Date:	Thu, 13 Dec 2012 13:38:28 +1100
From:	NeilBrown <neilb@...e.de>
To:	Thierry Reding <thierry.reding@...onic-design.de>
Cc:	Grant Erickson <marathon96@...il.com>, linux-omap@...r.kernel.org,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] OMAP: add pwm driver using dmtimers.

On Wed, 12 Dec 2012 12:31:45 +0100 Thierry Reding
<thierry.reding@...onic-design.de> wrote:

> On Wed, Dec 12, 2012 at 07:24:30PM +1100, NeilBrown wrote:
> > 
> > 
> > This patch is based on an earlier patch by Grant Erickson
> > which provided pwm devices using the 'legacy' interface.
> > 
> > This driver instead uses the new framework interface.
> 
> I'd prefer some kind of description about the driver here.

I'm not really sure what more there is to say.  There was a bit of text in a
comment at the top of the file which I've copied to the commit comment.


>                                                            Also the
> subject should be something like:
> 
> 	pwm: Add OMAP support using dual-mode timers
> 
> or
> 
> 	pwm: omap: Add PWM support using dual-mode timers

Done - I chose the second.

> 
> I take this description to mean that OMAP doesn't have dedicated PWM
> hardware? Otherwise it might be bad to call this pwm-omap.

Correct.  The timers can be used for a number of things which explicitly
includes PWM.

> 
> Also please use all-caps when referring to PWM devices in prose. A few
> other comments inline below.

OK.

> 
> > Cc: Grant Erickson <marathon96@...il.com>
> > Signed-off-by: NeilBrown <neilb@...e.de>
> > 
> > diff --git a/drivers/pwm/Kconfig b/drivers/pwm/Kconfig
> > index ed81720..7df573a 100644
> > --- a/drivers/pwm/Kconfig
> > +++ b/drivers/pwm/Kconfig
> > @@ -85,6 +85,15 @@ config PWM_MXS
> >  	  To compile this driver as a module, choose M here: the module
> >  	  will be called pwm-mxs.
> >  
> > +config PWM_OMAP
> > +	tristate "OMAP pwm support"
> 
> "OMAP PWM support"

Fixed.

> 
> > diff --git a/drivers/pwm/pwm-omap.c b/drivers/pwm/pwm-omap.c
> [...]
> > + *    The 'id' number for the device encodes the number of the dm timer
> > + *    to use, and the polarity of the output.
> > + *    lsb is '1' of active-high, and '0' for active low
> > + *    remaining bit a timer number and need to be shifted down before use.
> 
> I don't know if this is such a good idea. Usually you number platform
> devices sequentially, while this would leave gaps in the numbering. I
> know that adding platform data may sound a bit like overkill, but I
> really think the added clarity and consistency is worth it.

I guess so.  No other PWM driver seems to use platform data, and I needed so
little...
I'll see what I can do.


> 
> > +#define pr_fmt(fmt) "pwm-omap: " fmt
> 
> You don't seem to be using any of the pr_*() logging functions, so this
> isn't needed.

Gone now, thanks.


> 
> > +#include <linux/export.h>
> > +#include <linux/kernel.h>
> > +#include <linux/platform_device.h>
> > +#include <linux/slab.h>
> > +#include <linux/err.h>
> > +#include <linux/clk.h>
> > +#include <linux/io.h>
> > +#include <linux/pwm.h>
> > +#include <linux/module.h>
> > +
> > +#include <plat/dmtimer.h>
> > +
> > +#define DM_TIMER_LOAD_MIN		0xFFFFFFFE
> > +
> > +struct omap_chip {
> > +	struct platform_device	*pdev;
>
> I don't see this field being used anywhere.

No.  Gone.

> 
> > +	struct omap_dm_timer	*dm_timer;
> > +	unsigned int		polarity;
> 
> The PWM subsystem already has enum pwm_polarity for this.
> 

I'll use that then .... and as there  is a pwm_set_polarity() interface, that
probably means that I don't need to configure the polarity via the platform
data?  That would be a lot cleaner.


