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Date: Tue, 6 Feb 2024 18:38:29 +0100
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From: Mike Looijmans <mike.looijmans@topic.nl>
Subject: Re: [PATCH v3 2/2] iio: adc: ti-ads1298: Add driver
To: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
CC: devicetree@vger.kernel.org, linux-iio@vger.kernel.org,
 Jonathan Cameron <jic23@kernel.org>, Lars-Peter Clausen <lars@metafoo.de>,
 Liam Beguin <liambeguin@gmail.com>, Liam Girdwood <lgirdwood@gmail.com>,
 Maksim Kiselev <bigunclemax@gmail.com>,
 Marcus Folkesson <marcus.folkesson@gmail.com>,
 Marius Cristea <marius.cristea@microchip.com>,
 Mark Brown <broonie@kernel.org>, Niklas Schnelle <schnelle@linux.ibm.com>,
 Okan Sahin <okan.sahin@analog.com>, linux-kernel@vger.kernel.org
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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=20
lock assures that it's only called by one.

Usually the DRDY handler will call spi_async(). If the next DRDY arrives=20
before the spi_async transfer finishes, the SPI completion handler must=20
call spi_async() a.s.a.p. to also read the newly arrived sample. There's=20
no way to ask the chip whether there's data to read, so all the driver=20
can do is use the ISR to remember that DRDY did trigger.

The lock protects that the "busy" counter matches the actual pending=20
calls to spi_async, and also protects that only one handler will call=20
spi_async (and update the counter).

Maybe this picture helps:

DRDY ---+-----+-----+-----+-

SPI ------+------------+-+--

busy 00001100011111112211101


--=20
Mike Looijmans
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