linux-kernel - Re: [PATCH v3 2/2] iio: adc: ti-ads1298: Add driver

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Date: Wed, 14 Feb 2024 07:48:40 +0100
From: Mike Looijmans <mike.looijmans@...ic.nl>
To: Jonathan Cameron <jic23@...nel.org>
CC: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
 devicetree@...r.kernel.org, linux-iio@...r.kernel.org,
 Lars-Peter Clausen <lars@...afoo.de>, Liam Beguin <liambeguin@...il.com>,
 Liam Girdwood <lgirdwood@...il.com>, Maksim Kiselev <bigunclemax@...il.com>,
 Marcus Folkesson <marcus.folkesson@...il.com>,
 Marius Cristea <marius.cristea@...rochip.com>,
 Mark Brown <broonie@...nel.org>, Niklas Schnelle <schnelle@...ux.ibm.com>,
 Okan Sahin <okan.sahin@...log.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/2] iio: adc: ti-ads1298: Add driver

On 10-02-2024 17:27, Jonathan Cameron wrote:
> On Tue, 6 Feb 2024 18:38:29 +0100
> Mike Looijmans <mike.looijmans@...ic.nl> wrote:
> 
>> On 06-02-2024 17:32, Andy Shevchenko wrote:
>>> On Tue, Feb 06, 2024 at 04:44:03PM +0100, Mike Looijmans wrote:
>>>> On 06-02-2024 16:09, Andy Shevchenko wrote:
>>>>> On Tue, Feb 06, 2024 at 03:47:45PM +0100, Mike Looijmans wrote:
>>> ...
>>>   
>>>>> But it's up to you what to do with that.
>>>>> Maybe Jonathan can advice something different.
>>>>>   
>>>> The spinlock also protects the call to spi_async().
>>> I don't get this. Locks usually protect the data and not the code.
>>> Can you elaborate?
>>>   
>> Either the DRDY or SPI completion handler will call spi_async(), the
>> lock assures that it's only called by one.
> 
> Arguably it's protecting the destination buffer of the spi_async()
> call.  We don't really care if we issue two reads (it's a waste
> of time and we would store two sets of readings but meh), but we do
> care about being sure that don't issue a second read into a buffer
> that we are potentially simultaneously getting data back from.

Indeed, that.

> 
> There are comments where the release is to describe when it can
> be safely unlocked.
> 
> I'm not super keen on this whole structure but I don't really have a better
> idea.  Who builds a device where you have no latched way of seeing
> if there is new data? (some) Hardware folk love to assume they have a RTOS only
> talking to their device and that no pulse signals will ever be missed.
> 
> We get to educate them when ever the opportunity arises :)

Even on RTOS this chip was a pain - to get it to work reliably I had to set up 
a DMA controller to run the SPI transactions, which took some smart 
daisy-chaining (I recall having the DMA controller write to its own control 
registers to avoid involving the CPU).

It's probably possible to trick audio hardware (I2S controller) into grabbing 
the data (my chip doesn't have that) without involving the CPU.

As the code is now, I can grab data and display it with the IIO oscilloscope 
over network at 4kHz without losing samples on an A9 at 600Mhz.


-- 
Mike Looijmans
System Expert

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