Re: [PATCH 0/2] extend PWM framework to support PWM dead-times

[m18063]

On 22.08.2017 15:24, Benjamin Gaignard wrote:
> 2017-08-22 14:11 GMT+02:00 m18063 <Claudiu.Beznea@xxxxxxxxxxxxx>:
>> Hi Thierry,
>>
>> I added few other comments below. Please let me know what you think.
>>
>> Thank you,
>> Claudiu
>>
>> On 21.08.2017 14:25, Thierry Reding wrote:
>>> On Mon, Aug 21, 2017 at 01:23:08PM +0300, m18063 wrote:
>>>> Hi Thierry,
>>>>
>>>> Thank you for your response. I added few comments below.
>>>>
>>>> Thank you,
>>>> Claudiu
>>>>
>>>> On 21.08.2017 11:25, Thierry Reding wrote:
>>>>> On Tue, May 09, 2017 at 03:15:51PM +0300, Claudiu Beznea wrote:
>>>>>> Hi all,
>>>>>>
>>>>>> Please give feedback on these patches which extends the PWM
>>>>>> framework in order to support multiple PWM signal types.
>>>>>> Since I didn't receive any inputs on RFC series I'm resending it as
>>>>>> normal patch series.
>>>>>>
>>>>>> The current patch series add the following PWM signal types:
>>>>>> - PWM complementary signals
>>>>>> - PWM push-pull signal
>>>>>>
>>>>>> Definition of PWM complementary mode:
>>>>>> For a PWM controllers with more than one outputs per PWM channel,
>>>>>> e.g. with 2 outputs per PWM channels, the PWM complementary signals
>>>>>> have opposite levels, same duration and same starting times,
>>>>>> as in the following diagram:
>>>>>>
>>>>>>         __    __    __    __
>>>>>> PWMH __|  |__|  |__|  |__|  |__
>>>>>>      __    __    __    __    __
>>>>>> PWML   |__|  |__|  |__|  |__|
>>>>>>        <--T-->
>>>>>> Where T is the signal period.
>>>>>>
>>>>>> Definition of PWM push-pull mode:
>>>>>> For PWM controllers with more than one outputs per PWM channel,
>>>>>> e.g. with 2 outputs per PWM channel, the PWM push-pull signals
>>>>>> have same levels, same duration and are delayed until the begining
>>>>>> of the next period, as in the following diagram:
>>>>>>
>>>>>>         __          __
>>>>>> PWMH __|  |________|  |________
>>>>>>               __          __
>>>>>> PWML ________|  |________|  |__
>>>>>>        <--T-->
>>>>>>
>>>>>> Where T is the signal period.
>>>>>>
>>>>>> These output signals could be configured by setting PWM mode
>>>>>> (a new input in sysfs has been added in order to support this
>>>>>> operation).
>>>>>>
>>>>>> root@sama5d2-xplained:/sys/devices/platform/ahb/ahb:apb/f802c000.pwm/pwm/pwmchip0/pwm2# ls -l
>>>>>> -r--r--r--    1 root     root          4096 Feb  9 10:54 capture
>>>>>> -rw-r--r--    1 root     root          4096 Feb  9 10:54 duty_cycle
>>>>>> -rw-r--r--    1 root     root          4096 Feb  9 10:54 enable
>>>>>> -rw-r--r--    1 root     root          4096 Feb  9 10:54 mode
>>>>>> -rw-r--r--    1 root     root          4096 Feb  9 10:54 period
>>>>>> -rw-r--r--    1 root     root          4096 Feb  9 10:55 polarity
>>>>>> drwxr-xr-x    2 root     root             0 Feb  9 10:54 power
>>>>>> -rw-r--r--    1 root     root          4096 Feb  9 10:54 uevent
>>>>>>
>>>>>> The PWM push-pull mode could be usefull in applications like
>>>>>> half bridge converters.
>>>>>
>>>>> Sorry for taking an absurdly long time to get back to you on this.
