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SubjectRe: [PATCH v5 5/8] counter: 104-quad-8: Documentation: Add Generic Counter sysfs documentation
On Sat, Mar 24, 2018 at 05:21:40PM +0000, Jonathan Cameron wrote:
>On Fri, 9 Mar 2018 13:43:19 -0500
>William Breathitt Gray <vilhelm.gray@gmail.com> wrote:
>
>> This patch adds standard documentation for the Generic Counter interface
>> userspace sysfs attributes of the 104-QUAD-8 driver.
>>
>> Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
>A few minor comments inline...
>Some of this seems generic and common enough you should just put it in the
>main docs straight away rather that waiting for more devices to use it.
>
>Jonathan

That sounds reasonable so I'll move some of these into the main Generic
Counter sysfs documentation file.

Some comments inline follow.

William Breathitt Gray

>
>> ---
>> .../ABI/testing/sysfs-bus-counter-104-quad-8 | 115 +++++++++++++++++++++
>> MAINTAINERS | 1 +
>> 2 files changed, 116 insertions(+)
>> create mode 100644 Documentation/ABI/testing/sysfs-bus-counter-104-quad-8
>>
>> diff --git a/Documentation/ABI/testing/sysfs-bus-counter-104-quad-8 b/Documentation/ABI/testing/sysfs-bus-counter-104-quad-8
>> new file mode 100644
>> index 000000000000..4269b438185a
>> --- /dev/null
>> +++ b/Documentation/ABI/testing/sysfs-bus-counter-104-quad-8
>> @@ -0,0 +1,115 @@
>> +What: /sys/bus/counter/devices/counterX/countY_count_mode
>> +KernelVersion: 4.17
>> +Contact: linux-iio@vger.kernel.org
>> +Description:
>> + Count mode for channel Y. The preset value for channel Y is used
>> + by the count mode where required. The following count modes are
>> + available:
>> +
>> + Normal:
>> + Counting is continuous in either direction.
>> +
>> + Range Limit:
>> + An upper or lower limit is set, mimicking limit switches
>> + in the mechanical counterpart. The upper limit is set to
>> + the preset value, while the lower limit is set to 0. The
>> + counter freezes at count = preset when counting up, and
>> + at count = 0 when counting down. At either of these
>> + limits, the counting is resumed only when the count
>> + direction is reversed.
>> +
>> + Non-recycle:
>> + Counter is disabled whenever a 24-bit count overflow or
>> + underflow takes place. The counter is re-enabled when a
>> + new count value is loaded to the counter via a preset
>> + operation or write to raw.
>> +
>> + Modulo-N:
>> + A count boundary is set between 0 and the preset value.
>> + The counter is reset to 0 at count = preset when
>> + counting up, while the counter is set to the preset
>> + value at count = 0 when counting down; the counter does
>> + not freeze at the bundary points, but counts
>> + continuously throughout.
>This worries me a little in that you will end up with all sorts of subtle
>variations around these concepts and hence end up with an impossible to
>generalize userspace interface...

There is a potention for a deal of variations due to the diverse range
of counter devices out there -- in particular, I worry about the
confusion of similar functionality using slightly different names (e.g.
"Normal" may be named "Continuous" in another driver) and vice versa.
Perhaps I should standardize these count modes in the main Generic
Counter sysfs documentation file.

A difficulty with standardizing these count modes is how to handle
description cases such as "Non-recycle" mode whose limit range is that
of the count register size (24-bit). This limit range would be different
in a device of a different count register size; for example, the LS7366R
is a 32-bit version of this counter device with the same interface, but
which has a 32-bit limit in "Non-recycle" mode. However, this may not be
such a problem since the datasheet for these devices does use a more
generic description of this mode which I can utilize: "counter disabled
with carry or borrow, re-enabled with reset or load."

The count_mode attribute is core enough to Generic Counter paradigm that
I expect it to appear in many Counter drivers, so I'll move this to the
main Generic Counter sysfs documentation file for better standardization
of its modes.

>
>> +
>> +What: /sys/bus/counter/devices/counterX/countY_count_mode_available
>> +What: /sys/bus/counter/devices/counterX/countY_noise_error_available
>> +KernelVersion: 4.17
>> +Contact: linux-iio@vger.kernel.org
>> +Description:
>> + Discrete set of available values for the respective Count Y
>> + configuration are listed in this file.
>> +
>> +What: /sys/bus/counter/devices/counterX/countY_direction
>> +KernelVersion: 4.17
>> +Contact: linux-iio@vger.kernel.org
>> +Description:
>> + Read-only attribute that indicates the count direction of
>> + Count Y. Two count directions are available: Forward and
>> + Backward.
>Is this telling us which way it is currently counting? I would imagine
>it's generic inversion control, but this description doesn't make that clear.

The nature of quadrature encoding allows direction of movement to be
determined -- in terms of count data this direction represents whether
the count value is increasing or decreasing. For the 104-QUAD-8 device,
this "direction" is a read-only value provided by the hardware
evaluation of the A and B input lines.

