From: William Breathitt Gray <william.gray@linaro.org>
To: Julien Panis <jpanis@baylibre.com>
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 07:35:28 -0400 [thread overview]
Message-ID: <Yy2aAMv5PRjsJ4s2@fedora> (raw)
In-Reply-To: <2f3e5036-caab-f892-a4ad-b852f72db331@baylibre.com>
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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
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next prev parent reply other threads:[~2022-09-23 11:36 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 [this message]
2022-09-23 12:17 ` Julien Panis
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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