Re: [RESEND 02/11] pwm: sysfs: Add PWM Capture support

Lee Jones
2016-04-13T09:40:57Z

On Tue, 12 Apr 2016, Thierry Reding wrote:

> On Wed, Mar 02, 2016 at 03:32:00PM +0000, Lee Jones wrote:
> > Allow a user to read PWM Capture results from /sysfs. First,
> > the user must tell PWM Capture which channel they wish to
> > read from:
> >
> > $ echo 2 > $PWMCHIP/capture
> >
> > To start a capture and read the result, simply read the file:
> >
> > $ cat $PWMCHIP/capture
> >
> > The output format is left to the device.
> >
> > Signed-off-by: Lee Jones <lee.jones@linaro.org>
> > ---
> > drivers/pwm/sysfs.c | 28 ++++++++++++++++++++++++++++
> > 1 file changed, 28 insertions(+)
> >
> > diff --git a/drivers/pwm/sysfs.c b/drivers/pwm/sysfs.c
> > index 9c90886..3572ef4 100644
> > --- a/drivers/pwm/sysfs.c
> > +++ b/drivers/pwm/sysfs.c
> > @@ -23,6 +23,8 @@
> > #include <linux/kdev_t.h>
> > #include <linux/pwm.h>
> >
> > +static int capture_channel;
>
> Can't do that, this is very racy because it isn't protected by any lock.
> Fortunately I don't think the global variable is at all necessary. See
> below.
>
> > +
> > struct pwm_export {
> > struct device child;
> > struct pwm_device *pwm;
> > @@ -167,16 +169,42 @@ static ssize_t polarity_store(struct device *child,
> > return ret ? : size;
> > }
> >
> > +static ssize_t capture_show(struct device *child,
> > + struct device_attribute *attr,
> > + char *buf)
> > +{
> > + struct pwm_device *pwm = child_to_pwm_device(child);
> > +
> > + return pwm_capture(pwm, capture_channel, buf);
> > +}
> > +
> > +static ssize_t capture_store(struct device *child,
> > + struct device_attribute *attr,
> > + const char *buf, size_t size)
> > +{
> > + int val, ret;
> > +
> > + ret = kstrtoint(buf, 0, &val);
> > + if (ret)
> > + return ret;
> > +
> > + capture_channel = val;
> > +
> > + return size;
> > +}
> > +
> > static DEVICE_ATTR_RW(period);
> > static DEVICE_ATTR_RW(duty_cycle);
> > static DEVICE_ATTR_RW(enable);
> > static DEVICE_ATTR_RW(polarity);
> > +static DEVICE_ATTR_RW(capture);
>
> These are all per-PWM attributes and the specific PWM device that they
> are associated with can be retrieved using child_to_pwm_device(child)
> (see the other attributes' implementation for examples). So I don't
> think the capture attribute needs to be writable at all. You already
> implement capture_show() in almost the right way, and if you drop the
> channel parameter from pwm_capture() as I suggested in my reply to patch
> 1/11 this should resolve itself automatically.
>
> Of course capture_show() would become slightly more beefy if we return a
> standard result structure rather than leave it up to the drivers to fill
> out the sysfs string. The good thing is that it will be common code and
> therefore the sysfs interface would return the same format regardless of
> the driver.
>
> Perhaps something like
>
> struct pwm_device *pwm = child_to_pwm_device(child);
> struct pwm_capture result;
>
> err = pwm_capture(pwm, &result);
> if (err < 0)
> return err;
>
> return sprintf(buf, "%u %u\n", result.duty_cycle, result.period);
>
> would work?

Same reply as 1/11. Now I know that we should be treating each of our
channels, as *completely* separate devices, I think this method seems
reasonable.

--
Lee Jones
Linaro STMicroelectronics Landing Team Lead
Linaro.org │ Open source software for ARM SoCs
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