linux-kernel - Re: [PATCH RFC 0/9] iio: Introduce new timestamp grabbing APIs

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Message-ID: <f06d231d-297f-4c83-b457-b9eb242b9c93@baylibre.com>
Date: Mon, 19 May 2025 10:28:21 -0500
From: David Lechner <dlechner@...libre.com>
To: Gyeyoung Baek <gye976@...il.com>, Jonathan Cameron <jic23@...nel.org>,
 Nuno Sá <nuno.sa@...log.com>,
 Andy Shevchenko <andy@...nel.org>
Cc: linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC 0/9] iio: Introduce new timestamp grabbing APIs

On 5/19/25 9:25 AM, Gyeyoung Baek wrote:
> Support automatic timestamp grabbing by passing `true` to the `timestamp_enabled` parameter of `iio_triggered_buffer_setup_new()`.
> So consumer drivers don't need to set `iio_pollfunc_store_time()` as either the tophalf or bottomhalf manually.
> 
> For this, triggers must indicate whether they will call `poll()`, `poll_nested()`, or both before
> calling `iio_trigger_register()`. This is necessary because the consumer's handler does not know
> in advance which trigger will be attached.
> 
> Once `iio_trigger_attach_poll_func()` is called, a timestamp is grabbed in either the
> tophalf or bottomhalf based on the trigger's type (POLL or POLL_NESTED). If the trigger
> supports both (e.g., at91-sama5d2-adc.c), it is treated as POLL_NESTED since the consumer's
> tophalf is not invoked in poll_nested(), but the bottomhalf always is.
> 
> If the attached trigger supports timestamp grabbing itself, the consumer does not need to handle it.
> Instead, the consumer's `poll_func` pointer is passed to the trigger, which can then store the
> timestamp directly into consumer. Trigger drivers can pass timestamp values to consumers in a consistent
> interface using the new API `iio_trigger_store_time()`.

This is explaining what it does and how it works, but we really want to
know first _why_ we need this and why it is better that what we already
have or what sort of problem this is fixing that the current situation
can't handle.