> > +	const char		*label;
> 
> This isn't used anywhere either.

Gone.

> 
> > +
> > +	unsigned int		duty_ns, period_ns;
> > +	struct pwm_chip		chip;
> > +};
> > +
> > +#define to_omap_chip(chip)	container_of(chip, struct omap_chip, chip)
> > +
> > +#define	pwm_dbg(_pwm, msg...) dev_dbg(&(_pwm)->pdev->dev, msg)
> 
> This is never used.

:-)  There is a theme here.


> 
> > +
> > +/**
> > + * pwm_calc_value - determines the counter value for a clock rate and period.
> 
> Nit: You should either start the sentence with a capital or not
> terminate it with a full stop.

In this case the sentence really includes the function name which is
case-sensitive so cannot be capitalised ;-)
I'll rephrase a bit and find something to capitalise.

> 
> > + * @clk_rate: The clock rate, in Hz, of the PWM's clock source to compute the
> > + *            counter value for.
> > + * @ns: The period, in nanoseconds, to computer the counter value for.
> 
> "compute"

Yep.

> 
> > + *
> > + * Returns the PWM counter value for the specified clock rate and period.
> > + */
> > +static inline int pwm_calc_value(unsigned long clk_rate, int ns)
> > +{
> > +	const unsigned long nanoseconds_per_second = 1000000000;
> 
> Maybe use NSEC_PER_SEC instead?

Good idea, thanks.

> 
> > +	int cycles;
> > +	__u64 c;
> 
> I think for in-kernel use, the custom is to stick with simply u64.

It is, yes.


> 
> > +
> > +	c = (__u64)clk_rate * ns;
> > +	do_div(c, nanoseconds_per_second);
> > +	cycles = c;
> > +
> > +	return DM_TIMER_LOAD_MIN - cycles;
> 
> Can't you just do "DM_TIMER_LOAD_MIN - c" and get rid of the cycles
> variable altogether?

Yep.

> 
> > +static int omap_pwm_enable(struct pwm_chip *chip, struct pwm_device *pwm)
> > +{
> > +	struct omap_chip *omap = to_omap_chip(chip);
> > +	int status = 0;
> > +
> > +	/* Enable the counter--always--before attempting to write its
> > +	 * registers and then set the timer to its minimum load value to
> > +	 * ensure we get an overflow event right away once we start it.
> > +	 */
> 
> Block comments should be in the following format:
> 
> 	/*
> 	 * foo...
> 	 * bar...
> 	 */

OK, I fixed all of those.

> 
> > +
> > +	omap_dm_timer_enable(omap->dm_timer);
> > +	omap_dm_timer_write_counter(omap->dm_timer, DM_TIMER_LOAD_MIN);
> > +	omap_dm_timer_start(omap->dm_timer);
> > +	omap_dm_timer_disable(omap->dm_timer);
> 
> So omap_dm_timer_disable() doesn't actually stop the timer? It just
> disables the access to the registers?
> 
> > +	return status;
> 
> "return 0;" and drop the status variable.
> 

Done.

> > +static int omap_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm,
> > +			   int duty_ns, int period_ns)
> > +{
> > +	struct omap_chip *omap = to_omap_chip(chip);
> > +	int status = 0;
> 
> This one can be dropped as well.
> 
> > +	const bool enable = true;
> > +	const bool autoreload = true;
> > +	const bool toggle = true;
> > +	const int trigger = OMAP_TIMER_TRIGGER_OVERFLOW_AND_COMPARE;
> 
> I understand that these extra variables are supposed to "document" the
> parameters of the functions below. I'm not a huge fan of this approach
> because instead the API would better be designed to make it obvious what
> the parameters are. Or people can just look at the prototypes to find
> out the meaning. But if you really prefer this way I won't object.

