On 20/10/16 18:04, Nicolas Pitre wrote: > On Thu, 20 Oct 2016, Edward Cree wrote: >> Also, I don't think having any FOO=y should preclude BAZ=m. Suppose both >> FOO and FOO2 imply BAZ, FOO=y and FOO2=m. > Some people didn't like the fact that you could turn a driver from m to > y and silently lose some features if they were provided by a subsystem > that also used to be m, which arguably is not the same as being > explicitly disabled. With "select" this is not a problem as the target > symbol is also promoted to y in that case, so I wanted to preserve that > property. Right, but that's an argument for pushing the subsystem's default to y, not for preventing changing the subsystem back to m afterwards. >> Then if BAZ-features are only >> desired for driver FOO2, BAz=m makes sense. > In that case it would make more sense to add a config option related to > FOO asking if BAZ features are desired for that driver (there is already > one occurrence of that with PTP). Or you could simply drop the "imply" > statement from the FOO config entry. But the desire is a property of the user, not of the driver. If you're willing to add CONFIG_FOO_BAZ to every combination of (driver, subsystem) then "imply" becomes unnecessary, doesn't it? Conversely, if you *don't* want to have to do that, then "imply" needs to only ever deal in defaults, not in limitations. >> There is also the case of drivers with the ability to detect at runtime >> whether BAZ is present, rather than making the decision at build time, but >> I'm not sure how common that is. > Right now that's how PTP support is done. Drivers can optimize things > at build time, but most of them simply cope with a NULL return from > ptp_clock_register(). Hence the imply statement becomes a big > configuration hint rather than some hard build dependency. Right, so those drivers can use PTP if they're y and PTP is m, as long as the PTP module is loaded when they probe. But current "imply" semantics won't allow that...
I think that Josh's suggestion (have the UI warn you if you set BAZ to m while FOO=y) is the right approach, but I also think it should be done now rather than at some unspecified future time. Otherwise you forbid potentially valid configs. -Ed
