Hi Rory, On Thu, 7 Oct 2010 14:37:57 -0700, Rory Filer wrote: > I'm working on a proof-of-concept device which contains several chips > connected to the same adapter (/dev/ic2-0) on the I2C bus. Some of these have > kernel-space drivers (e.g. GPIO Expander) and some don't. For the ones that > don't I've opted to use the method described in the kernel > Documentation/i2c/dev-interface document. This works great and does all I > need for now. This is the general procedure I'm following as outlined in > kernel - Documentation/i2c/dev-interface. > > - open /dev/i2c-N > - Set the chip's address by calling ioctl(file, I2C_SLAVE, address) > - interact with the chip using read/write or smb calls, etc. > - close the file when done > > I'm assuming that a kernel space chip driver like our gpio expander > (pca953x.c) also uses the same adapter driver as my user-space application > since that expander chip is on the same bus.
Let's clarify right away: i2c-dev (which handles /dev/i2c-* nodes) is NOT an adapter driver. It is a bridge driver between user-space and your adapter driver. Your adapter driver is most likely one living in drivers/i2c/busses, and depends on the actual hardware you run on. > My question is, what happens if a kernel driver is using the bus at the same > time my user-mode application is? It will work fine. > Even if the user space app. may not be running when the chip driver runs, the > adapter has an address written into it via the ioctl() call, I noted above. Incorrect. The address isn't written to the adapter, it is a setting local to i2c-dev and even specific to the file handle you got when opening a /dev/i2c-* node. Each subsequent read, write or smbus call in your user-space driver automatically includes the slave address, which is only valid for the read, write or smbus call in question. > Can unpredictable results occur in this scenario? No. > I think I could protect against it by writing a thread to serialize accesses > to the I2C bus, but that only works if my entire application is running > inside one process. I'm hoping someone on the list might be able to advise. The i2c-core module already serializes all accesses to every given i2c adapter through i2c_transfer() and i2c_smbus_xfer(). -- Jean Delvare http://khali.linux-fr.org/wishlist.html -- To unsubscribe from this list: send the line "unsubscribe linux-i2c" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
