| Message ID | 1566630445-4599-1-git-send-email-wahrenst@gmx.net (mailing list archive) |
|---|---|
| Headers | show |
| Series | pwm: bcm2835: Minor fixes | expand |
Hello Stefan, On Sat, Aug 24, 2019 at 09:07:22AM +0200, Stefan Wahren wrote: > This small patch series contains minor fixes for clock handling and the > period range check. I'd like to understand the various different usecases of PWMs. The in-kernel consumers are kind of obvious, but sysfs users are not. It seems you are one of the latter. Would you mind sharing what you control using the PWM? Does it bother you that the sysfs interface doesn't allow atomic configuration? Assuming you have some interest in this driver: It still uses the legacy stuff implementing .config, .enable, .disable, .set_polarity. Are you willing to test (or even implement) a switch to .apply instead? Just from a quick lock into the driver I wonder a few things, maybe you can shed some light here. If there is publicly available documenation for this piece of hardware, feel free to point this out, then I might be able to work out some of the answers myself. The overall (and common) design of the PWM is an input clk, a counter, a duty and a period register. The slightly simplified logic in .config is: scaler = DIV_ROUND_CLOSEST(NSEC_PER_SEC, rate); writel(DIV_ROUND_CLOSEST(duty_ns, scaler), PWM_DUTY); writel(DIV_ROUND_CLOSEST(period_ns, scaler), PWM_PERIOD); This is loosing precision while the calculation could be cheaper (in CPU cycles) and more exact when using: writel(DIV_ROUND_CLOSEST(duty_ns * rate, NSEC_PER_SEC), PWM_DUTY); writel(DIV_ROUND_CLOSEST(period_ns * rate, NSEC_PER_SEC), PWM_PERIOD); This is only two divisions instead of three. And assuming a rate of 9.2 MHz and a request of duty_ns = 499945, period_ns = 999891 the original calculation yields DUTY = 4587 PERIOD = 9173 real_duty_ns = 498586.95652173914 real_period_ns = 997065.2173913043 error_duty_ns = 1358.0434782608645 error_period_ns = 2825.7826086956775 With my suggestion you'd get DUTY = 4599 PERIOD = 9199 real_duty_ns = 499891.3043478261 real_period_ns = 999891.304347826 error_duty_ns = 53.69565217389027 error_period_ns = -0.30434782605152577 (But having said this, I'd prefer to use rounding down instead of rounding closest). Also I wonder if reprogramming the hardware completes the currently running period and how the hardware behaves on disable. (In the latter case I'm interested in "Does it complete the running period?" and "Which state does the output stop in?") Best regards Uwe
Hi Uwe, Am 24.08.19 um 11:25 schrieb Uwe Kleine-König: > Hello Stefan, > > On Sat, Aug 24, 2019 at 09:07:22AM +0200, Stefan Wahren wrote: >> This small patch series contains minor fixes for clock handling and the >> period range check. > I'd like to understand the various different usecases of PWMs. The > in-kernel consumers are kind of obvious, but sysfs users are not. It > seems you are one of the latter. > > Would you mind sharing what you control using the PWM? not really a PWM user with BCM2835. It's more the motivation as a platform maintainer to keep the drivers in shape. At work we are using sysfs interface for user applications to control ventilation (via hwmon interface) and EV charging (IEC 61851) with a different SoC. > Does it bother > you that the sysfs interface doesn't allow atomic configuration? To be honest not really, but i think a lot of user could benefit and might stop using hacky Python scripts which manipulate the registers directly. > > Assuming you have some interest in this driver: It still uses the legacy > stuff implementing .config, .enable, .disable, .set_polarity. Are you > willing to test (or even implement) a switch to .apply instead? Yes, definitely. This is one of my never ending TODO list [1]. But i would be suprised that you wont have access to a Raspberry Pi. > > Just from a quick lock into the driver I wonder a few things, maybe you > can shed some light here. If there is publicly available documenation > for this piece of hardware, feel free to point this out, then I might be > able to work out some of the answers myself. Fortunately yes [2] > > The overall (and common) design of the PWM is an input clk, a counter, a > duty and a period register. > > The slightly simplified logic in .config is: > > scaler = DIV_ROUND_CLOSEST(NSEC_PER_SEC, rate); > writel(DIV_ROUND_CLOSEST(duty_ns, scaler), PWM_DUTY); > writel(DIV_ROUND_CLOSEST(period_ns, scaler), PWM_PERIOD); > > This is loosing precision while the calculation could be cheaper (in CPU > cycles) and more exact when using: > > writel(DIV_ROUND_CLOSEST(duty_ns * rate, NSEC_PER_SEC), PWM_DUTY); > writel(DIV_ROUND_CLOSEST(period_ns * rate, NSEC_PER_SEC), PWM_PERIOD); > > This is only two divisions instead of three. And assuming a rate of 9.2 > MHz and a