>>>>>
>>>>> One problem I see with this is that the PWM framework is built on the
>>>>> assumption that we have a single output per PWM channel. This becomes
>>>>> important when you start adding features such as this because now the
>>>>> users have no way of determining whether or not the PWM they retrieve
>>>>> will actually be able to do what they want.
>>>> I was thinking that the framework is there to offer support in configuring
>>>> the underlying hardware without taking into account how many outputs per
>>>> PWM channels are actually present. It is true I only worked with Atmel/Microchip
>>>> PWM controllers which for instance, SAMA5 SoC has a PWM controller
>>>> with 2 outputs per PWM channel. Taking into account that the driver is
>>>> already mainlined I though that the user is aware of what he is using
>>>> (either a one output per PWM channel controller, or 2 outputs or
>>>> more than 2) and about the underlying hardware support. I understand your
>>>> position on this. I'm just explaining myself on the approach I've chose
>>>> for this implementation.
>>>
>>> So currently we assume that there will be (at least) one output per PWM
>>> channel. However there are currently no drivers that actually use any
>>> output other than the "primary" one.
>>>
>>> That means, even if there are currently PWM controllers that support
>>> multiple outputs per PWM channel, we have no code making assumptions
>>> about it.
>>>
>>> Before we can add code that makes any assumptions about the number of
>>> outputs per PWM channel, we have to add facitilities for such code to
>>> detect capabilities, so that they can deal with PWMs that don't match
>>> what they need.
>>>
>>>>> For example, let's say you have a half bridge converter driver in the
>>>>> kernel and it does a pwm_get() to obtain a PWM to use. There's nothing
>>>>> preventing it from using a simple one-output PWM and there's no way to
>>>>> check that we're actually doing something wrong.
>>>> I understand your position here. I've chose this approach taking into
>>>> account that the user of the PWM will be aware of the capabilities
>>>> of the underlying hardware because I worked on a controller which already
>>>> has a mainlined driver and which has more than one output per PWM channel
>>>> and I believe there are also other controllers with this kind of capability
>>>> and with already mainlined driver (e.g. reading the code of STM32 PWM driver
>>>> I saw a bool type variable in driver specific data structure (struct stm32_pwm):
>>>> "bool have_complementary_output" which let me think that their controller
>>>> also could support more than one output per PWM channel (I will also try
>>>> to find the controller specific datasheet). At a first look I saw that also
>>>> TI ECAP PWM controller supports this (it is true that I am not aware of how
>>>> it is initialized in kernel, I need to check the datasheet, if it is public).
>>>
>>> Yes, it's quite possible that many of the controllers we currently
>>> support have this feature or not. But that's not really relevant. The
>>> only important bit is that none of the users know about it. We don't
>>> have the means to configure anything beyond just one output. So the only
>>> guarantee you will get with the current framework is that there will be
>>> one output signal per PWM channel.
>>>
>>>>> I think any in-kernel API would have to be more fully-featured,
>>>>> otherwise we're bound to run into problems. At the very least I think
>>>>> we'd have to expose some sort of capabilities list.
>>>> About the exposing of these capabilities would you prefer to have the
>>>> supported PWM modes registered in driver probe as a new field mask
>>>> in "struct pwm_chip". e.g.:
>>>> struct pwm_chip {
>>>>         struct device *dev;
>>>>         struct list_head list;
>>>>         const struct pwm_ops *ops;
>>>>         int base;
>>>>         unsigned int npwm;
>>>>      unsigned int pwm_modes_mask;    /* bitfield of supported signal types */
>>>>
>>>>         struct pwm_device *pwms;
>>>>
>>>>         struct pwm_device * (*of_xlate)(struct pwm_chip *pc,
>>>>                                         const struct of_phandle_args *args);
>>>>         unsigned int of_pwm_n_cells;
>>>> };
>>>>
>>>> And having in driver_probe() something like this:
>>>> driver_data->pwm_chip.pwm_modes = PWM_COMPLEMENTARY_MODE | PWM_PUSH_PULL_MODE;
>>>> pwmchip_add(&driver_data->chip);
>>>> Since the PWM push-pull mode imply more than one output per PWM channel. And
>>>> validate the supported PWM modes when trying to configure one.