I've removed the 'trigger' const as the name is self-descriptive.
I might leave the others.

> 
> > +	int load_value, match_value;
> > +	unsigned long clk_rate;
> > +
> > +	dev_dbg(chip->dev,
> > +		"duty cycle: %d, period %d\n",
> > +		duty_ns, period_ns);
> 
> This all fits on a single line, so no need to break it up.

Indeed.

> 
> > +
> > +	if (omap->duty_ns == duty_ns &&
> > +	    omap->period_ns == period_ns)
> > +		/* No change - don't cause any transients */
> > +		return 0;
> 
> Note to self: this might be a candidate to put in the core.

might be useful, though the core doesn't currently "know" the current values.

> 
> > +	clk_rate = clk_get_rate(omap_dm_timer_get_fclk(omap->dm_timer));
> > +
> > +	/* Calculate the appropriate load and match values based on the
> > +	 * specified period and duty cycle. The load value determines the
> > +	 * cycle time and the match value determines the duty cycle.
> > +	 */
> 
> Again wrong block comment style.
> 
> > +
> > +	load_value = pwm_calc_value(clk_rate, period_ns);
> > +	match_value = pwm_calc_value(clk_rate, period_ns - duty_ns);
> > +
> > +	/* We MUST enable yet stop the associated dual-mode timer before
> > +	 * attempting to write its registers.  Hopefully it is already
> > +	 * disabled, but call the (idempotent) pwm_disable just in case
> > +	 */
> 
> And here as well. While at it you might want to terminate the last
> sentence with a full stop.
> 

I check the sentences in other comments too.

> > +	pwm_disable(pwm);
> > +
> > +	omap_dm_timer_enable(omap->dm_timer);
> > +
> > +	omap_dm_timer_set_load(omap->dm_timer, autoreload, load_value);
> > +	omap_dm_timer_set_match(omap->dm_timer, enable, match_value);
> > +
> > +	dev_dbg(chip->dev,
> > +			"load value: %#08x (%d), "
> > +			"match value: %#08x (%d)\n",
> > +			load_value, load_value,
> > +			match_value, match_value);
> 
> Again this doesn't need so much wrapping.
> 

Nope.

> > +
> > +	omap_dm_timer_set_pwm(omap->dm_timer,
> > +			      !omap->polarity,
> > +			      toggle,
> > +			      trigger);
> 
> This doesn't either. Also you should be explicit about the polarity
> parameter, since enum pwm_polarity is an enum and therefore negating it
> isn't very nice (it should work though).
> 
> You could solve this by doing something like:
> 
> 	if (omap->polarity == PWM_POLARITY_NORMAL)
> 		polarity = 1;
> 	else
> 		polarity = 0;

(omap->polarity == PWM_POLARITY_NORMAL)

would have the same effect.

> 
> > +	/* Set the counter to generate an overflow event immediately. */
> > +
> > +	omap_dm_timer_write_counter(omap->dm_timer, DM_TIMER_LOAD_MIN);
> > +
> > +	/* Now that we're done configuring the dual-mode timer, disable it
> > +	 * again. We'll enable and start it later, when requested.
> > +	 */
> 
> Also wrong style.
> 
> > +	omap_dm_timer_disable(omap->dm_timer);
> > +	omap->duty_ns = duty_ns;
> > +	omap->period_ns = period_ns;
> > +
> > +	return status;
> 
> "return 0;"
> 
> > +}
> > +
> > +
> 
> Gratuitous newline.
> 
> > +static struct pwm_ops omap_pwm_ops = {
> 
> Should be "static const".
> 
> > +	.enable	= omap_pwm_enable,
> > +	.disable= omap_pwm_disable,
> 
> There should be a space between .disable and =.

OK.

