Messages in this thread | | | Date | Tue, 9 Apr 2024 22:48:28 +0800 | From | Jisheng Zhang <> | Subject | Re: [PATCH v2 2/3] clocksource/drivers/timer-clint: Add option to use CSR instead of mtime |
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On Tue, Apr 09, 2024 at 03:26:18PM +0100, Conor Dooley wrote: > On Sat, Apr 06, 2024 at 07:21:58PM +0800, Jisheng Zhang wrote: > > As pointed out by commit ca7810aecdba ("lib: utils/timer: mtimer: add a > > quirk for lacking mtime register") of opensbi: > > > > "T-Head developers surely have a different understanding of time CSR and > > CLINT's mtime register with SiFive ones, that they did not implement > > the mtime register at all -- as shown in openC906 source code, their > > time CSR value is just exposed at the top of their processor IP block > > and expects an external continous counter, which makes it not > > overrideable, and thus mtime register is not implemented, even not for > > reading. However, if CLINTEE is not enabled in T-Head's MXSTATUS > > extended CSR, these systems still rely on the mtimecmp registers to > > generate timer interrupts. This makes it necessary to implement T-Head > > C9xx CLINT support in OpenSBI MTIMER driver, which skips implementing > > reading mtime register and falls back to default code that reads time > > CSR." > > > > To use the clint in RISCV-M NOMMU env on Milkv Duo little core, we > > need to fall back to read time CSR instead of mtime register. Add the > > option for this purpose. > > > > Signed-off-by: Jisheng Zhang <jszhang@kernel.org> > > > diff --git a/drivers/clocksource/Kconfig b/drivers/clocksource/Kconfig > > index 34faa0320ece..7bbdbf2f96a8 100644 > > --- a/drivers/clocksource/Kconfig > > +++ b/drivers/clocksource/Kconfig > > @@ -650,6 +650,15 @@ config CLINT_TIMER > > This option enables the CLINT timer for RISC-V systems. The CLINT > > driver is usually used for NoMMU RISC-V systems. > > > > +config CLINT_USE_CSR_INSTEADOF_MTIME > > + bool "Use TIME CSR instead of the mtime register" > > + depends on CLINT_TIMER > > + help > > + Use TIME CSR instead of mtime register. Enable this option if > > + prefer TIME CSR over MTIME register, or if the implementation > > + doesn't implement the mtime register in CLINT, so fall back on > > + TIME CSR. > > This, as a Kconfig option, seems a bit strange to me. We know at runtime > if we are on a T-Head device without the mtime register and should be > able decide to use the CSR implementation dynamically in that case, > right?
Dynamically decision can be done in clocksource/clockevnt: I can patch clint_clocksource.read to point to different clint_rdtime() implementation.
But clint timer is also used in NOMMU RISCV-M's get_cycles(), this can't be dynamically chosen w/o an ugly "if (is_c900)" check, and I'm not sure whether this check in get_cycles() will introduce non-trival overhead or not. Or use code patching technology here?
Or introduce a function pointer such as unsigned long (*rdtime)(void) for RISCV_M_MODE, then point it to different implementation?
Any suggestion is welcome. Thanks
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