From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jean Delvare Subject: Re: I2C from interrupt context? Date: Fri, 7 Nov 2008 10:11:18 +0100 Message-ID: <20081107101118.286f3713@hyperion.delvare> References: <20081106.154754.02648408.davem@davemloft.net> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20081106.154754.02648408.davem-fT/PcQaiUtIeIZ0/mPfg9Q@public.gmane.org> Sender: linux-i2c-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: David Miller Cc: Krzysztof Halasa , linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-i2c-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-Id: linux-i2c@vger.kernel.org On Thu, 06 Nov 2008 15:47:54 -0800 (PST), David Miller wrote: > From: Krzysztof Halasa > 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