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

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Date:	Fri, 10 Jul 2015 17:02:39 +0300
From:	Grygorii Strashko <grygorii.strashko@...com>
To:	Wolfram Sang <wsa@...-dreams.de>,
	Alexander Sverdlin <alexander.sverdlin@...ia.com>,
	Felipe Balbi <balbi@...com>
CC:	ext Vignesh R <vigneshr@...com>, 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

Hi Wolfram,

On 07/10/2015 12:09 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.
> 
>> 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?
> 

I'd very appreciated if You would be able to clarify your point a bit, pls?
completion is used to indicate end of one message transfer (+check for msg timeout),
so .master_xfer()->omap_i2c_xfer could switch to next msg.
And there could be more than on IRQ triggered depending on msg length
while one message is being transfered.

-- 
regards,
-grygorii
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