devfreq()
>   ->get_target_freq()
>     devfreq_update_stats()
>         df->profile->get_dev_status()
>
> The dmc->curr_rate is also used from sysfs interface from devfreq.
> The local dmc lock serializes also this use case (when the HW freq
> has changed but not set yet into curr_rate.

These are different locks. You cannot protect dmc->curr_rate with
devfreq->lock in one place and dmc-lock in other place. This won't
protect it.

> > ---
> >   drivers/memory/samsung/exynos5422-dmc.c | 1 +
> >   1 file changed, 1 insertion(+)
> >
> > diff --git a/drivers/memory/samsung/exynos5422-dmc.c b/drivers/memory/samsung/exynos5422-dmc.c
> > index 93e9c2429c0d..0388066a7d96 100644
> > --- a/drivers/memory/samsung/exynos5422-dmc.c
> > +++ b/drivers/memory/samsung/exynos5422-dmc.c
> > @@ -114,6 +114,7 @@ struct exynos5_dmc {
> >     void __iomem *base_drexi0;
> >     void __iomem *base_drexi1;
> >     struct regmap *clk_regmap;
> > +   /* Protects curr_rate and frequency/voltage setting section */
> >     struct mutex lock;
> >     unsigned long curr_rate;
> >     unsigned long curr_volt;
> >
>
> I assume this missing comment for the lock was required by some scripts.
> In this case LGTM:
>
> Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>

Such comments are always useful. It is also pointed by strict
checkpatch:
CHECK: struct mutex definition without comment

Best regards,
Krzysztof

Re: [RFC] memory: exynos5422-dmc: Document mutex scope

[Lukasz Luba]

On 8/9/20 10:12 AM, Krzysztof Kozlowski wrote:
> On Tue, Aug 04, 2020 at 11:40:07AM +0100, Lukasz Luba wrote:
>> Hi Krzysztof,
>>
>> On 7/24/20 7:08 PM, Krzysztof Kozlowski wrote:
>>> Document scope of the mutex used by driver.
>>>
>>> Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
>>>
>>> ---
>>>
>>> It seems mutex was introduced to protect:
>>> 1. setting actual frequency/voltage,
>>> 2. dmc->curr_rate (in exynos5_dmc_get_cur_freq()).
>>>
>>> However dmc->curr_rate in exynos5_dmc_get_status() is not protected. Is
>>> it a bug?
>>
>> The callback get_dev_status() from devfreq->profile, which here is the
>> exynos5_dmc_get_status() should be already called with devfreq->lock
>> mutex hold, like e.g from simple_ondemand governor or directly
>> using update_devfreq exported function:
>> update_devfreq()
>>    ->get_target_freq()
>>      devfreq_update_stats()
>>          df->profile->get_dev_status()
>>
>> The dmc->curr_rate is also used from sysfs interface from devfreq.
>> The local dmc lock serializes also this use case (when the HW freq
>> has changed but not set yet into curr_rate.
> 
> These are different locks. You cannot protect dmc->curr_rate with
> devfreq->lock in one place and dmc-lock in other place. This won't
> protect it.

There are different paths that framework goes and mainly they are
protected by the df->lock.
But I tend to agree, I will send a patch which adds some locking.

Regards,
Lukasz