Date:   Wed, 12 Oct 2016 23:31:26 +0200
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Rich Felker <dalias@...c.org>
Cc:     devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-sh@...r.kernel.org, Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH v8 2/2] clocksource: add J-Core timer/clocksource driver

On Wed, Oct 12, 2016 at 01:02:36PM -0400, Rich Felker wrote:
> On Wed, Oct 12, 2016 at 11:27:11AM +0200, Daniel Lezcano wrote:
> > > > Are the CPUs on always-on power down ?
> > > 
> > > For now they are always on and don't even have the sleep instruction
> > > (i.e. stop cpu clock until interrupt) implemented. Adding sleep will
> > > be the first power-saving step, and perhaps the only one for now,
> > > since there doesn't seem to be any indication (according to the ppl
> > > working on the hardware) that a deeper sleep would provide significant
> > > additional savings.
> > 
> > Ok.
> > 
> > However, the 'sleep' state is not, in the power management terminology,
> > the idle state described above. It is called "clock gated" / "Wait for
> > Interrupt".
> > 
> > The 'sleep' state lose the CPU context.
> 
> I use the term "sleep" because that's the name of the SH instruction
> mnemonic for the opcode for entering the wait-for-interrupt state.
> Sorry it's confusing like the "RTC" all over again. :-)
> 
> > > > > A nanosecond-resolution clocksource is provided using the J-Core "RTC"
> > > > > registers, which give a 64-bit seconds count and 32-bit nanoseconds
> > > > > that wrap every second. The driver converts these to a full-range
> > > > > 32-bit nanoseconds count.
> > > > > 
> > > > > Signed-off-by: Rich Felker <dalias@...c.org>
> > > > > ---
> > > > >  drivers/clocksource/Kconfig     |  10 ++
> > > > >  drivers/clocksource/Makefile    |   1 +
> > > > >  drivers/clocksource/jcore-pit.c | 231 ++++++++++++++++++++++++++++++++++++++++
> > > > >  include/linux/cpuhotplug.h      |   1 +
> > > > >  4 files changed, 243 insertions(+)
> > > > >  create mode 100644 drivers/clocksource/jcore-pit.c
> > > > > 
> > > > > diff --git a/drivers/clocksource/Kconfig b/drivers/clocksource/Kconfig
> > > > > index 5677886..95dd78b 100644
> > > > > --- a/drivers/clocksource/Kconfig
> > > > > +++ b/drivers/clocksource/Kconfig
> > > > > @@ -407,6 +407,16 @@ config SYS_SUPPORTS_SH_TMU
> > > > >  config SYS_SUPPORTS_EM_STI
> > > > >          bool
> > > > >  
> > > > > +config CLKSRC_JCORE_PIT
> > > > > +	bool "J-Core PIT timer driver"
> > > > > +	depends on OF && (SUPERH || COMPILE_TEST)
> > > > 
> > > > Actually the idea is to have the SUPERH to select this timer, not create
> > > > a dependency on SUPERH from here.
> > > > 
> > > > We don't want to prompt in the configuration menu the drivers because it
> > > > would be impossible to anyone to know which timer comes with which
> > > > hardware, so we let the platform to select the timer it needs.
> > > 
> > > I thought we discussed this before. For users building a kernel for
> > > legacy SH systems, especially in the current state where they're only
> > > supported with hard-coded board files rather than device tree, it
> > > makes no sense to build drivers for J-core hardware. It would make
> > > sense to be on by default for CONFIG_SH_DEVICE_TREE with a compatible
> > > CPU selection, but at least at this time, not for SUPERH in general.
> > 
> > Probably I am missing the point but why the user would have to unselect
> > this driver manually ? The user wants a config file nothing more or a very
> > trivial option. Can you imagine someone can know every single IP block for
> > each boards of the same arch and be able to disable/enable the right ones ?
> 
> The common case I imagine is just accepting defaults that include more
> than you need, but for space-constrained (or more likely, fast boot,
> when the kernel is being loaded from a slow spi-based SD card
> interface) setups the user might be trying to minimize kernel size and
> turn off drivers they know they don't want. I'm not so worried about
> this driver, specifically, because it's small, but I am concerned
> about the general policy -- when we get rid of all the legacy board
> files and everything is move to device tree, will the user be stuck
> including a bunch of Renesas SH drivers when building a kernel the
> intend to use only on J-core?
> 
> > > Anyway I'd really like to do this non-invasively as long as we have a
> > > mix of legacy and new stuff and the legacy stuff is not readily
> > > testable. Once all of arch/sh is moved over to device tree, could we
> > > revisit this and make all the drivers follow a common policy (on by
> > > default if they're associated with boards/SoCs using a matching or
> > > compatible CPU model, or something like that, but still able to be
> > > disabled manually, since the user might be trying to get a tiny-ish
> > > embedded kernel)?
> > 
> > I understand the goal is to have one single configuration and everything
> > DT based and it sounds great but what is missing here is just a subarch,
> > not an option to enable/disable the timer.
> > 
> > Give a try with:
> > 
> > make ARCH=arm multi_v7_defconfig menuconfig
> > 
> > --> System Type
> > 
> > That is what you are looking for, a SUPERH config option selecting all the
> > common options and then a JCORE config option adding the different missing
> > bits, namely the CLKSRC_JCORE_PIT.
> 
> We do have something like "system type" in arch/sh, and it's what I'm
> trying to deprecate since it's the switch to select between all the
> hard-coded board files, _or_ device tree.
> 
> Since part of the goal of my DT overhaul is to be able (but not
> forced) to produce kernels that run on a wide range of hardware,
> rather than having a "system type (select one)" option, what about
> individual boolean options like:
> 
> config JCORE_SOC
> 	bool "Support for J-Core SoCs"
> 	select CLKSRC_JCORE_PIT
> 	select JCORE_AIC
> 	...

