linux-kernel - Re: [PATCH 3/4] devicetree: bindings: Add defeature-repeated-start property for Cadence I2C

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Date:	Tue, 2 Dec 2014 17:43:44 +0530
From:	Harini Katakam <harinikatakamlinux@...il.com>
To:	Mark Rutland <mark.rutland@....com>
Cc:	"wsa@...-dreams.de" <wsa@...-dreams.de>,
	"grant.likely@...aro.org" <grant.likely@...aro.org>,
	"robh+dt@...nel.org" <robh+dt@...nel.org>,
	Pawel Moll <Pawel.Moll@....com>,
	"ijc+devicetree@...lion.org.uk" <ijc+devicetree@...lion.org.uk>,
	"galak@...eaurora.org" <galak@...eaurora.org>,
	"michal.simek@...inx.com" <michal.simek@...inx.com>,
	"soren.brinkmann@...inx.com" <soren.brinkmann@...inx.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"vishnum@...inx.com" <vishnum@...inx.com>
Subject: Re: [PATCH 3/4] devicetree: bindings: Add defeature-repeated-start
 property for Cadence I2C

Hi Mark,

On Tue, Dec 2, 2014 at 4:49 PM, Mark Rutland <mark.rutland@....com> wrote:
> On Tue, Dec 02, 2014 at 10:05:48AM +0000, Harini Katakam wrote:
>> From: Vishnu Motghare <vishnum@...inx.com>
>>
>> This patch adds "defeature-repeated-start" property in i2c-cadence.txt.
>>
>> Signed-off-by: Vishnu Motghare <vishnum@...inx.com>
>> Signed-off-by: Harini Katakam <harinik@...inx.com>
>> ---
>>  .../devicetree/bindings/i2c/i2c-cadence.txt        |   11 +++++++++++
>>  1 file changed, 11 insertions(+)
>>
>> diff --git a/Documentation/devicetree/bindings/i2c/i2c-cadence.txt b/Documentation/devicetree/bindings/i2c/i2c-cadence.txt
>> index 7cb0b56..9d417a7 100644
>> --- a/Documentation/devicetree/bindings/i2c/i2c-cadence.txt
>> +++ b/Documentation/devicetree/bindings/i2c/i2c-cadence.txt
>> @@ -11,6 +11,17 @@ Required properties:
>>  Optional properties:
>>    - clock-frequency: Desired operating frequency, in Hz, of the bus.
>>    - clock-names: Input clock name, should be 'pclk'.
>> +  - defeature-repeated-start: Include this property to defeature repeated start
>> +                           This defeature is due to a few bugs in the
>> +                           I2C controller.
>> +                           Completion interrupt after a read/receive
>> +                           operation is NOT obtained if HOLD bit is set
>> +                           at that time. Because of this bug, repeated start
>> +                           will only work if there are no transfers following
>> +                           a read/receive transfer.
>> +                           If HOLD is held for long without a transfer,
>> +                           invalid read transactions are generated by the
>> +                           controller due to a HW timeout related bug.
>
> I'm not keen on the name; it sounds like we're disabling a feature
> rather than describing the problem (and "defeature" is not a common
> term in this sense, "disable" would be better).
>
> It sounds like there are two issues with staying in the HOLD state? Lost
> completion IRQs and a separate HW timeout bug? Or are the two related?
>

Yes, there are two issues here and they are not related.
But a combination of both is leading to not using repeated start.
The intention was to defeature except that it works in some scenarios
(such as a typical write+read in that order with repeated start)
and there are people who already use the driver with slaves that need this.

Regards,
Harini
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