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Subject: Re: [RFC v3 09/12] gpiolib: cdev: Add hardware timestamp clock type
To:     Kent Gibson <warthog618@gmail.com>
CC:     <thierry.reding@gmail.com>, <jonathanh@nvidia.com>,
        <linux-kernel@vger.kernel.org>, <linux-tegra@vger.kernel.org>,
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References: <20211123193039.25154-1-dipenp@nvidia.com>
 <20211123193039.25154-10-dipenp@nvidia.com> <20211126013137.GC10380@sol>
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 <20211201171638.GA31045@sol>
 <4c7c3db1-a1b3-1944-4278-cb37e8a4f373@nvidia.com> <20211202005349.GA7007@sol>
From:   Dipen Patel <dipenp@nvidia.com>
Message-ID: <498da060-b02d-528a-9dbc-81dd22fc13a0@nvidia.com>
Date:   Wed, 5 Jan 2022 15:00:24 -0800
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Hi,


On 12/1/21 4:53 PM, Kent Gibson wrote:
> On Wed, Dec 01, 2021 at 10:01:46AM -0800, Dipen Patel wrote:
>> Hi,
>>
>>
>> On 12/1/21 9:16 AM, Kent Gibson wrote:
>>> On Tue, Nov 30, 2021 at 07:29:20PM -0800, Dipen Patel wrote:
>>>> Hi,
>>>>
>>>> On 11/25/21 5:31 PM, Kent Gibson wrote:
>>>>> On Tue, Nov 23, 2021 at 11:30:36AM -0800, Dipen Patel wrote:
>>>>>> This patch adds new clock type for the GPIO controller which can
>>>>>> timestamp gpio lines in realtime using hardware means. To expose such
>>>>>> functionalities to the userspace, code has been added in this patch
>>>>>> where during line create call, it checks for new clock type and if
>>>>>> requested, calls hardware timestamp related API from gpiolib.c.
>>>>>> During line change event, the HTE subsystem pushes timestamp data
>>>>>> through callbacks.
>>>>>>
>>>>>> Signed-off-by: Dipen Patel <dipenp@nvidia.com>
>>>>>> Acked-by: Linus Walleij <linus.walleij@linaro.org>
>>>>>> ---
>>>>>> Changes in v2:
>>>>>> - Added hte_dir and static structure hte_ts_desc.
>>>>>> - Added callbacks which get invoked by HTE when new data is available.
>>>>>> - Better use of hte_dir and seq from hte_ts_desc.
>>>>>> - Modified sw debounce function to accommodate hardware timestamping.
>>>>>>
>>>>>>  drivers/gpio/gpiolib-cdev.c | 161 ++++++++++++++++++++++++++++++++++--
>>>>>>  include/uapi/linux/gpio.h   |   1 +
>>>>>>  2 files changed, 153 insertions(+), 9 deletions(-)
>>>>>>
>>>>>> [snip]
> The code here has to deal with the general case, not just the one example
> driver you have provided.  So in general there COULD be gaps in the
> ts->seq, right?
>
> I do see that using the ts-seq for sw debounced lines is problematic
> though. The debouncer itself will be discarding hte events, but that
> shouldn't be considered a lost event to the user.  You could track
> how many events are discarded by the debouncer and subtract those from
> the sequence numbers reported to userspace?

This could be little complicated, especially for "hybrid" scenario where

cdev debouncer receives partial events to discard and rest is dropped in hw fifo

in hte core. For example, if there were 5 events to discard before debounce period,

cdev receives only 3 events to discard and rest 2 is dropped in hte core. In this

case, when debounce period ends and work func gets executed, it will take 3rd event

as that was the last seen in cdev debouncer. This makes both ts and seq unstable and

out of sync. In the absence of actual hardware which can drop, its probably hard to

simulate and implement complete solution to tackle this.


>
>