On Wed, Mar 31, 2004 at 10:45:47AM +0200, Jean Delvare wrote: [...] > That's right. BTW, Aurelien Jarno and I decided to get rid of the > caching mechanism in the pcf8574 and pcf8591 drivers as we ported them > to 2.6. It seemed to be a problem not being able to get the values in > real-time for such devices, since they are used in very particular > conditions. I don't think it is a problem because such devices are not > present on motherboards and I don't expect many people to use these > drivers. And these devices don't seem to stop when you read from them > (actually it's the contrary, they sample the inputs as you read them. > > Aurelien, can you confirm this? I confirm that. For the pcf8574, the 8-bit input port is sampled during the first clock of the data byte, and for the pcf8591, each bit is sampled just before being sent. > That said there's still a problem. The pcf* drivers accept all chips > within their address ranges, because the chips are plain undetectable. > So if someone was to load them and they actually try to drive a > hardware monitoring chip instead, continuous readings from the sysfs > files could prevent the chip from working (left apart the fact that the > driver is likely to confuse the chip even without continuously reading, > but there's nothing we can do there). You're right that it can cause some problems, on my system I need to load the ds1621 module before the pcf8591 otherwise my ds1621 chip is recognized as a pcf8591. > Maybe we could add a security warning in the kernel configuration help > text for these two drivers? (something like "This driver is potentially > dangerous if used on inappropriate hardware. Including it into the > kernel is discouraged, module is prefered. Do not load this module > unless you know you need it.") That could be an idea. To be honest, I don't see other alternative as the chips are very stupids; the pcf8574 doesn't have any control register, and the pcf8591 has one control register but only writable. -- .''`. Aurelien Jarno GPG: 1024D/F1BCDB73 : :' : Debian GNU/Linux developer | Electrical Engineering Student `. `' aurel32 at debian.org | aurelien at aurel32.net `- people.debian.org/~aurel32 | www.aurel32.net
