Messages in this thread | | | Date | Tue, 24 Sep 2019 08:39:26 +0200 | From | Uwe Kleine-König <> | Subject | Re: [PATCH 1/2] pwm: Add different PWM output types support |
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On Mon, Sep 23, 2019 at 10:43:43PM -0700, Guru Das Srinagesh wrote: > On Mon, Sep 16, 2019 at 04:01:46PM +0200, Thierry Reding wrote: > > On Fri, Sep 13, 2019 at 03:57:43PM -0700, Guru Das Srinagesh wrote: > > > From: Fenglin Wu <fenglinw@codeaurora.org> > > > > > > Normally, PWM channel has fixed output until software request to change > > > its settings. There are some PWM devices which their outputs could be > > > changed autonomously according to a predefined pattern programmed in > > > hardware. Add pwm_output_type enum type to identify these two different > > > PWM types and add relevant helper functions to set and get PWM output > > > types and pattern. > > > > > > Change-Id: Ia1f914a45ab4f4dd7be037a395eeb89d0e65a80e > > > Signed-off-by: Fenglin Wu <fenglinw@codeaurora.org> > > > Signed-off-by: Guru Das Srinagesh <gurus@codeaurora.org> > > > --- > > > drivers/pwm/core.c | 26 ++++++++++++++++++++ > > > drivers/pwm/sysfs.c | 50 ++++++++++++++++++++++++++++++++++++++ > > > include/linux/pwm.h | 70 +++++++++++++++++++++++++++++++++++++++++++++++++++++ > > > 3 files changed, 146 insertions(+) > > > > This doesn't seem right to me. Are you describing a PWM pin that's > > actually driven in GPIO mode? We usually configure that using pinctrl. > > > > Thierry > > Sorry, let me clarify. > > Some Qualcomm PMICs have a PWM block called the Light Pulse Generator (LPG). > This block allows for the generation of a HW-controlled PWM "pattern", i.e. a > sequential altering of duty cycle, in addition to the normal PWM "fixed" duty > cycle operation, which is what the framework does currently. This pattern is > user-configurable in the form of a look-up table in the devicetree. The LPG's > registers have to be configured with the data in the look up table in order to > start the generation of the pattern. An example of a pattern is the "breath" > pattern, which simply ramps up the duty cycle and then ramps it down.
I'll try to describe it in my words to check if I got it right: So the mode you want to add needs a sequence of PWM states and the hardware is expected to apply them in turn, each for a configurable count of periods. If I understand this right, this is expected to be cyclic?
> This "pattern" mode is what has been defined as PWM_OUTPUT_MODULATED in this > patch. I see that the use of the term "modulated" is misleading - a more > accurate term would be PWM_OUTPUT_PATTERN perhaps.
Not sure "pattern" is better.
The PWM on the newer imx SoCs (using the imx27 driver) has a FIFO with length 4 that allows to program changing settings. Only the duty cycle can be modified and as repeat count only 1, 2, 4 and 8 are available. I assume the FIFO can be fed by the dma engine.
Note I only know this feature from reading the reference manual and never used it.
> This patch merely adds framework support to differentiate between the "fixed" > and "pattern" modes of operation. Actions such as configuring the LPG with the > devicetree pattern and setting it up for generating the pattern are performed > in the driver only if the output type is read as "pattern" and not otherwise.
Up to now I'm not convinced that this extension is a good one that can be supported by several PWM implementations. I'd say we should collect first some details about different implementations and what these could implement to get a picture what kind of API is sensible.
Best regards Uwe
-- Pengutronix e.K. | Uwe Kleine-König | Industrial Linux Solutions | http://www.pengutronix.de/ |
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