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From: David Lechner <david@lechnology.com>
To: William Breathitt Gray <vilhelm.gray@gmail.com>
Cc: jic23@kernel.org, kamel.bouhara@bootlin.com,
	gwendal@chromium.org, alexandre.belloni@bootlin.com,
	linux-iio@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-stm32@st-md-mailman.stormreply.com,
	linux-arm-kernel@lists.infradead.org, syednwaris@gmail.com,
	patrick.havelange@essensium.com, fabrice.gasnier@st.com,
	mcoquelin.stm32@gmail.com, alexandre.torgue@st.com
Subject: Re: [PATCH v5 3/5] counter: Add character device interface
Date: Sun, 25 Oct 2020 11:36:58 -0500
Message-ID: <e616b7f5-2e58-4b61-5a13-f64875c542d6@lechnology.com> (raw)
In-Reply-To: <20201025125557.GA3458@shinobu>


>>>>> diff --git a/drivers/counter/counter-sysfs.c b/drivers/counter/counter-sysfs.c
>>>>> index e66ed99dd5ea..cefef61f170d 100644
>>>>> --- a/drivers/counter/counter-sysfs.c
>>>>> +++ b/drivers/counter/counter-sysfs.c
>>>>
>>>>
>>>> Not sure why sysfs changes are in the chrdev patch. Are these
>>>> changes related somehow?
>>>
>>> Sorry, I forgot to explain this in the cover letter. The changes here
>>> are only useful for the character device interface. These changes
>>> introduce the extensionZ_name and extensionZ_width sysfs attributes.
>>>
>>> In the character device interface, extensions are selected by their id
>>> number, and the value returned depends on the type of data. The new
>>> sysfs attributes introduced here allow users to match the id of an
>>> extension with its name, as well as the bit width of the value returned
>>> so that the user knows whether to use the value_u8 or value_u64 union
>>> member in struct counter_event.
>>>
>>
>> Are we sure that all value types will always be CPU-endian unsigned
>> integers? Or should we make an enum to describe the data type instead
>> of just the width?
> 
> It should be safe to assume that the character device interface will
> only ever return CPU-endian unsigned integers. The device driver should
> handle the conversion of any strange endianness from the device before
> the character device interface, while userspace is the one responsible
> for interpreting the meaning of count in the context of the application.
> 
> Let's create a scenario for the sake of example. Suppose we want to use
> a counter device to track the vertical position of a component moved by
> a linear actuator. The operator considers some vertical position as the
> horizon, where anything above would be a positive position and anything
> below a negative position. The counter device stores its count in
> big-endian format; but the system CPU expects little-endian.
> 
> The flow of data for this scenario would look like the following (where
> BE = big-endian, LE = little-endian):
> 
> +----------+         +---------------+          +--------+
> | Raw data | - BE -> | Device driver | -> LE -> | chrdev | - u64 ->
> +----------+         +---------------+          +--------+
> 
> At this point, the userspace application would read the unsigned integer
> from the character device and determine how to interpret the position --
> whether the count be converted to a signed value to represent a negative
> physical position.
> 
> Whether or not a position should be considered negative is dependent on
> the user application and context. Because the character device does not
> know the context of the user application, it should only provide
> unsigned integers in order to ensure a standard interface for counter
> devices; userspace will be responsible for interpreting those counts to
> something meaningful for the context of their applications.
> 
> William Breathitt Gray
> 

Sounds reasonable to me.

  reply index

Thread overview: 35+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-27  2:18 [PATCH v5 0/5] Introduce the Counter " William Breathitt Gray
2020-09-27  2:18 ` [PATCH v5 2/5] docs: counter: Update to reflect sysfs internalization William Breathitt Gray
2020-09-27  2:18 ` [PATCH v5 3/5] counter: Add character device interface William Breathitt Gray
2020-10-14  1:40   ` David Lechner
2020-10-18 16:49     ` William Breathitt Gray
2020-10-20 15:53       ` David Lechner
2020-10-25 12:55         ` William Breathitt Gray
2020-10-25 16:36           ` David Lechner [this message]
2020-10-14 17:43   ` David Lechner
2020-10-14 19:05     ` William Breathitt Gray
2020-10-14 22:32       ` David Lechner
2020-10-14 22:40   ` David Lechner
2020-10-18 16:58     ` William Breathitt Gray
2020-10-20 16:06       ` David Lechner
2020-10-25 13:18         ` William Breathitt Gray
2020-10-25 16:34           ` David Lechner
2020-10-25 17:53             ` William Breathitt Gray
2020-09-27  2:18 ` [PATCH v5 4/5] docs: counter: Document " William Breathitt Gray
2020-10-08  8:09   ` Pavel Machek
2020-10-08 12:28     ` William Breathitt Gray
2020-10-12 17:04       ` David Lechner
2020-10-13 18:58         ` William Breathitt Gray
2020-10-13 19:08           ` David Lechner
2020-10-13 19:27             ` William Breathitt Gray
2020-09-27  2:18 ` [PATCH v5 5/5] counter: 104-quad-8: Add IRQ support for the ACCES 104-QUAD-8 William Breathitt Gray
2020-10-14  0:13   ` David Lechner
2020-10-18 14:50     ` William Breathitt Gray
2020-10-13  0:35 ` [PATCH v5 0/5] Introduce the Counter character device interface David Lechner
2020-10-18 14:14   ` William Breathitt Gray
     [not found] ` <e38f6dc3a08bf2510034334262776a6ed1df8b89.1601170670.git.vilhelm.gray@gmail.com>
2020-10-13  2:15   ` [PATCH v5 1/5] counter: Internalize sysfs interface code David Lechner
2020-10-18 14:49     ` William Breathitt Gray
2020-10-20 15:38       ` David Lechner
2020-10-23 13:12         ` William Breathitt Gray
2020-10-15  1:38   ` David Lechner
2020-10-18 17:00     ` William Breathitt Gray

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