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@xxxxxxxxxxxx>
> > 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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