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Date: Mon, 11 Dec 2023 13:18:07 +0100
From: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To: Thierry Reding <thierry.reding@...il.com>
Cc: linux-doc@...r.kernel.org, dri-devel@...ts.freedesktop.org,
	platform-driver-x86@...r.kernel.org,
	linux-hardening@...r.kernel.org, linux-riscv@...ts.infradead.org,
	linux-stm32@...md-mailman.stormreply.com,
	linux-leds@...r.kernel.org, chrome-platform@...ts.linux.dev,
	linux-samsung-soc@...r.kernel.org,
	Bartosz Golaszewski <brgl@...ev.pl>, linux-staging@...ts.linux.dev,
	linux-rockchip@...ts.infradead.org, linux-sunxi@...ts.linux.dev,
	linux-pwm@...r.kernel.org, greybus-dev@...ts.linaro.org,
	linux-mediatek@...ts.infradead.org,
	linux-rpi-kernel@...ts.infradead.org,
	linux-amlogic@...ts.infradead.org,
	linux-arm-kernel@...ts.infradead.org, linux-mips@...r.kernel.org,
	asahi@...ts.linux.dev, kernel@...gutronix.de
Subject: Re: [PATCH v4 000/115] pwm: Fix lifetime issues for pwm_chips

Hello Thierry,

On Mon, Dec 11, 2023 at 12:33:04PM +0100, Thierry Reding wrote:
> On Fri, Dec 08, 2023 at 07:50:33PM +0100, Uwe Kleine-König wrote:
> > The TL;DR; is essentially what I already wrote in my last reply to Bart
> > in the v3 thread[1]:
> > 
> >  - My approach needs more changes to the individual drivers (which I
> >    don't consider a relevant disadvantage given that the resulting code
> >    is better);
> >  - My approach works with less pointer dereferences which IMHO also
> >    simplifies understanding the code as all relevant data is in a single
> >    place.
> >  - My approach has a weaker separation between the core and the lowlevel
> >    drivers. That's ok in my book given that this doesn't complicate the
> >    lowlevel drivers and that hiding details considerably better doesn't
> >    work anyhow (see the drivers that need internal.h in your patch).
> > 
> > For me the single allocation issue is only an added bonus. The relevant
> > advantage of my approach is that the code is easier and (probably) more
> > efficient.
> 
> I happen to disagree. I think adding pwmchip_alloc() makes things much
> more complicated for low level drivers.

Looking at e.g.
https://lore.kernel.org/linux-pwm/2dda818b8bbbe8ba4b9df5ab54f960ff4a4f1ab5.1701860672.git.u.kleine-koenig@pengutronix.de/
I wonder where you see "much more complication". OK, there are two
pointers now for chip and private data, but I'd call that at most a
"mild" complication[1] which is more than balanced out by the
simplifications in the remaining parts of that patch.

Best regards
Uwe

[1] I'm not sure I'd refuse someone suggesting the following patch on
    top of today's next:

	diff --git a/drivers/pwm/pwm-microchip-core.c b/drivers/pwm/pwm-microchip-core.c
	index c0c53968f3e9..d32e65914599 100644
	--- a/drivers/pwm/pwm-microchip-core.c
	+++ b/drivers/pwm/pwm-microchip-core.c
	@@ -448,12 +448,14 @@ MODULE_DEVICE_TABLE(of, mchp_core_of_match);
	 static int mchp_core_pwm_probe(struct platform_device *pdev)
	 {
		struct mchp_core_pwm_chip *mchp_core_pwm;
	+	struct pwm_chip *chip;
		struct resource *regs;
		int ret;
	 
		mchp_core_pwm = devm_kzalloc(&pdev->dev, sizeof(*mchp_core_pwm), GFP_KERNEL);
		if (!mchp_core_pwm)
			return -ENOMEM;
	+	chip = &mchp_core_pwm->chip;
	 
		mchp_core_pwm->base = devm_platform_get_and_ioremap_resource(pdev, 0, &regs);
		if (IS_ERR(mchp_core_pwm->base))
	@@ -470,9 +472,9 @@ static int mchp_core_pwm_probe(struct platform_device *pdev)
	 
		mutex_init(&mchp_core_pwm->lock);
	 
	-	mchp_core_pwm->chip.dev = &pdev->dev;
	-	mchp_core_pwm->chip.ops = &mchp_core_pwm_ops;
	-	mchp_core_pwm->chip.npwm = 16;
	+	chip->dev = &pdev->dev;
	+	chip->ops = &mchp_core_pwm_ops;
	+	chip->npwm = 16;
	 
		mchp_core_pwm->channel_enabled = readb_relaxed(mchp_core_pwm->base + MCHPCOREPWM_EN(0));
		mchp_core_pwm->channel_enabled |=
	@@ -485,7 +487,7 @@ static int mchp_core_pwm_probe(struct platform_device *pdev)
		writel_relaxed(1U, mchp_core_pwm->base + MCHPCOREPWM_SYNC_UPD);
		mchp_core_pwm->update_timestamp = ktime_get();
	 
	-	ret = devm_pwmchip_add(&pdev->dev, &mchp_core_pwm->chip);
	+	ret = devm_pwmchip_add(&pdev->dev, chip);
		if (ret)
			return dev_err_probe(&pdev->dev, ret, "Failed to add pwmchip\n");
 
    With that applied before the above mentioned patch there is no
    complication at all in my eyes.


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

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