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

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Message-ID: <6f064bd8-7937-4634-8a39-13ee5bcd6193@bootlin.com>
Date: Fri, 12 Apr 2024 14:20:25 +0200
From: Bastien Curutchet <bastien.curutchet@...tlin.com>
To: Rodolfo Giometti <giometti@...eenne.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

Hi Rodolfo,

On 4/12/24 08:44, Rodolfo Giometti wrote:
> On 11/04/24 14:44, Bastien Curutchet wrote:
>>>
>>> However we should think very well about this modification since it 
>>> could be the case where we have a device sending both assert and 
>>> clear events but we wish to catch just the asserts... in this case we 
>>> will get doubled asserts!
>>>
>>
>> My understanding is that clear events are to be captured only when this
>> capture_clear boolean is set. If it is not set, the PPS_CAPTURECLEAR
>> flag is not added to pps_source_info->mode and get_irqf_trigger_flags()
>> will return only one edge flag (rising or falling depending on
>> assert-falling-edge DT property).
> 
> Yes. You are right.
> 
>> By the way, I see that the capture_clear is never set since the legacy
>> platform data support has been dropped (commit ee89646619ba).
> 
> I see, but it can be re-enabled in the future... In this scenario, I 
> think we should add a DT entry to enable this special behavior. Maybe we 
> can also add a warning as below: >
> static irqreturn_t pps_gpio_irq_handler(int irq, void *data)
> {
>          ...
>          if ((rising_edge && !info->assert_falling_edge) ||
>                          (!rising_edge && info->assert_falling_edge))
>                  pps_event(info->pps, &ts, PPS_CAPTUREASSERT, data);
>          else if (info->capture_clear &&
>                          ((rising_edge && info->assert_falling_edge) ||
>                          (!rising_edge && !info->assert_falling_edge)))
>                  pps_event(info->pps, &ts, PPS_CAPTURECLEAR, data);
>      else
>          dev_warn_ratelimited(dev, "no ASSERT or CAPTURE event? "
>              "Maybe you need support-tiny-assert-pulse?");
> 
>          return IRQ_HANDLED;
> }
> 

I'm not sure a DT entry is needed. IMO there are two cases:
  1) capture_clear is unset. We need to capture only assert events,
     interrupt will be triggered by assert edge only so there is no need
     to check GPIO state: we can use the bypass.
  2) capture_clear is set. We need to capture assert and/or clear
     events, interrupt will be triggered by both assert and clear edges
     so we can't avoid the GPIO state checking to distinguish clear
     events from assert events: we can't use the bypass.

So if we bypass the GPIO's state check when capture_clear is unset and
leave current behavior when capture_clear is set:
  - case 1) will be more efficient and we won't lose tiny pulses anymore
  - case 2) is unchanged: we still might lose tiny pulses but as bypass
can't be done here, I think that we can't do better.

I agree that adding warning when the handler is left without triggering
a pps event can be useful, I can add it in a V3 version.


Best regards,
Bastien