From: Julien Panis <jpanis@baylibre.com>
To: William Breathitt Gray <william.gray@linaro.org>
Cc: robh+dt@kernel.org, krzysztof.kozlowski+dt@linaro.org,
linux-iio@vger.kernel.org, linux-kernel@vger.kernel.org,
devicetree@vger.kernel.org, mranostay@ti.com
Subject: Re: [PATCH v8 3/4] counter: ti-ecap-capture: capture driver support for ECAP
Date: Fri, 23 Sep 2022 14:17:22 +0200 [thread overview]
Message-ID: <923f5876-d692-7e0e-f351-f0e05869ccd8@baylibre.com> (raw)
In-Reply-To: <Yy2aAMv5PRjsJ4s2@fedora>
On 23/09/2022 13:35, William Breathitt Gray wrote:
> On Fri, Sep 23, 2022 at 09:23:26AM +0200, Julien Panis wrote:
>>
>> On 23/09/2022 03:08, William Breathitt Gray wrote:
>>> On Thu, Sep 22, 2022 at 07:04:01PM +0200, Julien Panis wrote:
>>>> ECAP hardware on TI AM62x SoC supports capture feature. It can be used
>>>> to timestamp events (falling/rising edges) detected on input signal.
>>>>
>>>> This commit adds capture driver support for ECAP hardware on AM62x SoC.
>>>>
>>>> In the ECAP hardware, capture pin can also be configured to be in
>>>> PWM mode. Current implementation only supports capture operating mode.
>>>> Hardware also supports timebase sync between multiple instances, but
>>>> this driver supports simple independent capture functionality.
>>>>
>>>> Signed-off-by: Julien Panis <jpanis@baylibre.com>
>>> Hello Julien,
>>>
>>> Comments follow inline below.
>>>
>>>> +/**
>>>> + * struct ecap_cnt_dev - device private data structure
>>>> + * @enabled: device state
>>>> + * @clk: device clock
>>>> + * @regmap: device register map
>>>> + * @nb_ovf: number of overflows since capture start
>>>> + * @pm_ctx: device context for PM operations
>>>> + */
>>>> +struct ecap_cnt_dev {
>>>> + bool enabled;
>>>> + struct clk *clk;
>>>> + struct regmap *regmap;
>>>> + atomic_t nb_ovf;
>>>> + struct {
>>>> + u8 ev_mode;
>>>> + u32 time_cntr;
>>>> + } pm_ctx;
>>>> +};
>>> Provide documentation for the ev_mode and time_cntr members. You
>>> probably need a lock as well to protect access to this structure or
>>> you'll end up with race problems.
>> Hi William,
>>
>> How can I end up with race problems ? pm_ctx members are only accessed at
>> suspend (after capture/IRQ are disabled) and resume (before capture/IRQ are
>> re-enabled).
>> Is there any risk I did not identify ?
>>
>> Julien
> I was thinking of the ecap_cnt_dev enabled member. The Counter callbacks
> may execute in concurrent threads, so races can appear when you access
> members of the ecap_cnt_dev structure in these callbacks.
>
> Take for example this section of ecap_cnt_enable_write():
>
> if (enable == ecap_dev->enabled)
> return 0;
> if (enable)
> ecap_cnt_capture_enable(counter);
> else
> ecap_cnt_capture_disable(counter);
> ecap_dev->enabled = enable
>
> Suppose two threads try to enable the count capture. A race condition is
> present where the two threads could see ecap_dev->enabled as false and
> both proceed to call ecap_cnt_capture_enable(). This results in
> pm_runtime_get_sync() bumping the usage count twice and we're left with
> a mismatch the next time ecap_cnt_capture_disable() is called.
>
> William Breathitt Gray
OK, If I understand well there's the same problem with IO access with
regmap ?
Julien
next prev parent reply other threads:[~2022-09-23 12:22 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-22 17:03 [PATCH v8 0/4] ECAP support on TI AM62x SoC Julien Panis
2022-09-22 17:03 ` [PATCH v8 1/4] dt-bindings: counter: add ti,am62-ecap-capture.yaml Julien Panis
2022-09-22 17:04 ` [PATCH v8 2/4] Documentation: ABI: sysfs-bus-counter: add frequency & num_overflows items Julien Panis
2022-09-22 17:04 ` [PATCH v8 3/4] counter: ti-ecap-capture: capture driver support for ECAP Julien Panis
2022-09-23 1:08 ` William Breathitt Gray
2022-09-23 7:23 ` Julien Panis
2022-09-23 11:35 ` William Breathitt Gray
2022-09-23 12:17 ` Julien Panis [this message]
2022-09-23 12:32 ` Julien Panis
2022-09-23 13:09 ` William Breathitt Gray
2022-09-22 17:04 ` [PATCH v8 4/4] MAINTAINERS: add TI ECAP driver info Julien Panis
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