linux-kernel - Re: [PATCH] pps: fix poll support

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Message-ID: <636c5ad9-25ad-44f7-9454-a7787de7a6aa@enneenne.com>
Date: Thu, 3 Apr 2025 09:49:06 +0200
From: Rodolfo Giometti <giometti@...eenne.com>
To: Denis OSTERLAND-HEIM <denis.osterland@...hl.com>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH] pps: fix poll support

On 03/04/25 08:06, Denis OSTERLAND-HEIM wrote:
> Hi Rodolfo,
> 
> Do you think that there is a chance that this patch make it in the current merge window?

Honestly, I don't know, it's up to Greg or Andrew to proceed for inclusion.

Ciao,

Rodolfo

> -----Original Message-----
> From: Rodolfo Giometti <giometti@...eenne.com>
> Sent: Monday, March 3, 2025 12:54 PM
> To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>; Andrew Morton <akpm@...ux-foundation.org>
> Cc: linux-kernel@...r.kernel.org; Denis OSTERLAND-HEIM <denis.osterland@...hl.com>; stable@...r.kernel.org
> Subject: [EXT] Re: [PATCH] pps: fix poll support
> 
> [EXTERNAL EMAIL]
>   
> 
> On 03/03/25 09:02, Denis OSTERLAND-HEIM wrote:
>> [BUG]
>> A user space program that calls select/poll get always an immediate data
>> ready-to-read response. As a result the intended use to wait until next
>> data becomes ready does not work.
>>
>> User space snippet:
>>
>>       struct pollfd pollfd = {
>>         .fd = open("/dev/pps0", O_RDONLY),
>>         .events = POLLIN|POLLERR,
>>         .revents = 0 };
>>       while(1) {
>>         poll(&pollfd, 1, 2000/*ms*/); // returns immediate, but should wait
>>         if(revents & EPOLLIN) { // always true
>>           struct pps_fdata fdata;
>>           memset(&fdata, 0, sizeof(memdata));
>>           ioctl(PPS_FETCH, &fdata); // currently fetches data at max speed
>>         }
>>       }
>>
>> [CAUSE]
>> pps_cdev_poll() returns unconditionally EPOLLIN.
>>
>> [FIX]
>> Remember the last fetch event counter and compare this value in
>> pps_cdev_poll() with most recent event counter
>> and return 0 if they are equal.
>>
>> Signed-off-by: Denis OSTERLAND-HEIM <denis.osterland@...hl.com>
>> Co-developed-by: Rodolfo Giometti <giometti@...eenne.com>
>> Signed-off-by: Rodolfo Giometti <giometti@...eenne.com>
> 
> Acked-by: Rodolfo Giometti <giometti@...eenne.com>
> 
> If needed. :)
> 
>> Fixes: eae9d2ba0cfc ("LinuxPPS: core support")
>> CC: stable@...r.linux.org # 5.4+
>> ---
>>    drivers/pps/pps.c          | 11 +++++++++--
>>    include/linux/pps_kernel.h |  1 +
>>    2 files changed, 10 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/pps/pps.c b/drivers/pps/pps.c
>> index 6a02245ea35f..9463232af8d2 100644
>> --- a/drivers/pps/pps.c
>> +++ b/drivers/pps/pps.c
>> @@ -41,6 +41,9 @@ static __poll_t pps_cdev_poll(struct file *file, poll_table *wait)
>>    
>>    	poll_wait(file, &pps->queue, wait);
>>    
>> +	if (pps->last_fetched_ev == pps->last_ev)
>> +		return 0;
>> +
>>    	return EPOLLIN | EPOLLRDNORM;
>>    }
>>    
>> @@ -186,9 +189,11 @@ static long pps_cdev_ioctl(struct file *file,
>>    		if (err)
>>    			return err;
>>    
>> -		/* Return the fetched timestamp */
>> +		/* Return the fetched timestamp and save last fetched event  */
>>    		spin_lock_irq(&pps->lock);
>>    
>> +		pps->last_fetched_ev = pps->last_ev;
>> +
>>    		fdata.info.assert_sequence = pps->assert_sequence;
>>    		fdata.info.clear_sequence = pps->clear_sequence;
>>    		fdata.info.assert_tu = pps->assert_tu;
>> @@ -272,9 +277,11 @@ static long pps_cdev_compat_ioctl(struct file *file,
>>    		if (err)
>>    			return err;
>>    
>> -		/* Return the fetched timestamp */
>> +		/* Return the fetched timestamp and save last fetched event  */
>>    		spin_lock_irq(&pps->lock);
>>    
>> +		pps->last_fetched_ev = pps->last_ev;
>> +
>>    		compat.info.assert_sequence = pps->assert_sequence;
>>    		compat.info.clear_sequence = pps->clear_sequence;
>>    		compat.info.current_mode = pps->current_mode;
>> diff --git a/include/linux/pps_kernel.h b/include/linux/pps_kernel.h
>> index c7abce28ed29..aab0aebb529e 100644
>> --- a/include/linux/pps_kernel.h
>> +++ b/include/linux/pps_kernel.h
>> @@ -52,6 +52,7 @@ struct pps_device {
>>    	int current_mode;			/* PPS mode at event time */
>>    
>>    	unsigned int last_ev;			/* last PPS event id */
>> +	unsigned int last_fetched_ev;		/* last fetched PPS event id */
>>    	wait_queue_head_t queue;		/* PPS event queue */
>>    
>>    	unsigned int id;			/* PPS source unique ID */
> 

-- 
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