| Message ID | da60a97a0c6f754faa1f996334b3568ddb2f5ed0.1528137144.git.tommyhebb@gmail.com |
|---|---|
| State | Changes Requested |
| Headers | show |
| Series | [RESEND] pwm: berlin: Don't use broken prescaler values | expand |
On Mon, Jun 04, 2018 at 02:32:41PM -0400, Thomas Hebb wrote: > Six of the eight prescaler values available for Berlin PWM are not true > prescalers but rather internal shifts that throw away the high bits of > TCNT. Currently, we attempt to use those high bits, leading to erratic > behavior. Restrict the prescaler configurations we select to only the > two that respect the full range of TCNT. > > Tested on BG2CD. > > Signed-off-by: Thomas Hebb <tommyhebb@gmail.com> > --- > drivers/pwm/pwm-berlin.c | 45 ++++++++++++++++++++++------------------ > 1 file changed, 25 insertions(+), 20 deletions(-) Antoine, Jisheng, can you guys review this patch? I'm personally on the fence about this, even if we can technically do the shift in software, I don't necessarily see a reason why we can't "offload" to the hardware. Thierry
On 06/05/2018 05:10 AM, Thierry Reding wrote: > On Mon, Jun 04, 2018 at 02:32:41PM -0400, Thomas Hebb wrote: >> Six of the eight prescaler values available for Berlin PWM are not true >> prescalers but rather internal shifts that throw away the high bits of >> TCNT. Currently, we attempt to use those high bits, leading to erratic >> behavior. Restrict the prescaler configurations we select to only the >> two that respect the full range of TCNT. >> >> Tested on BG2CD. >> >> Signed-off-by: Thomas Hebb <tommyhebb@gmail.com> >> --- >> drivers/pwm/pwm-berlin.c | 45 ++++++++++++++++++++++------------------ >> 1 file changed, 25 insertions(+), 20 deletions(-) > > Antoine, Jisheng, > > can you guys review this patch? I'm personally on the fence about this, > even if we can technically do the shift in software, I don't necessarily > see a reason why we can't "offload" to the hardware. > > Thierry Sorry if my commit message was unclear: this patch doesn't just arbitrarily change the hw/sw division of responsibility. The driver in its current state is broken (at least on BG2CD), and this patch implements a fix. The reason the middle six prescaler values are useless is because they do not actually slow down the clock. Instead, they emulate slowing down the clock by internally multiplying TCNT. This would be a fine trick, if not for the fact that the internal TCNT value has no extra bits beyond the 16 already exposed to software by the register. What this means is that, for a prescaler of 4, the software must ensure that the top two bits of TCNT are not set, because hardware will chop them off; for a prescaler of 8, the top three bits must not be set, and so forth. Software does not currently ensure this, resulting in a TCNT several orders of magnitude lower than intended any time one of those six prescalers are selected. Because hardware chops off the high bits in its internal shift, the middle six prescalers don't actually allow *anything* that the first doesn't. In fact, they are strictly worse than the first, since the internal shift of TCNT prevents software from setting the low bits, decreasing the resolution, without providing any extra high bits. By skipping the useless prescalers entirely, this patch actually increases the driver's performance, since, when the 4096 prescaler is selected, it now does only a single shift rather than the seven successive divisions it did before. Let me know if any of this is still unclear, or if you'd like me to revise the commit message. -Tom -- To unsubscribe from this list: send the line "unsubscribe linux-pwm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Tue, Jun 05, 2018 at 12:48:51PM -0400, Tom Hebb wrote: > On 06/05/2018 05:10 AM, Thierry Reding wrote: > > On Mon, Jun 04, 2018 at 02:32:41PM -0400, Thomas Hebb wrote: > >> Six of the eight prescaler values available for Berlin PWM are not true > >> prescalers but rather internal shifts that throw away the high bits of > >> TCNT. Currently, we attempt to use those high bits, leading to erratic > >> behavior. Restrict the prescaler configurations we select to only the > >> two that respect the full range of TCNT. > >> > >> Tested on BG2CD. > >> > >> Signed-off-by: Thomas Hebb <tommyhebb@gmail.com> > >> --- > >> drivers/pwm/pwm-berlin.c | 45 ++++++++++++++++++++++------------------ > >> 1 file changed, 25 insertions(+), 20 deletions(-) > > > > Antoine, Jisheng, > > > > can you guys review this patch? I'm personally on the fence about this, > > even if we can technically do the shift in software, I don't necessarily > > see a reason why we can't "offload" to the hardware. > > > > Thierry > > Sorry if my commit message was unclear: this patch doesn't just > arbitrarily change the hw/sw division of responsibility. The driver in > its current state is broken (at least