linux-kernel - Re: [RFC PATCH v2 4/5] iio: accel: Support Kionix/ROHM KX022A accelerometer

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Date:   Sun, 9 Oct 2022 13:33:51 +0100
From:   Jonathan Cameron <jic23@...nel.org>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Matti Vaittinen <mazziesaccount@...il.com>,
        Matti Vaittinen <matti.vaittinen@...rohmeurope.com>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Jagath Jog J <jagathjog1996@...il.com>,
        Nikita Yushchenko <nikita.yoush@...entembedded.com>,
        Cosmin Tanislav <demonsingur@...il.com>,
        linux-iio@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 4/5] iio: accel: Support Kionix/ROHM KX022A
 accelerometer

On Thu, 6 Oct 2022 21:32:11 +0300
Andy Shevchenko <andriy.shevchenko@...ux.intel.com> wrote:

> On Thu, Oct 06, 2022 at 05:38:14PM +0300, Matti Vaittinen wrote:
> > KX022A is a 3-axis accelerometer from ROHM/Kionix. The senosr features
> > include variable ODRs, I2C and SPI control, FIFO/LIFO with watermark IRQ,
> > tap/motion detection, wake-up & back-to-sleep events, four acceleration
> > ranges (2, 4, 8 and 16g) and probably some other cool features.
> > 
> > Add support for the basic accelerometer features such as getting the
> > acceleration data via IIO. (raw reads, triggered buffer [data-ready] or
> > using the WMI IRQ).
> > 
> > Important things to be added include the double-tap, motion
> > detection and wake-up as well as the runtime power management.
> > 
> > NOTE: Filling-up the hardware FIFO should be avoided. During my testing
> > I noticed that filling up the hardware FIFO might mess-up the sample
> > count. My sensor ended up in a state where the amount of data in FIFO was
> > reported to be 0xff bytes, which equals to 42,5 samples. Specification
> > says the FIFO can hold a maximum of 41 samples in HiRes mode. Also, at
> > least once the FIFO was stuck in a state where reading data from
> > hardware FIFO did not decrease the amount of data reported to be in the
> > FIFO - eg. FIFO was "stuck". The code has now an error count and 10
> > reads with invalid FIFO data count will cause the fifo contents to be
> > dropped.  
> 

> ...
...



> ...
> 
> > +module_param(g_kx022a_use_buffer, bool, 0);
> > +MODULE_PARM_DESC(g_kx022a_use_buffer,
> > +		 "Buffer samples. Use at own risk. Fifo must not overflow");  
> 
> Why?! We usually do not allow module parameters in the new code.
> 

Badly broken hardware - was my suggestion.  Alternatives if there are usecases
that need to use the fifo, but it can wedge hard in a fashion that is impossible
to prevent from the driver?  My gut feeling is still drop the support entirely
with a strong comment in the code that the hardware is broken in a fashion we don't
know how to work around.

Jonathan