Re: [PATCH v7 1/9] irqchip: add Amlogic Meson GPIO irqchip driver

[Marc Zyngier <marc.zyngier@xxxxxxx>]

On 15/06/17 16:24, Heiner Kallweit wrote:
> Am 15.06.2017 um 15:27 schrieb Marc Zyngier:
>> On 15/06/17 14:10, Heiner Kallweit wrote:
>>> Am 13.06.2017 um 10:31 schrieb Marc Zyngier:
>>>> On 10/06/17 22:57, Heiner Kallweit wrote:
>>>>> Add a driver supporting the GPIO interrupt controller on certain
>>>>> Amlogic meson SoC's.
>>>>>
>>>>> Signed-off-by: Heiner Kallweit <hkallweit1@xxxxxxxxx>
>>>>> ---
>>>>> v5:
>>>>> - changed Kconfig entry based on Neil's suggestion
>>>>> - added authors
>>>>> - extended explanation why 2 * n hwirqs are used
>>>>> v6:
>>>>> - change DT property parent-interrupts to interrupts
>>>>> v7:
>>>>> - no changes
>>>>> ---
>>>>>  drivers/irqchip/Kconfig          |   5 +
>>>>>  drivers/irqchip/Makefile         |   1 +
>>>>>  drivers/irqchip/irq-meson-gpio.c | 295 +++++++++++++++++++++++++++++++++++++++
>>>>>  3 files changed, 301 insertions(+)
>>>>>  create mode 100644 drivers/irqchip/irq-meson-gpio.c
>>>>>
>>>>> diff --git a/drivers/irqchip/Kconfig b/drivers/irqchip/Kconfig
>>>>> index 478f8ace..bdc86e14 100644
>>>>> --- a/drivers/irqchip/Kconfig
>>>>> +++ b/drivers/irqchip/Kconfig
>>>>> @@ -301,3 +301,8 @@ config QCOM_IRQ_COMBINER
>>>>>  	help
>>>>>  	  Say yes here to add support for the IRQ combiner devices embedded
>>>>>  	  in Qualcomm Technologies chips.
>>>>> +
>>>>> +config MESON_GPIO_INTC
>>>>> +	bool
>>>>> +	depends on ARCH_MESON
>>>>> +	default y
>>>>> diff --git a/drivers/irqchip/Makefile b/drivers/irqchip/Makefile
>>>>> index b64c59b8..1be482bd 100644
>>>>> --- a/drivers/irqchip/Makefile
>>>>> +++ b/drivers/irqchip/Makefile
>>>>> @@ -76,3 +76,4 @@ obj-$(CONFIG_EZNPS_GIC)			+= irq-eznps.o
>>>>>  obj-$(CONFIG_ARCH_ASPEED)		+= irq-aspeed-vic.o
>>>>>  obj-$(CONFIG_STM32_EXTI) 		+= irq-stm32-exti.o
>>>>>  obj-$(CONFIG_QCOM_IRQ_COMBINER)		+= qcom-irq-combiner.o
>>>>> +obj-$(CONFIG_MESON_GPIO_INTC)		+= irq-meson-gpio.o
>>>>> diff --git a/drivers/irqchip/irq-meson-gpio.c b/drivers/irqchip/irq-meson-gpio.c
>>>>> new file mode 100644
>>>>> index 00000000..925d00c2
>>>>> --- /dev/null
>>>>> +++ b/drivers/irqchip/irq-meson-gpio.c
>>>>> @@ -0,0 +1,295 @@
>>>>> +/*
>>>>> + * Amlogic Meson GPIO IRQ chip driver
>>>>> + *
>>>>> + * Copyright (c) 2015 Endless Mobile, Inc.
>>>>> + * Author: Carlo Caione <carlo@xxxxxxxxxxxx>
>>>>> + * Copyright (c) 2016 BayLibre, SAS.
>>>>> + * Author: Jerome Brunet <jbrunet@xxxxxxxxxxxx>
>>>>> + * Copyright (c) 2017 Heiner Kallweit <hkallweit1@xxxxxxxxx>
>>>>> + *
>>>>> + * This program is free software; you can redistribute it and/or
>>>>> + * modify it under the terms of the GNU General Public License as
>>>>> + * published by the Free Software Foundation, version 2.
>>>>> + */
>>>>> +
>>>>> +#include <linux/device.h>
>>>>> +#include <linux/init.h>
>>>>> +#include <linux/interrupt.h>
>>>>> +#include <linux/irqchip.h>
>>>>> +#include <linux/of.h>
>>>>> +#include <linux/of_irq.h>
>>>>> +#include <linux/of_address.h>
>>>>> +#include <linux/regmap.h>
>>>>> +
>>>>> +#define REG_EDGE_POL		0x00
>>>>> +#define REG_PIN_03_SEL		0x04
>>>>> +#define REG_PIN_47_SEL		0x08
>>>>> +#define REG_FILTER_SEL		0x0c
>>>>> +
>>>>> +#define REG_EDGE_POL_MASK(x)	(BIT(x) | BIT(16 + (x)))
>>>>> +#define REG_EDGE_POL_EDGE(x)	BIT(x)
>>>>> +#define REG_EDGE_POL_LOW(x)	BIT(16 + (x))
>>>>> +
>>>>> +#define MAX_PARENT_IRQ_NUM	8
>>>>> +
