linux-kernel - Re: [PATCH 1/1] pps: clients: gpio: Bypass edge's direction check when not needed

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Date: Wed, 10 Apr 2024 17:24:36 +0200
From: Rodolfo Giometti <giometti@...eenne.com>
To: Bastien Curutchet <bastien.curutchet@...tlin.com>
Cc: linux-kernel@...r.kernel.org,
 Thomas Petazzoni <thomas.petazzoni@...tlin.com>, herve.codina@...tlin.com,
 christophercordahi@...ometrics.ca
Subject: Re: [PATCH 1/1] pps: clients: gpio: Bypass edge's direction check
 when not needed

On 10/04/24 16:46, Bastien Curutchet wrote:
> Hi Rodolfo,
> 
> On 4/10/24 16:23, Rodolfo Giometti wrote:
>> On 10/04/24 13:35, Bastien Curutchet wrote:
>>> In the IRQ handler, the GPIO's state is read to verify the direction of
>>> the edge that triggered the interruption before generating the PPS event.
>>> If a pulse is too short, the GPIO line can reach back its original state
>>> before this verification and the PPS event is lost.
>>>
>>> This check is needed when info->capture_clear is set because it needs
>>> interruptions on both rising and falling edges. When info->capture_clear
>>> is not set, interruption is triggered by one edge only so this check can
>>> be omitted.
>>>
>>> Bypass the edge's direction verification when info->capture_clear is not
>>> set.
>>>
>>> Signed-off-by: Bastien Curutchet <bastien.curutchet@...tlin.com>
>>> ---
>>>   drivers/pps/clients/pps-gpio.c | 9 +++++++++
>>>   1 file changed, 9 insertions(+)
>>>
>>> diff --git a/drivers/pps/clients/pps-gpio.c b/drivers/pps/clients/pps-gpio.c
>>> index 2f4b11b4dfcd..c2a96e3e3836 100644
>>> --- a/drivers/pps/clients/pps-gpio.c
>>> +++ b/drivers/pps/clients/pps-gpio.c
>>> @@ -52,6 +52,15 @@ static irqreturn_t pps_gpio_irq_handler(int irq, void *data)
>>>       info = data;
>>> +    if (!info->capture_clear) {
>>> +        /*
>>> +         * If capture_clear is unset, IRQ is triggered by one edge only.
>>> +         * So the check on edge direction is not needed here
>>> +         */
>>> +        pps_event(info->pps, &ts, PPS_CAPTUREASSERT, data);
>>> +        return IRQ_HANDLED;
>>> +    }
>>> +
>>>       rising_edge = gpiod_get_value(info->gpio_pin);
>>>       if ((rising_edge && !info->assert_falling_edge) ||
>>>               (!rising_edge && info->assert_falling_edge))
>>
>> Apart the code duplication, which are the real benefits of doing so?
>>
> 
> It prevents from losing a PPS event when the pulse is so short (or the
> kernel so busy) that the trailing edge of the pulse occurs before the
> interrupt handler can read the state of the GPIO pin.

Have you a real case when this happens?

In any cases we should avoid code duplication... so I think we should do 
something as below:

diff --git a/drivers/pps/clients/pps-gpio.c b/drivers/pps/clients/pps-gpio.c
index 2f4b11b4dfcd..f05fb15ed7f4 100644
--- a/drivers/pps/clients/pps-gpio.c
+++ b/drivers/pps/clients/pps-gpio.c
@@ -52,7 +52,9 @@ static irqreturn_t pps_gpio_irq_handler(int irq, void *data)

         info = data;

-       rising_edge = gpiod_get_value(info->gpio_pin);
+       rising_edge = info->capture_clear ? \
+                       gpiod_get_value(info->gpio_pin) : \
+                       !info->assert_falling_edge;
         if ((rising_edge && !info->assert_falling_edge) ||
                         (!rising_edge && info->assert_falling_edge))
                 pps_event(info->pps, &ts, PPS_CAPTUREASSERT, data);

Please, review and test it before resubmitting. :)

Ciao,

Rodolfo

-- 
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