Date: Fri, 7 Nov 2008 10:11:18 +0100
From: Jean Delvare <khali@linux-fr.org>
To: David Miller <davem@davemloft.net>
Cc: Krzysztof Halasa <khc@pm.waw.pl>, linux-kernel@vger.kernel.org,
       linux-i2c@vger.kernel.org
Subject: Re: I2C from interrupt context?
Message-ID: <20081107101118.286f3713@hyperion.delvare>
In-Reply-To: <20081106.154754.02648408.davem@davemloft.net>
References: <m3bpws8ln3.fsf@maximus.localdomain>
	<20081106.154754.02648408.davem@davemloft.net>
Mime-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1791
Lines: 38

On Thu, 06 Nov 2008 15:47:54 -0800 (PST), David Miller wrote:
> From: Krzysztof Halasa <khc@pm.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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/