> 
> > +	.config	= omap_pwm_config,
> > +	.owner	= THIS_MODULE,
> > +};
> > +
> > +/**
> > + * omap_pwm_probe - check for the PWM and bind it to the driver.
> > + * @pdev: A pointer to the platform device node associated with the
> > + *        PWM instance to be probed for driver binding.
> > + *
> > + * Returns 0 if the PWM instance was successfully bound to the driver;
> > + * otherwise, < 0 on error.
> > + */
> 
> I'm not sure how useful this kerneldoc comment really is. It isn't about
> an exported function and developers usually know what the .probe() does.

True.  Gone.

> 
> > +static int __devinit omap_pwm_probe(struct platform_device *pdev)
> 
> No more __devinit, please.

If you say so (having no idea what it did :-)


> 
> > +{
> > +	struct device *dev = &pdev->dev;
> > +	struct omap_chip *omap;
> > +	int status = 0;
> > +	unsigned int id = pdev->id;
> > +	unsigned int timer = id >> 1; /* lsb is polarity */
> 
> I've said this before, I don't think it's a good idea.
> 
> > +
> > +	omap = kzalloc(sizeof(struct pwm_device), GFP_KERNEL);
> > +
> 
> Gratuituous newline.
> 
> > +	if (omap == NULL) {
> > +		dev_err(dev, "Could not allocate memory.\n");
> > +		status = -ENOMEM;
> > +		goto done;
> > +	}
> > +
> > +	/* Request the OMAP dual-mode timer that will be bound to and
> > +	 * associated with this generic PWM.
> > +	 */
> 
> Block comment style again.
> 
> > +
> > +	omap->dm_timer = omap_dm_timer_request_specific(timer);
> > +
> 
> Gratuitous newline.

Gone.

> 
> > +	if (omap->dm_timer == NULL) {
> > +		status = -EPROBE_DEFER;
> > +		goto err_free;
> > +	}
> > +
> > +	/* Configure the source for the dual-mode timer backing this
> > +	 * generic PWM device. The clock source will ultimately determine
> > +	 * how small or large the PWM frequency can be.
> > +	 *
> > +	 * At some point, it's probably worth revisiting moving this to
> > +	 * the configure method and choosing either the slow- or
> > +	 * system-clock source as appropriate for the desired PWM period.
> > +	 */
> 
> And again.
> 
> > +
> > +	omap_dm_timer_set_source(omap->dm_timer, OMAP_TIMER_SRC_SYS_CLK);
> > +
> > +	/* Cache away other miscellaneous driver-private data and state
> > +	 * information and add the driver-private data to the platform
> > +	 * device.
> > +	 */
> 
> And again. =)
> 
> > +
> > +	omap->chip.dev = dev;
> > +	omap->chip.ops = &omap_pwm_ops;
> > +	omap->chip.base = -1;
> > +	omap->chip.npwm = 1;
> > +	omap->polarity = id & 1;
> > +
> > +	status = pwmchip_add(&omap->chip);
> > +	if (status < 0) {
> > +		dev_err(dev, "failed to register pwm\n");
> > +		omap_dm_timer_free(omap->dm_timer);
> > +		goto err_free;
> > +	}
> > +
> > +	platform_set_drvdata(pdev, omap);
> > +
> > +	status = 0;
> > +	goto done;
> 
> This can just be "return 0;".

Yep.

> 
> > +
> > + err_free:
> > +	kfree(omap);
> > + done:
> > +	return status;
> > +}
> > +
> > +/**
> > + * omap_pwm_remove - unbind the specified PWM platform device from the driver.
> > + * @pdev: A pointer to the platform device node associated with the
> > + *        PWM instance to be unbound/removed.
> > + *
> > + * Returns 0 if the PWM was successfully removed as a platform device;
> > + * otherwise, < 0 on error.


> > + */
> > +static int __devexit omap_pwm_remove(struct platform_device *pdev)
> 
> No __devexit, please.
> 
> > +{
> > +	struct omap_chip *omap = platform_get_drvdata(pdev);
> > +	int status = 0;
> 
> Just drop this.
> 
> > +
> > +	status = pwmchip_remove(&omap->chip);
> > +	if (status < 0)
> > +		goto done;
> 
> "return status;"

Fixed.

