Op 2 nov. 2012, om 10:42 heeft Russ Dill <Russ.Dill@xxxxxx> het volgende geschreven: > 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. Worse, things like early revisions of the picoDLP projector will erase their firmware if you do a linear scan through all addresses. regards, Koen-- 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
