linux-kernel - Re: [PATCH v5 1/1] gpio: add sloppy logic analyzer using polling

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Message-ID: <CACRpkdYJqP7WJuhS9G65abCZHK1_LX9hkXU6o+k10t2LXw100w@mail.gmail.com>
Date:   Wed, 24 Nov 2021 01:35:34 +0100
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Wolfram Sang <wsa+renesas@...g-engineering.com>
Cc:     linux-gpio@...r.kernel.org, linux-renesas-soc@...r.kernel.org,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        linux-kernel@...r.kernel.org, Jonathan Corbet <corbet@....net>,
        Bartosz Golaszewski <brgl@...ev.pl>, linux-doc@...r.kernel.org
Subject: Re: [PATCH v5 1/1] gpio: add sloppy logic analyzer using polling

Hi Wolfram!

I like this patch.

On Tue, Nov 23, 2021 at 5:49 PM Wolfram Sang
<wsa+renesas@...g-engineering.com> wrote:

> +Introduction
> +============
> +
> +This document briefly describes how to run the GPIO based in-kernel sloppy
> +logic analyzer running on an isolated CPU.
> +
> +Note that this is a last resort analyzer which can be affected by latencies,
> +non-deterministic code paths and non-maskable interrupts. It is called 'sloppy'
> +for a reason. However, for e.g. remote development, it may be useful to get a
> +first view and aid further debugging.

Maybe a small paragraph first saying what this is, the usecase (feel
free to steal,
rewrite etc):

The sloppy logic analyzer will utilize a few GPIO lines in input mode on a
system to rapidly sample these digital lines, which will, if the
Nyquist criteria
is met, result in a time series log with approximate waveforms as they appeared
on these lines.

One way to use it is to analyze external traffic connected to these GPIO
lines with wires (i.e. digital probes), acting as a common logic analyzer.

Another thing it can do is to snoop on on-chip peripherals if the I/O cells of
these peripherals can be used in GPIO input mode at the same time as they
are being used as inputs or outputs for the peripheral, for example it would be
possible to scale down the speed of a certain MMC controller and snoop
the traffic between the MMC controller and the SD card by the sloppy
logic analyzer. In the pin control subsystem such pin controllers are
called "non-strict": a certain pin can be used with a certain peripheral and
as a GPIO input line at the same time.

Yours,
Linus Walleij