On Wed, Oct 7, 2015 at 11:47 AM, Måns Rullgård <mans@xxxxxxxxx> wrote: > Mark Rutland <mark.rutland@xxxxxxx> writes: > >> On Wed, Oct 07, 2015 at 04:37:13PM +0100, Mans Rullgard wrote: >>> This adds a DT binding for a generic mmio clocksource as implemented >>> by clocksource_mmio_init(). >>> >>> Signed-off-by: Mans Rullgard <mans@xxxxxxxxx> >>> --- >>> Changed in v2: >>> - added sched_clock support >>> --- >>> .../devicetree/bindings/timer/clocksource-mmio.txt | 28 ++++++++++++++++++++++ >>> 1 file changed, 28 insertions(+) >>> create mode 100644 Documentation/devicetree/bindings/timer/clocksource-mmio.txt >>> >>> diff --git a/Documentation/devicetree/bindings/timer/clocksource-mmio.txt b/Documentation/devicetree/bindings/timer/clocksource-mmio.txt >>> new file mode 100644 >>> index 0000000..cfb3601 >>> --- /dev/null >>> +++ b/Documentation/devicetree/bindings/timer/clocksource-mmio.txt >>> @@ -0,0 +1,28 @@ >>> +Generic MMIO clocksource >>> + >>> +Required properties: >>> + >>> +- compatible: should be "clocksource-mmio" I'm doubtful this matches any h/w. This would assume there is no other control like enabling, reset, clock dividers, etc. Who is your user of this? >>> +- reg: the physical address of the counter register >>> +- reg-io-width: size of counter register in bytes, should be 2 or 4 >> >> Can this not be inferred from the reg? > > You're right, it can. > >> What about 8 byte counters? > > The existing code only supports 2 or 4, but that of course doesn't > matter here. > >>> +- clocks: phandle to the source clock >> >> Is the frequency expected to be exactly the source clock frequency? I >> imagine it's possible for there to be a divisor. > > There could of course be, though there isn't in the hardware I'm dealing > with. Is specifying it here preferable using a fixed-factor-clock? > >> We can add properties for that later, but we should be explcit as to >> what we currently expect the relationship between the clock and the >> clocksource to be. >> >>> +- clocksource-bits: number of valid bits >>> +- clocksource-rating: rating of the clocksource >> >> NAK. This has no meaning w.r.t. the hardware. This should not be in the >> DT. If there are criteria that bias this (e.g. frequency, latency), they >> should either be described in the DT or determined dynamically. > > I had a bad feeling about this. How would you suggest determining a > suitable value from actual hardware parameters? I wouldn't. I think there is general agreement that rating is broken. This has come up a few times before in context of given a pile of timer h/w how do you select one. Chosen properties have been one example (I think that actually went in on Integrator, but we since removed it). Think about what the properties of a timer are and then you can decide based on properties for those. OMAP timers are a good example. > >>> +- linux,sched-clock: boolean, register clocksource as sched_clock >> >> Likewise, this property doesn't belong in the DT for the same reasons as >> clocksource-rating. > > What would be a proper way to select a sched_clock source? I realise > it's a Linux-specific thing and DT is supposed to be generic, but the > information must be provided somehow. The kernel already has some logic to do this. Most number of bits followed by highest frequency will be the winning sched_clock. You might also want to look at things like always on or not. Rob > > -- > Måns Rullgård > mans@xxxxxxxxx -- 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
