On Tuesday 29 September 2015 13:11:55 Peter Griffin wrote: > Hi Arnd, > > On Tue, 29 Sep 2015, Arnd Bergmann wrote: > > > On Tuesday 29 September 2015 11:04:40 Peter Griffin wrote: > > > > > > "The hardware is identical, and different firmware is used to apply > > > it in different ways." > > > > > > Which is the case with fdma. By encoding the "way you wish to apply it" into the > > > compatible string, it causes problems if you want to change for example fdma0 > > > to do some other function other than audio. > > > > > > You then require a DT update, (when the hardware hasn't changed, just the > > > firmware) which is the same problem as using the filename directly in DT. > > > > > > Therefore I believe it is important that the DT binding does *not* encode the > > > way the hardware is to be applied into the binding in *any* way, and defers this > > > decision to the driver. > > > That is the rationale / reasoning behind choosing the fdma instance number. > > > > > > Assuming you agree with my arguments above, then the choice becomes between > > > having a fdma instance DT property, or having lots of compatibles where the only > > > difference is the appending of the instance number. I think out of the two I prefer > > > my original approach. > > > > > > Any thoughts from the DT folks? > > > > To me both approaches sound wrong: basing the firmware name on the instance > > number requires that each instance is always used in the same way, which > > is not guaranteed to be the case, > > Does it? I didn't think it did. > > Using the instance number as a DT property defers the decision over what firmware to > load to the driver, which can choose whatever firmware name it wishes. > > e.g. in v4.3 it could load xyz.elf, in v4.4 it could choose abc.elf. The DT will remain > unchanged, but the use of that fdma instance has changed. > > We currently only have one firmware for each instance with the "use" compiled into it. > If in the future we had two firmwares with different "uses" for the same instance some extra > logic would be required in the driver to make a decision on which firmware to load. Ok, I probably need some more background about what the firmware on this device does, and what it could do with a different firmware. Could you elaborate? > > and you correctly describe the problem with > > using the compatible string for the firmware name if the driver for the FDMA > > does not actually care what firmware is being used here. > > > > Whatever code makes the decision as to how the FDMA is used should also > > decide on the name of the firmware file. > > The code which makes this decision currently is the st_fdma.c driver. However it does > need to know which fdma controller it is operating on to make this decision correctly. > > Apart from passing the fdma instance number in DT, how else can we determine which > controller we are? > > I guess we could infer it by having a table in the driver containing the base addresses > of the controllers for a given SoC, and match that against what DT passes us in the > reg property. But that seems ugly, and is encoding the same information in two > different places. > > I'm open to suggestions if there is a better way to do this. Using the address would be the same thing, that doesn't change the fundamental logic. Can you explain why it matters which instance a firmware is used on for this driver? Arnd -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