However, I can imagine some simple counter devices permitting write
operations to configure a direction as a form of inversion control for
the counter. This attribute is generic enough to include in the main
Generic Counter sysfs documentation file, so I'll move it there and add
a more detailed explanation of its read and write functionality.

>
>> +
>> +What: /sys/bus/counter/devices/counterX/countY_enable
>> +KernelVersion: 4.17
>> +Contact: linux-iio@vger.kernel.org
>> +Description:
>> + Whether channel Y inputs A and B are enabled. Valid attribute
>> + values are boolean.
>Why would you disable them? I'm unclear on what userspace would do with this.

Truthfully, I haven't found much use for this functionality in my
applications, but several devices provide it so I have exposed it here.
I believe the intention is to allow users to pause a counter temporarily
(i.e. by ignoring changes on the A and B inputs) and then pick up where
they left off.

I think a possible real life use case would look like this: a conveyor
system tracks total movement, reaches the end of a production run and
pauses the counter, rehomes the conveyor belt, then restarts the counter
and continues counting where it left off.

This count enable functionality is generic enough that I can also move
it to the main Generic Counter sysfs documentation file, but I'll give
it a more generic description without referencing the A and B input
lines.

>
>> +
>> +What: /sys/bus/counter/devices/counterX/countY_noise_error
>> +KernelVersion: 4.17
>> +Contact: linux-iio@vger.kernel.org
>> +Description:
>> + Read-only attribute that indicates whether excessive noise is
>> + present at the channel Y count inputs in quadrature clock mode;
>> + irrelevant in non-quadrature (Pulse-Direction) clock mode.
>If you are going to report errors like this I would suggest trying to have
>a generic form that is easy for userspace to match.
>countY_error_noise would allow easy presentation of all errors of the
>form
>countY_error_<error type> as a list based on <error type>
>
>> +
>> +What: /sys/bus/counter/devices/counterX/countY_preset
>> +KernelVersion: 4.17
>> +Contact: linux-iio@vger.kernel.org
>> +Description:
>> + If the counter device supports preset registers, the preset
>> + count for channel Y is provided by this attribute.
>> +
>> +What: /sys/bus/counter/devices/counterX/countY_preset_enable
>> +KernelVersion: 4.17
>> +Contact: linux-iio@vger.kernel.org
>> +Description:
>> + Whether to set channel Y counter with channel Y preset value
>> + when channel Y index input is active, or continuously count.
>> + Valid attribute values are boolean.
>> +
>> +What: /sys/bus/counter/devices/counterX/signalY_index_polarity
>> +KernelVersion: 4.17
>> +Contact: linux-iio@vger.kernel.org
>> +Description:
>> + Active level of channel Y-16 index input; irrelevant in
>> + non-synchronous load mode.
>This seems like a generic control that should be in the main docs?
>don't use Y-16 to identify the channel. Use "the associated channel to signal Y".
>
>> +
>> +What: /sys/bus/counter/devices/counterX/signalY_index_polarity_available
>> +What: /sys/bus/counter/devices/counterX/signalY_synchronous_mode_available
>> +KernelVersion: 4.17
>> +Contact: linux-iio@vger.kernel.org
>> +Description:
>> + Discrete set of available values for the respective Signal Y
>> + configuration are listed in this file.
>> +
>> +What: /sys/bus/counter/devices/counterX/signalY_synchronous_mode
>> +KernelVersion: 4.17
>> +Contact: linux-iio@vger.kernel.org
>> +Description:
>> + Configure channel Y-16 counter for non-synchronous or
>> + synchronous load mode. Synchronous load mode cannot be selected
>> + in non-quadrature (Pulse-Direction) clock mode.
>> +
>> + Non-synchronous:
>> + A logic low level is the active level at this index
>> + input. The index function (as enabled via preset_enable)
>> + is performed directly on the active level of the index
>> + input.
>> +
>> + Synchronous:
>> + Intended for interfacing with encoder Index output in
>> + quadrature clock mode. The active level is configured
>> + via index_polarity. The index function (as enabled via
>> + preset_enable) is performed synchronously with the
>> + quadrature clock on the active level of the index input.
>> diff --git a/MAINTAINERS b/MAINTAINERS
>> index febe27a9962e..8134050d175a 100644
>> --- a/MAINTAINERS
>> +++ b/MAINTAINERS
>> @@ -270,6 +270,7 @@ ACCES 104-QUAD-8 DRIVER
>> M: William Breathitt Gray <vilhelm.gray@gmail.com>
>> L: linux-iio@vger.kernel.org
>> S: Maintained
>> +F: Documentation/ABI/testing/sysfs-bus-counter-104-quad-8
>> F: Documentation/ABI/testing/sysfs-bus-iio-counter-104-quad-8
>> F: drivers/counter/104-quad-8.c
>>
>

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