request of duty_ns = 499945, period_ns = 999891 the original > calculation yields > > DUTY = 4587 > PERIOD = 9173 > > real_duty_ns = 498586.95652173914 > real_period_ns = 997065.2173913043 > > error_duty_ns = 1358.0434782608645 > error_period_ns = 2825.7826086956775 > > With my suggestion you'd get > > DUTY = 4599 > PERIOD = 9199 > > real_duty_ns = 499891.3043478261 > real_period_ns = 999891.304347826 > > error_duty_ns = 53.69565217389027 > error_period_ns = -0.30434782605152577 > > (But having said this, I'd prefer to use rounding down instead of > rounding closest). > > Also I wonder if reprogramming the hardware completes the currently > running period and how the hardware behaves on disable. (In the latter > case I'm interested in "Does it complete the running period?" and "Which > state does the output stop in?") I need to make logic analyzer traces to make any statement. Thanks [1] - https://github.com/lategoodbye/rpi-zero/issues/16 [2] - https://www.raspberrypi.org/app/uploads/2012/02/BCM2835-ARM-Peripherals.pdf > > Best regards > Uwe >
Hello Stefan, On Sat, Aug 24, 2019 at 12:05:00PM +0200, Stefan Wahren wrote: > Am 24.08.19 um 11:25 schrieb Uwe Kleine-König: > > Hello Stefan, > > > > On Sat, Aug 24, 2019 at 09:07:22AM +0200, Stefan Wahren wrote: > >> This small patch series contains minor fixes for clock handling and the > >> period range check. > > > > I'd like to understand the various different usecases of PWMs. The > > in-kernel consumers are kind of obvious, but sysfs users are not. It > > seems you are one of the latter. > > > > Would you mind sharing what you control using the PWM? > > not really a PWM user with BCM2835. It's more the motivation as a > platform maintainer to keep the drivers in shape. At work we are using > sysfs interface for user applications to control ventilation (via hwmon > interface) and EV charging (IEC 61851) with a different SoC. I don't understand how you use the sysfs interface and still interact with the hwmon interface. Other than that, thanks for sharing. > > Assuming you have some interest in this driver: It still uses the legacy > > stuff implementing .config, .enable, .disable, .set_polarity. Are you > > willing to test (or even implement) a switch to .apply instead? > > Yes, definitely. This is one of my never ending TODO list [1]. But i > would be suprised that you wont have access to a Raspberry Pi. So be surprised :-) > > Just from a quick lock into the driver I wonder a few things, maybe you > > can shed some light here. If there is publicly available documenation > > for this piece of hardware, feel free to point this out, then I might be > > able to work out some of the answers myself. > > Fortunately yes [2] Care to add a link to this document in the driver for others to easily find it? Best regards Uwe
Hi Uwe, Am 24.08.19 um 12:56 schrieb Uwe Kleine-König: > Hello Stefan, > > On Sat, Aug 24, 2019 at 12:05:00PM +0200, Stefan Wahren wrote: >> Am 24.08.19 um 11:25 schrieb Uwe Kleine-König: >>> Hello Stefan, >>> >>> On Sat, Aug 24, 2019 at 09:07:22AM +0200, Stefan Wahren wrote: >>>> This small patch series contains minor fixes for clock handling and the >>>> period range check. >>> I'd like to understand the various different usecases of PWMs. The >>> in-kernel consumers are kind of obvious, but sysfs users are not. It >>> seems you are one of the latter. >>> >>> Would you mind sharing what you control using the PWM? >> not really a PWM user with BCM2835. It's more the motivation as a >> platform maintainer to keep the drivers in shape. At work we are using >> sysfs interface for user applications to control ventilation (via hwmon >> interface) and EV charging (IEC 61851) with a different SoC. > I don't understand how you use the sysfs interface and still interact > with the hwmon interface. Other than that, thanks for sharing. I meant the hwmon sysfs interface. Sure using with pwm sysfs in parallel isn't possible. > > >>> Assuming you have some interest in this driver: It still uses the legacy >>> stuff implementing .config, .enable, .disable, .set_polarity. Are you >>> willing to test (or even implement) a switch to .apply instead? >> Yes, definitely. This is one of my never ending TODO list [1]. But i >> would be suprised that you wont have access to a Raspberry Pi. > So be surprised :-) > >>> Just from a quick lock into the driver I wonder a few things, maybe you >>> can shed some light here. If there is publicly available documenation >>> for this piece of hardware, feel free to point this out, then I might be >>> able to work out some of the answers myself. >> Fortunately yes [2] > Care to add a link to this document in the driver for others to easily > find it? I don't think it's necessary. This document is easy to find via "bcm2835 datasheet". > > Best regards > Uwe >