>>>>> +/* maximum number of GPIO IRQs on supported platforms */
>>>>> +#define MAX_NUM_GPIO_IRQ	133
>>>>
>>>> Why aren't these values coming from DT? I bet that a future revision of
>>>> the same HW will double these. Or at least, you could match it on the
>>>> compatible string.
>>>>
>>> Alternatively this value can be set to 255. The GPIO source is an 8 bit
>>> value with 255 being reserved for "no interrupt source assigned".
>>
>> Who is reserving it? The HW? Or is that your own defined convention?
>>
>>> This way we cover all chips based on the same IP.
>>
>> Why? Where is that 8bit limit coming from?
>>
> The 8 bit limit is in the HW.
> 
>>> I think what we could gain by introducing an additional DT property
>>> (saving a few bytes in the irqdomain mapping table) isn't worth the effort.
>>
>> It is not about saving or wasting memory. It is about making the driver
>> and its binding able to span multiple generation of the HW without too
>> much churn. Which is why I'm suggesting that you either define these
>> properties in DT *or* match the compatible string to obtain these values.
>>
>>>>> +
>>>>> +/*
>>>>> + * In case of IRQ_TYPE_EDGE_BOTH we need two parent interrupts, one for each
>>>>> + * edge. That's due to HW constraints.
>>>>> + * We use format 2 * GPIO_HWIRQ +(0|1) for the hwirq, so we can have one
>>>>> + * GPIO_HWIRQ twice and derive the GPIO_HWIRQ from hwirq by shifting hwirq
>>>>> + * one bit to the right.
>>>>
>>>> Please expand on how you expect this to work, specially when a random
>>>> driver expects a single interrupt.
>>>>
>>> The gpio interrupt controller in this chip doesn't have native support for
>>> IRQ_TYPE_EDGE_BOTH. As a workaround we would need to assign the same gpio
>>> to two parent interrupts, one for each edge.
>>
>> No, that's horrible, racy, and impractical. It has been proposed in the
>> past (for the same HW), and we're not going there again.
>>
> IIRC what has been proposed before is to re-program the polarity of edge
> detection withing the ISR. This would match your concern that it is racy.
> 
> Here it's about using two parent irq's, one programmed to react on the
> rising edge whilst the other is triggered in case of falling edge.
> Would you consider this to be racy too?

How do you reconcile two interrupts to make look like a single one for a
random, pre-existing driver?

[...]

>>>> So you reject EDGE_BOTH? So what's the deal with the beginning of the patch?
>>>>
>>> We reject it in the initial version of the patch set as there's no consensus
>>> yet on some details of the workaround needed for EDGE_BOTH support.
>>
>> There is a consensus: The HW doesn't support this feature.
>>
> Means what? There is no acceptable way to support EDGE_BOTH on this HW?
> In this case I could stop here as for me this feature is important.

Answer my question above, which I asked in my initial review: How do you
make two interrupts appear as one for a driver that wants to get
signaled on each edge, using the existing API.

> 
> Would be somewhat a pity as there is a working solution. If it doesn't fit
> into the current irq(chip) framework, shouldn't this be considered a gap
> in the framework?

Again, this is not about the framework, which we can almost twist as we
see fit. It is about the API that a driver expects. At the moment, you
haven't proposed anything.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...
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