linux-kernel - Re: [PATCH] gpiolib: debugfs: display gpios requested as irq only

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Date:	Thu, 21 May 2015 23:33:01 +0300
From:	"Grygorii.Strashko@...aro.org" <grygorii.strashko@...aro.org>
To:	Johan Hovold <johan@...nel.org>,
	Linus Walleij <linus.walleij@...aro.org>
CC:	Grygorii Strashko <grygorii.strashko@...aro.org>,
	Alexandre Courbot <gnurou@...il.com>,
	"linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] gpiolib: debugfs: display gpios requested as irq only

On 05/21/2015 05:25 PM, Johan Hovold wrote:
> On Tue, May 19, 2015 at 04:28:55PM +0200, Linus Walleij wrote:
>> On Mon, May 18, 2015 at 1:02 PM, Johan Hovold <johan@...nel.org> wrote:
>>> On Fri, May 15, 2015 at 04:25:21PM +0300, grygorii.strashko@...aro.org wrote:
>>
>>>> GPIOs 192-223, platform/48051000.gpio, gpio:
>>>>   gpio-203 (vtt_fixed           ) out hi requested
>>>
>>> This is backwards. All gpios *should* be requested. *If* we are to
>>> include not-requested gpios in the debug output, then it is those pins
>>> that need to be marked as not-requested.
>>
>> It depends, really. As concluded in earlier discussions when we
>> introduced gpiochip_[un]lock_as_irq() the gpiolib and irqchip APIs
>> are essentially orthogonal.
> 
> [...]
> 
>> So to atleast try to safeguard from a scenario such as
>>
>> - Client A requests IRQ from the irqchip side of the API
>>    and sets up a level active-low IRQ on it
>>
>> - Client B request the same line as GPIO
>>
>> - Client B sets it to output and drivers it low.
>>
>> - Client A crashes in an infinite IRQ loop as that line
>>    is not hammered low and will generate IRQs until
>>    the end of time.
>>
>> I introduced the gpiochip_[un]lock_as_irq() calls so we
>> could safeguard against this. Notably that blocks client A
>> from setting the line as output. I hope.
> 
> A problem with the current implementation is that it uses as a flag
> rather than a refcount. As I pointed out elsewhere in this thread, it is
> possible to request a shared IRQ (e.g. via the sysfs interface) and
> release it, thereby making it possible to change the direction of the
> pin while still in use for irq.

Yes (checked). And this is an issue which need to be fixed.
- gpio sysfs should not call gpiochip_un/lock_as_irq()
- gpio drivers should use gpiochip API or implement 
  .irq_release/request_resources() callbacks

in this case case IRQ core will do just what is required. Right?

> 
>> I thought this would mean the line would only be used as IRQ
>> in this case, so I could block any gpiod_get() calls to that
>> line but *of course* some driver is using the IRQ and snooping
>> into the GPIO value at the same time. So can't simplify things
>> like so either.
>>
>> Maybe I'm smashing open doors here...
> 
> No, I understand that use case. But there are some issues with how it's
> currently implemented. Besides the example above, nothing pins a gpio
> chip driver in memory unless it has requested gpios, specifically,
> requesting a pin for irq use is not enough.

ok. An issue. Possible fix below:

diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
index ea11706..64392ad 100644
--- a/drivers/gpio/gpiolib.c
+++ b/drivers/gpio/gpiolib.c
@@ -514,6 +514,9 @@ static int gpiochip_irq_reqres(struct irq_data *d)
 {
        struct gpio_chip *chip = irq_data_get_irq_chip_data(d);
 
+       if (!try_module_get(chip->owner))
+               return -ENODEV;
+
        if (gpiochip_lock_as_irq(chip, d->hwirq)) {
                chip_err(chip,
                        "unable to lock HW IRQ %lu for IRQ\n",
@@ -528,6 +531,7 @@ static void gpiochip_irq_relres(struct irq_data *d)
        struct gpio_chip *chip = irq_data_get_irq_chip_data(d);
 
        gpiochip_unlock_as_irq(chip, d->hwirq);
+       module_put(chip->owner);
 }


> 
>> Anyway to get back to the original statement:
>>
>>> This is backwards. All gpios *should* be requested. *If* we are to
>>> include not-requested gpios in the debug output, then it is those pins
>>> that need to be marked as not-requested.
>>
>> This is correct, all GPIOs accessed *as gpios* should be
>> requested, no matter if they are then cast to IRQs by gpiod_to_irq
>> or not. However if the same hardware is used as only "some IRQ"
>> through it's irqchip interface, it needs not be requested, but
>> that is by definition not a GPIO, it is an IRQ.
> 
> True. And since it is not a GPIO, should it show up in
> /sys/kernel/debug/gpio? ;)

"Nice" idea :)
This information needed for debugging and testing which includes
checking of pin state (hi/lo) - especially useful during board's
bring-up when a lot of mistakes are detected related to wrong usage
of IRQ/GPIO numbers.

-- 
regards,
-grygorii
--
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