Hi; this patchset proposes a couple of new APIs for Clock, which I found I needed/wanted for a work-in-progress patchset that includes a model of a new timer device, the "System Timer" documented in the Arm SSE-123 Example Subsystem Technical Reference Manual: https://developer.arm.com/documentation/101370/0000/system-time-components/system-timer?lang=en
It's going to be a little while before I post the patchseries with the new timer device implementation, because I need to complete a model of the SSE-300 SoC and an MPS board model that uses it; so I wanted to put these out as an RFC to see if people agree on the APIs I'm suggesting. The problem the first two patches are trying to solve is that I found that I wanted the old value of the Clock's period when my device got a notification about a frequency/period change. The current ClockCallback API calls you after the period has changed, so the Clock is already using the new period. I wanted this because my timer device has a register that's basically a free-running up-counter; the value of that counter can be calculated with: s->ticks_then + clock_ns_to_ticks(s->clk, now - s->ns_then); where (ns_then, ticks_then) are a tuple of a QEMU_CLOCK_VIRTUAL time and the tick count at that point. Whenever the clock frequency changes we calculate a new (ns_then, ticks_then) to use as the baseline for future counter value reads, but in order to do that we must calculate ticks_then using the *old* clock period. My solution to this is to add a ClockEvent argument to the callback function, which is an enum: ClockPreUpdate : callback called before period change ClockUpdate : callback called after period change All callback functions are called for all events, and they look at the 'event' argument to see whether they need to do anything. This means that the patch has to update every ClockCallback in the codebase to take the new argument and do nothing if it is not ClockUpdate, but luckily there aren't many of them; this seemed better than trying to manage multiple separate callback pointers. The problem the third patch addresses is that we don't have a function for "tell me how many times this clock would tick in this length of time". clock_ns_to_ticks() does the inverse of the clock_ticks_to_ns() that we already have. Two points in particular where comment would be useful: * I chose to make the overflow case (where a clock has a very short period and the specified length of time is very long, so the clock would tick more than UINT64_MAX times) just let the value wrap around, on the basis that usually this is being used to calculate a guest register value that's in a 64 bit or 32 bit register, and so wrap-around is the right behaviour. But I'm not 100% set on this if somebody has a better idea. * The calculation needs to do a 96-bit / 64 bit => 64 bit division, for which the best thing we have is divu128(). This is particularly painful on 32-bit hosts. I don't suppose there's anything clever we can do to make this better ? Patch 4 just uses clock_ns_to_ticks() in the one place in the current codebase where we're currently using clock_ticks_to_ns() and manual calculation. Side note: there is currently no MAINTAINERS entry for the clock framework. Any volunteers? It would cover F: include/hw/clock.h F: include/hw/qdev-clock.h F: hw/core/clock.c F: hw/core/qdev-clock.c F: docs/devel/clocks.rst thanks -- PMM Peter Maydell (4): clock: Add ClockEvent parameter to callbacks clock: Add ClockPreUpdate callback event type clock: Add clock_ns_to_ticks() function hw/timer/npcm7xx_timer: Use new clock_ns_to_ticks() docs/devel/clocks.rst | 54 ++++++++++++++++++++++++++++++-- include/hw/clock.h | 52 +++++++++++++++++++++++++++++- hw/arm/armsse.c | 8 +++-- hw/char/cadence_uart.c | 5 ++- hw/char/ibex_uart.c | 5 ++- hw/char/pl011.c | 5 ++- hw/core/clock.c | 5 ++- hw/misc/bcm2835_cprman.c | 20 +++++++++--- hw/misc/npcm7xx_clk.c | 31 ++++++++++++++++-- hw/misc/zynq_slcr.c | 6 +++- hw/timer/cmsdk-apb-dualtimer.c | 5 ++- hw/timer/cmsdk-apb-timer.c | 5 ++- hw/timer/npcm7xx_timer.c | 4 +-- hw/watchdog/cmsdk-apb-watchdog.c | 5 ++- 14 files changed, 188 insertions(+), 22 deletions(-) -- 2.20.1
