linux-kernel - Re: I2C from interrupt context?

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Date:	Fri, 7 Nov 2008 10:11:18 +0100
From:	Jean Delvare <khali@...ux-fr.org>
To:	David Miller <davem@...emloft.net>
Cc:	Krzysztof Halasa <khc@...waw.pl>, linux-kernel@...r.kernel.org,
	linux-i2c@...r.kernel.org
Subject: Re: I2C from interrupt context?

On Thu, 06 Nov 2008 15:47:54 -0800 (PST), David Miller wrote:
> From: Krzysztof Halasa <khc@...waw.pl>
> Date: Fri, 07 Nov 2008 00:44:48 +0100
> 
> [ Fixed i2c list address, it's now hosted at vger ]
> 
> > I'm updating my EEPROM console logger and have encountered a problem
> > - the logger (as any console) can be called with hardware interrupts
> > disabled and/or from interrupt context. It needs to write to I^2C
> > (using ARM (Xscale) GPIO) and possibly SMBUS-only EEPROM chip. Is it
> > at all supposed to be possible?
> 
> Not really.  The I2C operations need to be able to sleep and that's
> not allowed in interrupt context.

That's not totally correct. Since kernel 2.6.25, i2c_transfer()
supports being called in contexts where it cannot sleep. Of course, for
it to work, the underlying i2c adapter driver must also not sleep. As I
recall, only the i2c-pxa driver was explicitly modified to support this
at the moment (see member use_pio of struct pxa_i2c) but other drivers
could be modified in a similar way (and some i2c adapter drivers may be
naturally non-sleeping - everything based on i2c-algo-bit is likely to
fall into this category.)

The situation is far from perfect though. For one thing, I seem to
recall that Andrew Morton didn't like the approach taken in
i2c_transfer(). For another, i2c_smbus_xfer() was not yet modified so
at this point only I2C-level transactions can be non-sleeping,
SMBus-level transactions can't. But all this could be fixed by anyone
who cares about these specific issues.

-- 
Jean Delvare
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