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Date:   Wed, 13 Feb 2019 10:39:46 +0000
From:   Lee Jones <lee.jones@linaro.org>
To:     Bartosz Golaszewski <bgolaszewski@baylibre.com>
Cc:     Bartosz Golaszewski <brgl@bgdev.pl>,
        Rob Herring <robh+dt@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Linus Walleij <linus.walleij@linaro.org>,
        Dmitry Torokhov <dmitry.torokhov@gmail.com>,
        Jacek Anaszewski <jacek.anaszewski@gmail.com>,
        Pavel Machek <pavel@ucw.cz>,
        Sebastian Reichel <sre@kernel.org>,
        Liam Girdwood <lgirdwood@gmail.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@vger.kernel.org>,
        devicetree <devicetree@vger.kernel.org>,
        Linux Input <linux-input@vger.kernel.org>,
        Linux LED Subsystem <linux-leds@vger.kernel.org>,
        Linux PM list <linux-pm@vger.kernel.org>,
        Mark Brown <broonie@kernel.org>
Subject: Re: [PATCH v4 05/10] mfd: max77650: new core mfd driver
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On Wed, 13 Feb 2019, Bartosz Golaszewski wrote:

> śr., 13 lut 2019 o 10:53 Lee Jones <lee.jones@linaro.org> napisał(a):
> >
> > On Wed, 13 Feb 2019, Bartosz Golaszewski wrote:
> >
> > > śr., 13 lut 2019 o 10:25 Lee Jones <lee.jones@linaro.org> napisał(a):
> > > >
> > > > On Tue, 12 Feb 2019, Lee Jones wrote:
> > > >
> > > > > On Tue, 12 Feb 2019, Bartosz Golaszewski wrote:
> > > > >
> > > > > > wt., 12 lut 2019 o 12:14 Lee Jones <lee.jones@linaro.org> napisał(a):
> > > > > > >
> > > > > > > On Tue, 12 Feb 2019, Bartosz Golaszewski wrote:
> > > > > > >
> > > > > > > > wt., 12 lut 2019 o 11:18 Lee Jones <lee.jones@linaro.org> napisał(a):
> > > > > > > > >
> > > > > > > > > On Tue, 12 Feb 2019, Bartosz Golaszewski wrote:
> > > > > > > > >
> > > > > > > > > > wt., 12 lut 2019 o 10:55 Lee Jones <lee.jones@linaro.org> napisał(a):
> > > > > > > > > > >
> > > > > > > > > > >  * The declaration of a superfluous struct
> > > > > > > > > > >  * 100 lines of additional/avoidable code
> > > > > > > > > > >  * Hacky hoop jumping trying to fudge VIRQs into resources
> > > > > > > > > > >  * Resources were designed for HWIRQs (unless a domain is present)
> > > > > > > > > > >  * Loads of additional/avoidable CPU cycles setting all this up
> > > > > > > > > >
> > > > > > > > > > While the above may be right, this one is negligible and you know it. :)
> > > > > > > > >
> > > > > > > > > You have nested for() loops.  You *are* wasting lots of cycles.
> > > > > > > > >
> > > > > > > > > > > Need I go on? :)
> > > > > > > > > > >
> > > > > > > > > > > Surely the fact that you are using both sides of an API
> > > > > > > > > > > (devm_regmap_init_i2c and regmap_irq_get_*) in the same driver, must
> > > > > > > > > > > set some alarm bells ringing?
> > > > > > > > > > >
> > > > > > > > > > > This whole HWIRQ setting, VIRQ getting, resource hacking is a mess.
> > > > > > > > > > >
> > > > > > > > > > > And for what?  To avoid passing IRQ data to a child driver?
> > > > > > > > > >
> > > > > > > > > > What do you propose? Should I go back to the approach in v1 and pass
> > > > > > > > > > the regmap_irq_chip_data to child drivers?
> > > > > > > > >
> > > > > > > > > I'm saying you should remove all of this hackery and pass IRQs as they
> > > > > > > > > are supposed to be passed (like everyone else does).
> > > > > > > >
> > > > > > > > I'm not sure what you mean by "like everyone else does" - different
> > > > > > > > mfd drivers seem to be doing different things. Is a simple struct
> > > > > > > > containing virtual irq numbers passed to sub-drivers fine?
> > > > > > >
> > > > > > > How do you plan on deriving the VIRQs to place into the struct?
> > > > > >
> > > > > > Exampe:
> > > > > >
> > > > > > struct max77650_gpio_pdata {
> > > > > >     int gpi_irq;
> > > > > > };
> > > > > >
> > > > > > In MFD driver:
> > > > > >
> > > > > > struct max77650_gpio_pdata *gpio_data = devm_kmalloc(dev, sizeof(*gpio_data));
> > > > > >
> > > > > > gpio_data->gpi_irq = regmap_irq_get_virq(irqchip_data, GPI_NUM);
> > > > > >
> > > > > > gpio_cell.platform_data = gpio_data;
> > > > > >
> > > > > > In GPIO driver:
> > > > > >
> > > > > > struct max77650_gpio_pdata *gpio_data = pdev->dev.platform_data;
> > > > > >
> > > > > > int irq = gpio_data->gpi_irq;
> > > > >
> > > > > Definitely not.  What you're trying to do is a hack.
> > > > >
> > > > > If you're using Regmap to handle your IRQs, then you should use Regmap
> > > > > in the client to pull them out.  Setting them via Regmap, then pulling
> > > > > them out again in the *same driver*, only to store them in platform
> > > > > data to be passed to a child device is bonkers.
> > > > >
> > > > > *Either* use the MFD provided platform-data helpers *or* pass and
> > > > > handle them via the Regmap APIs, *not* both.
> > > >
> > > > Right, a plan has been formed.
> > > >
> > > > Hopefully this works and you can avoid all this dancing around.
> > > >
> > > > Firstly, you need to make a small change to:
> > > >
> > > >   drivers/base/regmap/regmap-irq.c
> > > >
> > > > Add the following function:
> > > >
> > > >   struct irq_domain *regmap_irq_get_domain(struct regmap *map)
> > >
> > > We already do have such function and a lot of mfd drivers actually use it.
> >
> > Even better.
> >
> > > > As you can see, it will return the IRQ Domain for the chip.
> > > >
> > > > You can then pass this IRQ domain to mfd_add_devices() and it will do
> > > > the HWIRQ => VIRQ mapping for you on the fly.  Meaning that you can
> > > > remove all the nastiness in max77650_setup_irqs() and have the Input
> > > > device use the standard (e.g. platform_get_irq()) APIs.
> > > >
> > > > How does that Sound?
> > >
> > > This does sound better! Why didn't you lead with that in the first place?
> >
> > I'm not even going to dignify that stupid question with a response.
> 
> It's not a stupid question and you're being unnecessarily rude. As an
> expert in the subsystem you maintain you could have answered simply
> with a "this is wrong, retrieve the irq domain from the regmap
> irq_chip and pass it over to mfd_add_devices, the mfd core will create
> appropriate mappings".

