On Fri, Nov 2, 2012 at 1:57 AM, Felipe Balbi <balbi@xxxxxx> wrote: > Hi, > > On Thu, Nov 01, 2012 at 04:49:23PM -0700, Russ Dill wrote: >> On Thu, Nov 1, 2012 at 3:05 PM, Felipe Balbi <balbi@xxxxxx> wrote: >> > HI, >> > >> > On Thu, Nov 01, 2012 at 03:59:50PM +0200, Pantelis Antoniou wrote: >> >> Hi Alan, >> >> >> >> On Nov 1, 2012, at 3:51 PM, Alan Cox wrote: >> >> >> >> >> What they want, and what every user wants, is I plug this board in, and >> >> >> the driver make sure everything is loaded and ready. No, the end users >> >> >> don't want to see any of the implementation details of how the bitfile >> >> >> is transported; the driver can handle it. >> >> > >> >> > That doesn't necessarily make it a bus merely some kind of hotplug >> >> > enumeration of devices. That should all work properly both for devices >> >> > and busses with spi and i²c as the final bits needed for it got fixed >> >> > some time ago. >> >> > >> >> > In an ideal world you don't want to be writing custom drivers for stuff. >> >> > If your cape routes an i²c serial device to the existing system i²c >> >> > busses then you want to just create an instance of any existing driver on >> >> > the existing i²c bus not create a whole new layer of goop. >> >> > >> >> > It does need to do the plumbing and resource management for the plumbing >> >> > but thats not the same as being a bus. >> >> > >> >> > Alan >> >> >> >> >> >> Fair enough. But there's no such thing a 'hotplug enumeration >> >> construct' in Linux yet, and a bus is the closest thing to it. It does >> >> take advantage of the nice way device code matches drivers and devices >> >> though. >> >> >> >> I'm afraid that having the I2C/SPI drivers doing the detection won't >> >> work. The capes can have arbitrary I2C/SPI devices (and even more >> >> weird components). There is no way to assure for example that the I2C >> >> device responding to address 'foo' in cape A is the same I2C device >> >> responding to the same address in cape B. >> > >> > your ->detect() method should take care of that. >> >> There isn't some magical serial number in I²C devices that a >> ->detect() method can read and the implementation of I²C is somewhat >> flexible. One devices read may be another devices write. A detect > > look at what other drivers do. You can read a revision register, you can > write a command and see if the device responds as expected, it doesn't > matter. Since a "revision" register isn't required by the I²C spec, it isn't implemented on a huge number of chips. Also, having a few dozen probe routines come though and write to random address of every single I²C device can a) take a really long time, and b) have quite a few unintended side effects. >> method that only performs reads could easily toggle a gpio that resets >> the board, rewrite and eeprom, or set the printer on fire. If you > > how ? It's just a read. Because the I²C spec is incredibly flexible. For a lot of devices, reading from a register is done by writing the register address, and then reading the contents. For devices that don't implement registers in that way (such as many eeproms), this is just a write. >> browse through various detect functions, yes, some of them key off an >> ID, but a lot of them just check various registers to see if certain >> bits are zero, or certain bits are one. A lot of I²C devices I've >> dealt with have no good way of probing them, especially GPIO chips >> (you'll notice none of the I²C GPIO expanders have detect functions) > > it doesn't mean it can't be done. Really? Please, do tell how you would write a detect function for a PCA9534. It has 4 registers, an input port registers, an output port register, a polarity inversion register, and a configuration register. And don't forget, since we are probing, every detect routine for every I²C driver will have to run with every I²C address on every bus, possibly with both address formats. >> On top of all this the detect routine does not tell you if the alert >> pin is connected to some IRQ, or in the case of a GPIO expander, what >> those GPIOs are connected to, etc, etc. > > so what ? All you want to do with detect is figure out if the far end is > who you think it is, not what it's doing. If we already knew who was there, we wouldn't need a detect routine. -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