>>>>
>>>> Or registering, as a field in pwm_chip, how many outputs per channel are actually
>>>> supported by the PWM controller and then validate the supported PWM mode based
>>>> on this?
>>>> e.g.:
>>>> in "struct pwm_chip". e.g.:
>>>> struct pwm_chip {
>>>>         struct device *dev;
>>>>         struct list_head list;
>>>>         const struct pwm_ops *ops;
>>>>         int base;
>>>>         unsigned int npwm;
>>>>      unsigned int pwm_outputs;       /* Number of supported outputs per channel */>>
>>>>         struct pwm_device *pwms;
>>>>
>>>>         struct pwm_device * (*of_xlate)(struct pwm_chip *pc,
>>>>                                         const struct of_phandle_args *args);
>>>>         unsigned int of_pwm_n_cells;
>>>> };
>>>>
>>>> In driver_probe():
>>>> //...
>>>> driver_data->pwm_chip.pwm_outputs = 2; /* 2 outputs per PWM channel. */
>>>> pwmchip_add(&driver_data->chip);
>>>
>>> I think I'd prefer something more flexible. I also think this can't be
>>> per-chip, but really has to be at the granularity of single PWM
>>> channels. How about something like this:
>>>
>>>       struct pwm_caps {
>>>               ...
>>>       };
>>>
>>>       int pwm_get_caps(struct pwm_device *pwm, struct pwm_caps *caps);
>>>
>>> From a high-level perspective that would give users an easy way to
>>> obtain the capabilities of the PWM they're handed.
>> I forgot to mention, I don't know if you had time to look over the other
>> patch series I send to extend the PWM framework:
>> "[PATCH v2 0/2] extends PWM framework to support PWM dead-times"
>> (v2 since the auto build robot prompted me some compilation errors).
>> In that series I proposed some extensions to be able to configure
>> dead-times for PWM channels. Those changes are also specific to PWM
>> controllers which has more than 1 output per PWM channel. Taking into
>> account that:
>> - this is also specific to PWM controllers with more than 1 output per
>> PWM channel and
>> - your proposed below to introduce capabilities specific to more than
>> one output per PWM channel and not to introduce something to specify that
>> the PWM chip or PWM device supports a number of X outputs per PWM channel and
>> - PWM dead-time is, in my opinion, some kind of PWM push-pull mode with
>> lower delays b/w the edges of complementary outputs:
>> Do you think it would be ok (in order to also have a way to configure
>> PWM dead-times) to merge the changes of
>> "[PATCH v2 0/2] extends PWM framework to support PWM dead-times" with the
>> changes that I will do for push-pull mode (with your below proposals)
>> and have a single PWM mode for both PWM push-pull and PWM dead-times
>> (e.g. the PWM_MODE_PUSH_PULL) and to have an interface which could be used
>> to configure the dead-times that will work only when the PWM device supports
>> push-pull mode and is configured in push-pull mode?
