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. > 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. > 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. > 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. -- balbi
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