linux-kernel - Re: [RFC PATCH] i2c: busses: i2c-omap: Increase timeout for i2c interrupt

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Message-ID: <559FC5D7.3000108@ti.com>
Date:	Fri, 10 Jul 2015 18:47:11 +0530
From:	Vignesh R <vigneshr@...com>
To:	Wolfram Sang <wsa@...-dreams.de>,
	Alexander Sverdlin <alexander.sverdlin@...ia.com>,
	Felipe Balbi <balbi@...com>
CC:	Tony Lindgren <tony@...mide.com>, <linux-omap@...r.kernel.org>,
	<linux-i2c@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] i2c: busses: i2c-omap: Increase timeout for i2c interrupt



On 07/10/2015 02:39 PM, Wolfram Sang wrote:
> 
>> 60 s sounds way too much and actually I simply don't believe this is
>> the root cause. If I take a look into the driver, then I see, that
> 
> I agree, this is just a workaround.
> 

Yes, this is a workaround. I thought this is simpler change and can go
into -rc while I work on the better fix. As you can see, the other
suggestions need quite a significant change to the isr code.

>> the design is not really the best. The whole IRQ handling could be
>> actually performed in hard IRQ handler, without threading overhead.
>> Putting even 2 bytes in the controller FIFO should not be too heavy
>> for the hard IRQ handler. Then these ridiculous spin_lock()s. What
>> is the reason behind? The IRQ is flagged with ONESHOT, so thread and
>> hardirq handler are anyway mutually excluded. But if this thread
>> ever runs longer than it's allowed in IRQ context, then it anyway
>> produces this IRQ latency because it locks spin_lock_irqsave() for
>> the whole time! So the whole point of threaded interrupt is missing.
> 
> Furthermore, this combination of threaded_irq and struct completion seems
> bogus to me. If you just want to ensure the irq happened before timeout,
> you just complete when the irq happened and do the "bottom half" after the
> completion returned?

This sounds good to me. I will try to implement this option.
Thanks for the suggestion.

> 
>> I would propose you to throw away spinlocks. Convert threaded IRQ to
>> just one hardirq handler. And continue debugging. You will reduce the
>> load of the system with the above measures, maybe it will not happen
>> any more, maybe you'll figure out that problem is somewhere else.
> 
> Or this.

I am not convinced with moving entire code at hardirq context. I believe
its better to keep hardirq as small as possible.

-- 
Regards
Vignesh
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