Re: [PATCH v8 3/4] counter: ti-ecap-capture: capture driver support for ECAP

[Julien Panis]

On 23/09/2022 14:17, Julien Panis wrote:
> 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@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
>
> OK, If I understand well there's the same problem with IO access with 
> regmap ?
> Julien

[ERRATUM] It seems that some spinlock is already used by regmap API.
So, only the 'enabled' member needs a lock.