>> Something like this:
>>
>> root@sama5d2-xplained:/sys/devices/platform/ahb/ahb:apb/f802c000.pwm/pwm/pwmchip0/pwm2# ls -l
>> -r--r--r--    1 root     root          4096 Feb  9 10:54 capture
>> -rw-r--r--    1 root     root          4096 Feb  9 10:54 duty_cycle
>> -rw-r--r--    1 root     root          4096 Feb  9 10:54 deadtime_re    <<<< interface for configuring dead-time rising edge delay
>> -rw-r--r--    1 root     root          4096 Feb  9 10:54 deadtime_fe    <<<< interface for configuring dead-time falling edge delay
>> -rw-r--r--    1 root     root          4096 Feb  9 10:54 enable
>> -rw-r--r--    1 root     root          4096 Feb  9 10:54 mode           <<<< interface for PWM mode configuration
>> -rw-r--r--    1 root     root          4096 Feb  9 10:54 period
>> -rw-r--r--    1 root     root          4096 Feb  9 10:55 polarity
>> drwxr-xr-x    2 root     root             0 Feb  9 10:54 power
>> -rw-r--r--    1 root     root          4096 Feb  9 10:54 uevent
>>
>> When user switch to push-pull mode it could start configure
>> the dead-times (in nanoseconds) to obtain a signal of a form similar to:
>>                       ____0    D____P     ____      ____      ____
>>         PWM signal __|    |____|    |____|    |____|    |____|    |___
>>                         __        __        __        __        __
>>               PWMH ____|  |____re|  |______|  |______|  |______|  |___
>>                    __        __        __        __        __        __
>>               PWML   |______|  |____fe|  |______|  |______|  |______|
>>
>> (see "[PATCH v2 0/2] extends PWM framework to support PWM dead-times" cover
>> letter),
>> and starting from a driver specific dead-time value (e.g. on Atmel/Microchip
>> PWM controller starting with a dead-time value of ~0.05 microseconds) the PWM
>> outputs to actually switch to PWM push-pull mode (in this mode there is a delay
>> of a period introduce on a PWM ouptut) to obtain a signal similar to this one:
>>          __          __
>>  PWMH __|  |________|  |________
>>                __          __
>>  PWML ________|  |________|  |__
>>         <--T-->
>>
>> Then we can start
>>> filling in that structure.
>>>
>>>       struct pwm_caps {
>>>               unsigned long modes;
>>>       };
>>>
>>>       #define PWM_MODE_NORMAL (1 << 0)
>>>       #define PWM_MODE_COMPLEMENTARY (1 << 1)
>>>       #define PWM_MODE_PUSH_PULL (1 << 2)
>>>
>>>       static inline bool pwm_caps_supports_mode(const struct pwm_caps *caps,
>>>                                                 unsigned long mode)
>>>       {
>>>               return (caps->modes & mode) != 0;
>>>       }
>>>
>>> and maybe something like this as a shortcut:
>>>
>>>       static inline bool pwm_supports_mode(struct pwm_device *pwm,
>>>                                            unsigned long mode)
>>>       {
>>>               struct pwm_caps caps;
>>>               int err;
>>>
>>>               err = pwm_get_caps(pwm, &caps);
>>>               if (err < 0)
>>>                       return false;
>>>
>>>               return pwm_caps_supports_mode(&caps, mode);
>>>       }
>>>
>>> I think that gives us a lot of flexibility and easy extensibility for
>>> other capabilities. The implementation of pwm_get_caps() should probably
>>> call back into the driver.>
>>> But that would cause a lot of boilerplate for simple cases, and would be
>>> a lot of work to convert all existing drivers up front. So I think we
>>> can add helpers to solve the normal cases. Something along these lines
>>> perhaps:
>>>
>>>       struct pwm_chip {
>>>               ...
>>>
>>>               int (*get_caps)(struct pwm_device *pwm, struct pwm_caps *caps);
>>>
>>>               const struct pwm_caps *caps;
>>>       };
>>>
>>>       static int pwm_simple_get_caps(struct pwm_device *pwm, struct pwm_caps *caps)
>>>       {
>>>               if (!pwm || !caps || !pwm->chip->caps)
>>>                       return -EINVAL;
>>>
>>>               *caps = *pwm->chip->caps;
>>>
>>>               return 0;
>>>       }
>>>
>>> And then perhaps even do something like this:
>>>
>>>       const struct pwm_caps pwm_default_caps = {
>>>               .modes = PWM_MODE_NORMAL,
>>>       };
>>>> Now we can either have a patch that sets pwm_default_caps and
>>> pwm_simple_get_caps() for all existing drivers, or even easier add this
>>> to the pwmchip_add_with_polarity() function:
>>>
>>>       int pwmchip_add_with_polarity(struct pwm_chip *chip,
>>>                                     enum pwm_polarity polarity)
>>>       {
>>>               ...
>>>
>>>               if (!chip->get_caps && !chip->caps) {
>>>                       chip->get_caps = pwm_simple_get_caps;
>>>                       chip->caps = pwm_default_caps;
>>>               }
>>>
>>>               ...
