linux-kernel - Re: [PATCH v4] media: rc: pwm-ir-tx: Switch to atomic PWM API

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Message-ID: <20211029110602.uugnbm5vtfpghiwh@pengutronix.de>
Date:   Fri, 29 Oct 2021 13:06:02 +0200
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Sean Young <sean@...s.org>
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

On Fri, Oct 29, 2021 at 08:16:08AM +0100, Sean Young wrote:
> On Thu, Oct 28, 2021 at 08:05:16PM +0200, Uwe Kleine-König wrote:
> > On Thu, Oct 28, 2021 at 01:26:10PM +0100, Sean Young wrote:
> > > > 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.
> > 
> > ack. This is because the compiler inlines the division which accounts
> > for > 100 bytes.
> 
> I'm surprised it's that large. This is on 32 bit?

Yes, it's a 64 bit division on 32 bit ARM.

> > > > 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? 
> > 
> > With my changes pwm_ir_tx got smaller and { pwm_ir_probe,
> > pwm_ir_set_carrier, pwm_ir_set_duty_cycle } got bigger. Now if for a
> > typical runtime pattern pwm_ir_probe and pwm_ir_set_carrier run once and
> > pwm_ir_set_duty_cycle 100 times and pwm_ir_tx 1000 times (no idea if
> > that is realistic) it might be a net win in sum.
> 
> The two most common programs for sending IR are
> 
> ir-ctl: https://git.linuxtv.org/v4l-utils.git/tree/utils/ir-ctl/ir-ctl.c#n1041
> lircd: https://sourceforge.net/p/lirc/git/ci/master/tree/lib/transmit.c
> 
> For each transmission, the carrier is set. If the duty cyle is specified,
> then that is set too. Then the transmit itself is done. Both of them
> set the carrier and duty cycle (if required) for every transmission: setting
> the carrier and duty cycle is a cheap operation, and it is device property
> which can be overriden by another process. 
> 
> This means with your changes, if the carrier and duty cycle are both set
> for each transmission, then we're doing more work. If only the carrier
> is set for each transmission, then there is no net gain/loss (I think),
> but the code size has increased.

OK, then I discard my patch.

While reading that I wondered if it makes sense to have a callback that
sets both carrier and duty cycle and then remove the other two.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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