[linux-pm] sleepy linux self-test

David Brownell david-b at pacbell.net
Sat Feb 2 11:47:44 PST 2008

On Saturday 02 February 2008, Ingo Molnar wrote:
> * David Brownell <david-b at pacbell.net> wrote:
> > On Saturday 02 February 2008, Ingo Molnar wrote:
> > > 
> > > i'd really love to have a /dev/rtc device compatibility APIs, both 
> > > inside and outside the kernel.
> > 
> > Unfortunately the /dev/rtc code became a legacy API for good reasons.
> > 
> > Like not recognizing that all the world's not a PC, with a single RTC 
> > that clones a long-obsolete chip from Motorola ... and not having been 
> > specified in a hardware-neutral manner.  Oh, and of course not all 
> > systems actually used the same RTC driver anyway; it's not like there 
> > was just *one* such programming interface to worry about.
> i dont get it - please give me specific technological reasons

In a word:  -ENOPATCH.  ;)

> why on my  
> PC /dev/rtc couldnt be mapped to /dev/rtc0 - without requiring any 
> user-space changes. The APIs seem mostly covered, or at least mappable. 
> Why should the transition to a new driver require user-level changes? 
> (beyond the obvious extensions, but those should show up as extensions.) 

So far as I'm aware, the only issue visible to userspace relate to
the legacy driver's use of "/dev/rtc" not "/dev/rtc0" ... which has
previously been "solved" by symlinking "rtc" -> "rtc0", possibly with
assistance from udev.  (Related:  the major/minor number of /dev/rtc.)
Is that your understanding too?

The "why" is that nobody has been sufficiently bothered by the need
to symlink that they produced a kernel patch to compensate.

> In fact i detest the old RTC code with a vengence, so dont understand 
> this as some invitation to flame or something - i simply want YOUR new 
> code to be utilized more!

Good to know!  :)

But so you're clear ... not "my" code, mostly.  Alessandro Zummo started
this framework, based in part on Russell King's framework for RTCs that
were integrated into ARM based SOCs.  I contributed a bunch, including
rtc-cmos to let the PC side of Linux join the effort.

> I just dont see the specific technological  
> reasons of why there is no .config switch to switch the legacy /dev/rtc 
> over to the new RTC driver and be done with it.

Initially:  because that idea hadn't been suggested.  And because that
sort of code migration on PC hardware needs to be done slowly enough
that the migration issues have a real chance to surface.  Issues like:

 - Change to the ACPI suspend/resume interactions broke RTC wakeup
   for a couple releases in the new RTC framework; now fixed.

 - HPET stuff.  I think the recently merged HPET update may imply
   that rtc-cmos needs an HPET hook, but I've not looked at details.

 - Minor bugfixes, which have been resolved over time.

 - Your desire to keep using old /dev/rtc nodes (no symlink, so new
   kernels and old non-udev fileystems can mix).  New issue, no patch.

Nothing else come to mind.

> I'd enable it in a  
> heartbeat and would encourage distros to do so. Are there missing APIs? 
> Is the ioctl API totally different? It's impossible to wrap it?

See above.  Once the HPET thing is resolved, I think distros can
convert given some resolution of the /dev/rtc issue.

> I'm not really interested in "this isnt a PC" arguments.

Maybe because for you, it *is* a PC.  ;)

> The incompatibility is  
> such an obvious migration barrier to me - do you really not see it?

Let's just say that all my PCs run the new code just fine, and not
all of them have /dev/rtc symlinked to /dev/rtc0 ... but I can very
easily imagine it look bit different from a distro perpective.

- Dave

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