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

From: Wolfram Sang <wsa+renesas@sang-engineering.com>
To: Linus Walleij <linus.walleij@linaro.org>
Cc: linux-gpio@vger.kernel.org, linux-renesas-soc@vger.kernel.org,
	Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	linux-kernel@vger.kernel.org, Jonathan Corbet <corbet@lwn.net>,
	Bartosz Golaszewski <brgl@bgdev.pl>,
	linux-doc@vger.kernel.org
Subject: Re: [PATCH v5 1/1] gpio: add sloppy logic analyzer using polling
Date: Fri, 10 Dec 2021 12:18:41 +0100	[thread overview]
Message-ID: <YbM3kcEW7m9Ado1e@ninjato> (raw)
In-Reply-To: <CACRpkdYJqP7WJuhS9G65abCZHK1_LX9hkXU6o+k10t2LXw100w@mail.gmail.com>

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Hi Linus!

> I like this patch.

I am glad!

> 
> 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.

Well, frankly, with the driver depending on EXPERT, this paragraph seems
a tad superfluous to me. But as it came for free and won't hurt, I took
the liberty to add this to the beginning of the documentation.

> 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.

Thanks for pointing out the 'strict' mode. I actually did snoop pins
muxed to I2C but I had to use a gpiolib hack for it assuming this was
not supported otherwise. Now, I have a one-liner for the Renesas pinctrl
driver which makes things work as well. I will work this out with Geert
hopefully. Would be really great to have this feature without my hack!

That being said, the paragraph above is a bit too long for my taste,
I'll see if I can make it more concise. But it should be there, yes.

Thanks for your support!

Happy hacking,

   Wolfram

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