linux-kernel - Re: [RFC 08/11] gpiolib: cdev: Add hardware timestamp clock type

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Date:   Thu, 29 Jul 2021 20:16:04 -0700
From:   Dipen Patel <dipenp@...dia.com>
To:     Linus Walleij <linus.walleij@...aro.org>
CC:     "thierry.reding@...il.com" <thierry.reding@...il.com>,
        Jon Hunter <jonathanh@...dia.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-tegra <linux-tegra@...r.kernel.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Kent Gibson <warthog618@...il.com>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        Rob Herring <robh+dt@...nel.org>
Subject: Re: [RFC 08/11] gpiolib: cdev: Add hardware timestamp clock type


On 6/27/21 4:49 AM, Linus Walleij wrote:
> On Sat, Jun 26, 2021 at 1:48 AM Dipen Patel <dipenp@...dia.com> wrote:
>
> Just a quick question about this:
>
>> +        GPIO_V2_LINE_FLAG_EVENT_CLOCK_HARDWARE | \
> Is the usage intended to be such that since hardware timestamp
> can not be guaranteed we need to ask for it and fail and if that
> fails maybe the software wants to fall back to the realtime or
> common timestamp?
>
> I'm thinking from the view of libgpiod or similar apps that abstract
> this and they will be "I want to use hardware timestamps if and
> only if it is available, otherwise I want to use this other timestamp"
> or is that use case uncommon, such that either you know exactly
> what you want or you should not be messing with hardware
> timestamps?


The way currently is implemented, if you have requested

FLAG_EVENT_CLOCK_HARDWARE and it fails, control will return

to userspace with an error. There is no fallback.

>
> Yours,
> Linus Walleij