On Fri, Aug 16, 2013 at 07:12:46PM +0100, Benoit Cousson wrote: > Hi Mark, > > On 16/08/2013 19:20, Mark Rutland wrote: > > Hi Benoit, > > > > On Fri, Aug 16, 2013 at 03:15:57PM +0100, Benoit Cousson wrote: > >> Hi Gururaja, > >> > >> On 16/08/2013 13:36, Hebbar, Gururaja wrote: > >>> The syntax of compatible property in DT is to mention the Most specific > >>> match to most generic match. > >>> > >>> Since AM335x is the platform with latest IP revision, add it 1st in > >>> the device id table. > >> > >> I don't understand why? The order should not matter at all. > >> > >> I've tried to follow the thread you had with Mark on the v2, but AFAIK, > >> you've never answered to his latest question. > >> > >> Moreover, checking the differences between the Davinci and the am3352 > >> RTC IP, I would not claim that both are compatible. > >> > >> Sure you can use the am3352 with the Davinci driver, but you will lose > >> the wakeup functionality without even being notify about that. > > > > Could you describe the wakeup functionality, and how it differs between > > the am3352-rtc and the da830-rtc? > > AFAIK, da830-rtc does not have that functionality at all. This is > something that was added to the am3352-rtc. Ok. So the am3352-rtc can be driven with the full functionality of the da830-rtc (ie. it's compatible with the da830-rtc programming model), or it can be driven as an am3352-rtc, for the OS to gain wakeup functionality in addition to the da830-rtc features. :) > > > As I understand it, the am3352 functionality is a superset of the da830 > > functionality. You can use the old driver, and get some functionality, > > or use the new driver and get it all. > > Mmm, what your are saying now seems to make sense to me as well. So I'm > even more confused :-) I'll convince you yet :) > > > That means that am3352-rtc is compatible with da830. As long as the > > kernel first checks for am3352-rtc, there will be *no* loss of > > functionality. All this does is enable older kernels to use the hardware > > in some fashion, and given the older kernel didn't have support for the > > am3352-rtc features, this is a *gain* in functionality, not a loss. > > > >> > >> For my point of view, compatible mean that the HW will still be fully > >> functional with both versions of the driver, which is not the case here. > > > > What? A driver for any entry in the compatible list should be able to > > drive the hardware to *some* level of functionality. We list from > > most-specific to most-general to allow a graceful degradation from fully > > supported to bare minimum functionality. > > OK, but where is it written in the DT spec that this is what the > compatible is supposed to mean? > > I'm quoting it again: > " > The compatible property value consists of one or more strings that > define the specific programming model for the device. This list of > strings should be used by a client program for device driver selection. > The property value consists of a concatenated list of null terminated > strings, from most specific to most general. They allow a device to > express its compatibility with a family of similar devices, potentially > allowing a single device driver to match against several devices. > " > > The graceful degradation or the loss of functionality is not something > that I really understand in that text. I think it's implicit in the example that follows, where a failure to match against a specific device results in the OS falling back to a "more general" device. The "more general" device may not have all the features of a more specific device (conversely, the more general device may have more optional features that a more specific device is known not to implement). > > Anyway, I'm probably too tired... I'll go back home, and think about > that after the week-end. Ok, let me know what you think. :) Thanks, Mark. -- 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