> 
> > +
> > +	omap_dm_timer_free(omap->dm_timer);
> > +
> > +	kfree(omap);
> > +
> > + done:
> > +	return status;
> 
> Just "return 0;". No label required.
> 
> > +}
> > +
> > +#if CONFIG_PM
> > +static int omap_pwm_suspend(struct platform_device *pdev, pm_message_t state)
> > +{
> > +	struct omap_chip *omap = platform_get_drvdata(pdev);
> > +	/* No one preserve these values during suspend so reset them
> > +	 * Otherwise driver leaves PWM unconfigured if same values
> > +	 * passed to pwm_config
> > +	 */
> > +	omap->period_ns = 0;
> > +	omap->duty_ns = 0;
> > +
> > +	return 0;
> > +}
> > +#else
> > +#define omap_pwm_suspend	NULL
> > +#endif
> 
> This doesn't look right. You should implement .resume() if you really
> care, in which case the resume callback would have to reconfigure with
> the cached values. In that case maybe you should switch to dev_pm_ops
> and SIMPLE_DEV_PM_OPS() as well.
> 
> If you don't, just resetting these values will not make the PWM work
> properly after resume either since it will have to be explicitly
> reconfigured.

I just copied that from pwm-samsung.c

I think the point is to avoid the "no transients" short-circuit in
omap_pwm_config if the config is unchanged.

The assumption is that pwm_disable() will be called before suspend and
pwm_config()/pwm_enable() after resume.  So there is no point actually
configuring anything in .resume() - it makes sense to wait until pwm_config()
is called (if ever).  But we want to make sure that pwm_config actually does
something.

But yes, I should change to dev_pm_ops.

> 
> > +static struct platform_driver omap_pwm_driver = {
> > +	.driver.name	= "omap-pwm",
> > +	.driver.owner	= THIS_MODULE,
> 
> The more idiomatic way of writing this is:
> 
> 	.driver = {
> 		.name = "omap-pwm",
> 		.owner = THIS_MODULE,
> 	},
> 

I guess... though I think my way is a lot neater.  But conformity is good.

> > +	.probe		= omap_pwm_probe,
> > +	.remove		= __devexit_p(omap_pwm_remove),
> 
> No __devexit_p anymore.
> 

OK.

> > +	.suspend	= omap_pwm_suspend,
> > +	.resume		= NULL,
> > +};
> > +
> > +static int __init omap_pwm_init(void)
> > +{
> > +	return platform_driver_register(&omap_pwm_driver);
> > +}
> > +
> > +static void __exit omap_pwm_exit(void)
> > +{
> > +	platform_driver_unregister(&omap_pwm_driver);
> > +}
> > +
> > +arch_initcall(omap_pwm_init);
> 
> This should probably be module_init() instead. You already return
> -EPROBE_DEFER if the dual-mode timer isn't ready yet so you should bite
> the bullet and get all the dependencies to behave properly as well. New
> code shouldn't be using this kind of dependency handling.
> 
> In that case you could also just run module_platform_driver().
> 
> Thierry
> 
> > +module_exit(omap_pwm_exit);
> > +
> > +MODULE_AUTHOR("Grant Erickson <marathon96@...il.com>");
> > +MODULE_AUTHOR("NeilBrown <neilb@...e.de>");
> 
> Shouldn't this be "Neil Brown"? I noticed you use the concatenated form
> in the email address as well, so maybe that's on purpose?

Yes, it is on purpose.  With a surname like "Brown", one likes finding ways
to be distinctive :-)

> 
> > +MODULE_LICENSE("GPLv2");
> 
> This should be "GPL v2". Maybe MODULE_DESCRIPTION() would be nice too.

Fixed.

> 
> Thierry

Thanks for your very thorough review!  I'll send an updated version once I've
resolved other comments and tested again.

NeilBrown

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