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

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Date: Tue, 6 Feb 2024 18:38:29 +0100
From: Mike Looijmans <mike.looijmans@...ic.nl>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
CC: devicetree@...r.kernel.org, linux-iio@...r.kernel.org,
 Jonathan Cameron <jic23@...nel.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 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.

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

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

Maybe this picture helps:

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

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

busy 00001100011111112211101


-- 
Mike Looijmans
System Expert

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