>>>       }
>>>
>> What about having a new member in "struct pwm_ops" something like this:
>> struct pwm_ops {
>> // ...
>>         int (*get_caps)(struct pwm_chip *chip, struct pwm_device *pwm, struct pwm_caps *caps);
>> // ...
>> };
>> to get from the driver the per PWM capabilities.
>>
>> And maybe to have, in core.c:
>> int pwm_get_caps(struct pwm_device *pwm, struct pwm_caps *caps)
>> {
>>         struct pwm_chip *chip = pwm->chip;
>>         int ret = 0;
>>
>>         if (!pwm || !chip || !chip->ops || !caps)
>>                 return -EINVAL;
>>
>>         if (chip->ops->get_caps)
>>                 ret = chip->ops->get_caps(chip, pwm, caps);
>>         else
>>                 caps = &pwm_default_caps;
>>
>>         return ret;
>> }
>>
>>> That way, every driver, unless upgraded with a custom ->get_caps() would
>>> get the defaults, which encode the current assumptions of the framework.
>>>
>>> I think what we'd also want to do is make sure that every driver always
>>> at least implement NORMAL mode and perhaps even fail to register those
>>> that don't. Not sure we want to go that far, but those are the current
>>> assumptions, so devices must be supporting it already anyway.
>>>
>>>>> A possibly simpler approach might be to restrict this to the driver that
>>>>> you need this for. It looks like you will be mainly using this via sysfs
>>>>> and in that case you might be able to just extend what information is
>>>>> exposed in sysfs for the particular PWM you're dealing with.
>>>> By this you mean exporting the sysfs attributes only for my driver or possibly
>>>> other drivers interested in this new feature? I may have another in kernel driver
>>>> which may try to use this feature.
>>>
>>> If you've got another driver that will want to use this, I'm leaning
>>> towards going with the fully featured approach.
>> I'm thinking at PWM regulator driver. I worked lately on adding code to PWM
>> framework in order to support what our PWM controller calls PWM triggers.
>> These are external input received directly by the PWM controller which let
>> it change the PWM output waveform, as long as the trigger is active, only
>> at the controller level (no interaction with kernel code interaction -
>> no IRQ received on trigger activation).
>> This could be used, e.g., in regulators with feedback. I'm thinking that this
>> could be useful for PWM regulator driver when it is using a PWM channel
>> in push-pull mode and with trigger configuration.
>>
>> I have create that code on top of push-pull mode modifications since
>> I want to take advantage of push-pull mode while using PWM triggers.
>> Of course this is something I didn't introduce (I can give more details
>> on this if you want).
> 
> STM32 PWM have the same kind of triggers features but, since the harware
> block can do more than PWM, we have implemented the trigger part in IIO.
> It allow use to controls (start/stop) PWMs on events or levels.
It is true! I am aware of that and followed latest changes on that STM32 PWM
driver. Atmel/Microchip PWM hardware is only PWM aware. I though at taking advantage
of the IIO subsystem and integrate the PWM in there but it looked complicated
to integrate both subsystems.
Basically, on the Atmel/Microchip PWM controller setting a specific trigger
involves setting a particular register bit. And, yes, as you said, based
on external SoC events, the PWM output waveforms is disabled/enabled (based
on the external events).

Thank you,
Claudiu
> 
>>
>> Thank you,
>> Claudiu
>>
>> Ideally you would try to
>>> implement both at once so we can get a better feeling about whether or
>>> not the framework changes are adequate to cover at least the first two
>>> cases, which is much more reassuring than just being able to cover a
>>> single case.
>>>
>>> One thing to consider is still that if you only use this from sysfs,
>>> it'll be quite a lot of work to add in-kernel infrastructure when it's
>>> never needed. There's still an advantage to do it anyway because it
>>> allows us to abstract everything away properly from the start and avoid
>>> fragmentation of the sysfs interface.
>>>
>>> Thierry
>>>
>>
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> 
> 
>