linux-kernel - Re: [PATCH v2 05/10] gpio: dwapb: Convert driver to using the GPIO-lib-based IRQ-chip

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Date:   Thu, 30 Jul 2020 17:38:20 +0300
From:   Serge Semin <Sergey.Semin@...kalelectronics.ru>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
CC:     Serge Semin <fancer.lancer@...il.com>,
        Hoan Tran <hoan@...amperecomputing.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>,
        Pavel Parkhomenko <Pavel.Parkhomenko@...kalelectronics.ru>,
        Rob Herring <robh+dt@...nel.org>, <linux-gpio@...r.kernel.org>,
        <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 05/10] gpio: dwapb: Convert driver to using the
 GPIO-lib-based IRQ-chip

On Thu, Jul 30, 2020 at 05:26:18PM +0300, Andy Shevchenko wrote:
> On Thu, Jul 30, 2020 at 04:55:31PM +0300, Serge Semin wrote:
> > GPIO-lib provides a ready-to-use interface to initialize an IRQ-chip on
> > top of a GPIO chip. It's better from maintainability and readability
> > point of view to use one instead of supporting a hand-written Generic
> > IRQ-chip-based implementation. Moreover the new implementation won't
> > cause much functional overhead but will provide a cleaner driver code.
> > All of that makes the DW APB GPIO driver conversion pretty much justified
> > especially seeing a tendency of the other GPIO drivers getting converted
> > too.
> > 
> > Here is what we do in the framework of this commit to convert the driver
> > to using the GPIO-lib-based IRQ-chip interface:
> > 1) IRQ ack, mask and unmask callbacks are locally defined instead of
> > using the Generic IRQ-chip ones.
> 

> Easy to read if you put blank lines in between of items.

Ok.

> 
> > 2) An irq_chip structure instance is embedded into the dwapb_gpio
> > private data. Note we can't have a static instance of that structure since
> > GPIO-lib will add some hooks into it by calling gpiochip_set_irq_hooks().
> > A warning about that would have been printed by the GPIO-lib code if we
> > used a single irq_chip structure instance for multiple DW APB GPIO
> > controllers.
> > 3) Initialize the gpio_irq_chip structure embedded into the gpio_chip
> > descriptor. By default there is no IRQ enabled so any event raised will be
> > handled by the handle_bad_irq() IRQ flow handler. If DW APB GPIO IP-core
> > is synthesized to have non-shared reference IRQ-lines, then as before the
> > hierarchical and cascaded cases are distinguished by checking how many
> > parental IRQs are defined. (Note irq_set_chained_handler_and_data() won't
> > initialize IRQs, which descriptors couldn't be found.) If DW APB GPIO IP
> > is used on a platform with shared IRQ line, then we simply won't let the
> > GPIO-lib to initialize the parental IRQs, but will handle them locally in
> > the driver.
> > 4) Discard linear IRQ-domain and Generic IRQ-chip initialization, since
> > GPIO-lib IRQ-chip interface will create a new domain and accept a standard
> > IRQ-chip structure pointer based on the setting we provided in the
> > gpio_irq_chip structure.
> > 5) Manually select a proper IRQ flow handler directly in the
> > irq_set_type() callback by calling irq_set_handler_locked() method, since
> > an ordinary (not Generic) irq_chip descriptor is now utilized. Note this
> > shalln't give any regression
> > 6) Alter CONFIG_GPIO_DWAPB kernel config to select
> > CONFIG_GPIOLIB_IRQCHIP instead of CONFIG_GENERIC_IRQ_CHIP.
> > 
> > Note neither 4) nor 5) shall cause a regression of commit 6a2f4b7dadd5
> > ("gpio: dwapb: use a second irq chip"), since the later isn't properly
> > used here anyway.
> 
> ...
> 
> >  struct dwapb_gpio_port {
> >  	struct gpio_chip	gc;
> > +	unsigned int		nr_irqs;
> > +	unsigned int		irq[DWAPB_MAX_GPIOS];
> > +	struct irq_chip		irqchip;
> >  	bool			is_registered;
> >  	struct dwapb_gpio	*gpio;
> 

> Isn't it too much wasted memory (imagine 4 port controller)?
> 
> What if we have it in another structure and allocate dynamically?
> 
> struct dwapb_gpio_port_irqchip {
> 	struct irq_chip		irqchip;
> 	unsigned int		nr_irqs;
> 	unsigned int		irq[DWAPB_MAX_GPIOS];
> };

Agree. I have to send a new revision of the series anyway. I'll do that shortly.

-Sergey

> 
> 	...
> 	struct dwapb_gpio_port_irqchip *pirq;
> 	...
> 
> (I agree that IRQ chip is rather property of a port than controller)
> 
> -- 
> With Best Regards,
> Andy Shevchenko
> 
>