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Date: Thu, 28 Oct 2021 13:26:10 +0100
From: Sean Young <sean@...s.org>
To: Uwe Kleine-König
<u.kleine-koenig@...gutronix.de>
Cc: Maíra Canal <maira.canal@....br>, lkp@...el.com,
mchehab@...nel.org, thierry.reding@...il.com, lee.jones@...aro.org,
llvm@...ts.linux.dev, kbuild-all@...ts.01.org,
linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-pwm@...r.kernel.org
Subject: Re: [PATCH v4] media: rc: pwm-ir-tx: Switch to atomic PWM API
Hi Uwe,
On Thu, Oct 28, 2021 at 01:15:35PM +0200, Uwe Kleine-König wrote:
> On Thu, Oct 28, 2021 at 10:14:42AM +0100, Sean Young wrote:
> > On Thu, Oct 28, 2021 at 08:45:13AM +0200, Uwe Kleine-König wrote:
> > > The conversion is right (I think),
> >
> > We still have the problem that the pwm drivers calculate the period
> > incorrectly by rounding down (except pwm-bcm2835). So the period is not
> > as good as it could be in most cases, but this driver can't do anything
> > about that.
>
> Yeah, some time ago I started coding a round_state function
> (wip at
> https://git.pengutronix.de/cgit/ukl/linux/commit/?h=pwm-wip&id=ae348eb6a55d6526f30ef4a49819197d9616391e)
> but this was pushed down on my todo-list by more important stuff.
That looks great, thank you for working on that!
> If you want to experiment with that ...
I will have a look.
> > > note this could be optimized a bit
> > > further: state.period only depends on carrier which rarely changes, so
> > > the calculation could be done in pwm_ir_set_carrier(). Ditto for duty
> > > which only depends on state.period and pwm_ir->duty_cycle. (This is for
> > > a separate commit though.)
> >
> > I'm not sure what caching this is much of a win. The calculation is a few
> > instructions, so you're not winning in the way of speed. On the flip side
> > you use more memory since pwm_state has to be kmalloc() rather than existing
>
> I tested a bit with this patch on top of Maíra's:
>
> diff --git a/drivers/media/rc/pwm-ir-tx.c b/drivers/media/rc/pwm-ir-tx.c
> index 105a9c24f1e3..7585c21775bc 100644
> --- a/drivers/media/rc/pwm-ir-tx.c
> +++ b/drivers/media/rc/pwm-ir-tx.c
> @@ -17,7 +17,7 @@
>
> struct pwm_ir {
> struct pwm_device *pwm;
> - unsigned int carrier;
> + struct pwm_state state;
> unsigned int duty_cycle;
> };
>
> @@ -32,6 +32,7 @@ static int pwm_ir_set_duty_cycle(struct rc_dev *dev, u32 duty_cycle)
> struct pwm_ir *pwm_ir = dev->priv;
>
> pwm_ir->duty_cycle = duty_cycle;
> + pwm_set_relative_duty_cycle(&pwm_ir->state, pwm_ir->duty_cycle, 100);
>
> return 0;
> }
> @@ -43,7 +44,8 @@ static int pwm_ir_set_carrier(struct rc_dev *dev, u32 carrier)
> if (!carrier)
> return -EINVAL;
>
> - pwm_ir->carrier = carrier;
> + pwm_ir->state.period = DIV_ROUND_CLOSEST(NSEC_PER_SEC, carrier);
> + pwm_set_relative_duty_cycle(&pwm_ir->state, pwm_ir->duty_cycle, 100);
>
> return 0;
> }
> @@ -53,21 +55,15 @@ static int pwm_ir_tx(struct rc_dev *dev, unsigned int *txbuf,
> {
> struct pwm_ir *pwm_ir = dev->priv;
> struct pwm_device *pwm = pwm_ir->pwm;
> - struct pwm_state state;
> int i;
> ktime_t edge;
> long delta;
>
> - pwm_init_state(pwm, &state);
> -
> - state.period = DIV_ROUND_CLOSEST(NSEC_PER_SEC, pwm_ir->carrier);
> - pwm_set_relative_duty_cycle(&state, pwm_ir->duty_cycle, 100);
> -
> edge = ktime_get();
>
> for (i = 0; i < count; i++) {
> - state.enabled = !(i % 2);
> - pwm_apply_state(pwm, &state);
> + pwm_ir->state.enabled = !(i % 2);
> + pwm_apply_state(pwm, &pwm_ir->state);
>
> edge = ktime_add_us(edge, txbuf[i]);
> delta = ktime_us_delta(edge, ktime_get());
> @@ -75,8 +71,8 @@ static int pwm_ir_tx(struct rc_dev *dev, unsigned int *txbuf,
> usleep_range(delta, delta + 10);
> }
>
> - state.enabled = false;
> - pwm_apply_state(pwm, &state);
> + pwm_ir->state.enabled = false;
> + pwm_apply_state(pwm, &pwm_ir->state);
>
> return count;
> }
> @@ -95,8 +91,9 @@ static int pwm_ir_probe(struct platform_device *pdev)
> if (IS_ERR(pwm_ir->pwm))
> return PTR_ERR(pwm_ir->pwm);
>
> - pwm_ir->carrier = 38000;
> - pwm_ir->duty_cycle = 50;
> + pwm_ir->state.duty_cycle = 50;
> + pwm_init_state(pwm_ir->pwm, &pwm_ir->state);
> + pwm_ir_set_carrier(rcdev, 38000);
>
> rcdev = devm_rc_allocate_device(&pdev->dev, RC_DRIVER_IR_RAW_TX);
> if (!rcdev)
>
> bloat-o-meter reports (for an arm allmodconfig build)
>
> add/remove: 0/0 grow/shrink: 3/1 up/down: 644/-396 (248)
> Function old new delta
> pwm_ir_probe 372 676 +304
> pwm_ir_set_carrier 108 292 +184
> pwm_ir_set_duty_cycle 68 224 +156
> pwm_ir_tx 908 512 -396
> Total: Before=2302, After=2550, chg +10.77%
So 248 bytes more after your changes.
> struct pwm_ir increases from 12 bytes to 40 bytes.
>
> The stack space required by pwm_ir_tx decreases from 60 to 36
>
> I don't know exactly how kmalloc works internally. Maybe allocating a
> structure of size 40 bytes doesn't need more memory than a structure of
> size 12?
>
> I didn't check how runtimes change, but the size decrease of pwm_ir_tx()
> is nice and might save a bit of runtime.
I'm not following, how is this decreasing runtime?
> Probably it depends on your focus if this change is good for you or not.
Decreasing size is of course a good thing.
> > just on the stack, and both ioctl handlers and the probe function need to
> > recalculate the period/duty cycle, so there is a slight increase in code size.
> >
> > This change does not improve anything measurably and only increases code
> > complexity.
>
> You did measure?
Thanks for prototyping this.
Sean
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