Could be culture clash, but I found the question offensive which is
why I chose not to answer it.  The reason is actually explained below:

 "It's only the craziness in this patch which forced me to look into how
  Regmap handles IRQs and come up with a subsequent solution which fits
  your use-case."

Thus the fact that a) Regmap used IRQ domains and b) the IRQ domain
could be fetched and reused here didn't enter my thought process until
I delved into the inner workings of Regmap.

Yes, I know MFD pretty well, but I only tend to deep-dive into other
subsystems, particularly ones as complicated as Regmap, when it's
necessary to do so.  Now I know a little more about it, I can provide
the feedback you suggest going forward.

> > > It's a pity it's not documented, I had to look at the code to find out
> > > irq resources would get translated in mfd_add_devices() if a domain is
> > > present.
> >
> > Where is it likely to be documented?  MFD is pretty simple and seldom
> > needs explanation.  A 3 second look at the API you're trying to use
> > (instead of blind copy & paste) would have told you that it's possible
> > to take an IRQ domain as an argument.
> >
> > It's only the craziness in this patch which forced me to look into how
> > Regmap handles IRQs and come up with a subsequent solution which fits
> > your use-case.
> >
> > > In that case - I really don't see a reason to create a superfluous
> > > structure to only hold the main regmap - we can very well get it from
> > > the parent device in sub-drivers as I do now.
> >
> > The whole point of this solution is that you don't need to pass
> > anything non-standard (i.e. not provided by the current APIs) to the
> > child device.
> 
> I don't understand what you're saying here.

I'm saying that the structure you speak of is no longer required.

-- 
Lee Jones [李琼斯]
Linaro Services Technical Lead
Linaro.org │ Open source software for ARM SoCs
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