on BG2CD), and this patch > implements a fix. > > The reason the middle six prescaler values are useless is because they > do not actually slow down the clock. Instead, they emulate slowing down > the clock by internally multiplying TCNT. > > This would be a fine trick, if not for the fact that the internal TCNT > value has no extra bits beyond the 16 already exposed to software by the > register. What this means is that, for a prescaler of 4, the software > must ensure that the top two bits of TCNT are not set, because hardware > will chop them off; for a prescaler of 8, the top three bits must not be > set, and so forth. Software does not currently ensure this, resulting in > a TCNT several orders of magnitude lower than intended any time one of > those six prescalers are selected. > > Because hardware chops off the high bits in its internal shift, the > middle six prescalers don't actually allow *anything* that the first > doesn't. In fact, they are strictly worse than the first, since the > internal shift of TCNT prevents software from setting the low bits, > decreasing the resolution, without providing any extra high bits. > > By skipping the useless prescalers entirely, this patch actually > increases the driver's performance, since, when the 4096 prescaler is > selected, it now does only a single shift rather than the seven > successive divisions it did before. > > Let me know if any of this is still unclear, or if you'd like me to > revise the commit message. Perfect, the above, with slight adaptations, would make a great commit message. =) Thierry
diff --git a/drivers/pwm/pwm-berlin.c b/drivers/pwm/pwm-berlin.c index 771859aca4be..7c8d6a168ceb 100644 --- a/drivers/pwm/pwm-berlin.c +++ b/drivers/pwm/pwm-berlin.c @@ -21,8 +21,18 @@ #define BERLIN_PWM_EN 0x0 #define BERLIN_PWM_ENABLE BIT(0) #define BERLIN_PWM_CONTROL 0x4 -#define BERLIN_PWM_PRESCALE_MASK 0x7 -#define BERLIN_PWM_PRESCALE_MAX 4096 +/* + * The prescaler claims to support 8 different moduli, configured using the + * low three bits of PWM_CONTROL. (Sequentially, they are 1, 4, 8, 16, 64, + * 256, 1024, and 4096.) However, the moduli from 4 to 1024 appear to be + * implemented by internally shifting TCNT left without adding additional + * bits. So, the max TCNT that actually works for a modulus of 4 is 0x3fff; + * for 8, 0x1fff; and so on. This means that those moduli are entirely + * useless, as we could just do the shift ourselves. The 4096 modulus is + * implemented with a real prescaler, so we do use that, but we treat it + * as a flag instead of pretending the modulus is actually configurable. + */ +#define BERLIN_PWM_PRESCALE_4096 0x7 #define BERLIN_PWM_INVERT_POLARITY BIT(3) #define BERLIN_PWM_DUTY 0x8 #define BERLIN_PWM_TCNT 0xc @@ -46,10 +56,6 @@ static inline struct berlin_pwm_chip *to_berlin_pwm_chip(struct pwm_chip *chip) return container_of(chip, struct berlin_pwm_chip, chip); } -static const u32 prescaler_table[] = { - 1, 4, 8, 16, 64, 256, 1024, 4096 -}; - static inline u32 berlin_pwm_readl(struct berlin_pwm_chip *chip, unsigned int channel, unsigned long offset) { @@ -86,33 +92,32 @@ static int berlin_pwm_config(struct pwm_chip *chip, struct pwm_device *pwm_dev, int duty_ns, int period_ns) { struct berlin_pwm_chip *pwm = to_berlin_pwm_chip(chip); - unsigned int prescale; + bool prescale_4096 = false; u32 value, duty, period; - u64 cycles, tmp; + u64 cycles; cycles = clk_get_rate(pwm->clk); cycles *= period_ns; do_div(cycles, NSEC_PER_SEC); - for (prescale = 0; prescale < ARRAY_SIZE(prescaler_table); prescale++) { - tmp = cycles; - do_div(tmp, prescaler_table[prescale]); + if (cycles > BERLIN_PWM_MAX_TCNT) { + prescale_4096 = true; + cycles >>= 12; // Prescaled by 4096 - if (tmp <= BERLIN_PWM_MAX_TCNT) - break; + if (cycles > BERLIN_PWM_MAX_TCNT) + return -ERANGE; } - if (tmp > BERLIN_PWM_MAX_TCNT) - return -ERANGE; - - period = tmp; - cycles = tmp * duty_ns; + period = cycles; + cycles *= duty_ns; do_div(cycles, period_ns); duty = cycles; value = berlin_pwm_readl(pwm, pwm_dev->hwpwm, BERLIN_PWM_CONTROL); - value &= ~BERLIN_PWM_PRESCALE_MASK; - value |= prescale; + if (prescale_4096) + value |= BERLIN_PWM_PRESCALE_4096; + else + value &= ~BERLIN_PWM_PRESCALE_4096; berlin_pwm_writel(pwm, pwm_dev->hwpwm, value, BERLIN_PWM_CONTROL); berlin_pwm_writel(pwm, pwm_dev->hwpwm, duty, BERLIN_PWM_DUTY);
Six of the eight prescaler values available for Berlin PWM are not true prescalers but rather internal shifts that throw away the high bits of TCNT. Currently, we attempt to use those high bits, leading to erratic behavior. Restrict the prescaler configurations we select to only the two that respect the full range of TCNT. Tested on BG2CD. Signed-off-by: Thomas Hebb <tommyhebb@gmail.com> --- drivers/pwm/pwm-berlin.c | 45 ++++++++++++++++++++++------------------ 1 file changed, 25 insertions(+), 20 deletions(-)