I'm perfectly fine with this.
 
> Note that there are other drivers that should probably be optional
> even if you have JCORE_SOC enabled, like the SPI controller, DMA
> controller (not implemented yet), Ethernet (not submitted upstream
> yet), etc. Maybe they could depend on JCORE_SOC and be default-yes but
> configurable if available?

That sounds fine also.

> In any case, the SoC support is supposedly there in the current kernel
> release (4.8) but not working because of missing essential drivers, so
> I'd really like to fix that without making the fix dependent on
> restructuring the arch/sh system type handling, which is an ongoing,
> independent project for which I'm waiting for help converting and
> testing the conversions of legacy board support. My preference would
> be to keep the Kconfig stuff the way I submitted it for now --
> j2_defconfig already handles enabling thse right drivers -- and do
> something more user-friendly as part of the bigger arch overhaul
> project.

I prefer the move the option to config JCORE_SOC. That is not a big deal
to add this bool in the sh's Kconfig and select the timer from there.
 
> > > > > +	/*
> > > > > +	 * The J-Core PIT is not hard-wired to a particular IRQ, but
> > > > > +	 * integrated with the interrupt controller such that the IRQ it
> > > > > +	 * generates is programmable. The programming interface has a
> > > > > +	 * legacy field which was an interrupt priority for AIC1, but
> > > > > +	 * which is OR'd onto bits 2-5 of the generated IRQ number when
> > > > > +	 * used with J-Core AIC2, so set it to match these bits.
> > > > > +	 */
> > > > > +	hwirq = irq_get_irq_data(pit_irq)->hwirq;
> > > > > +	irqprio = (hwirq >> 2) & PIT_PRIO_MASK;
> > > > > +	enable_val = (1U << PIT_ENABLE_SHIFT)
> > > > > +		   | (hwirq << PIT_IRQ_SHIFT)
> > > > > +		   | (irqprio << PIT_PRIO_SHIFT);
> > > > > +
> > > > 
> > > > Why mention AIC1 if there is not test to check if AIC1 || AIC2 ?
> > > > 
> > > > Will be the same information available if the irqchip is AIC1 ?
> > > 
> > > The bit layout of the PIT enable register is:
> > > 
> > > 	.....e..ppppiiiiiiii............
> > > 
> > > where the .'s indicate unrelated/unused bits, e is enable, p is
> > > priority, and i is hard irq number.
> > > 
> > > For the PIT included in AIC1 (obsolete but still in use), any hard irq
> > > (trap number) can be programmed via the 8 iiiiiiii bits, and a
> > > priority (0-15) is programmable separately in the pppp bits.
> > > 
> > > For the PIT included in AIC2 (current), the programming interface is
> > > equivalent modulo interrupt mapping. This is why a different
> > > compatible tag was not used. However only traps 64-127 (the ones
> > > actually intended to be used for interrupts, rather than
> > > syscalls/exceptions/etc.) can be programmed (the high 2 bits of i are
> > > ignored) and the priority pppp is <<2'd and or'd onto the irq number.
> > > This was a poor decision made on the hardware engineering side based
> > > on a wrong assumption that preserving old priority mapping of outdated
> > > software was important, whereas priorities weren't/aren't even being
> > > used.
> > > 
> > > When we do the next round of interrupt controller improvements (AIC3)
> > > the PIT programming interface should remain compatible with the
> > > driver; likely the priority bits will just be ignored.
> > > 
> > > If we do want to change the programming interface beyond this at some
> > > point (that maay be a good idea, since we have identified several
> > > things that are less than ideal for Linux, like the sechi/seclo/ns
> > > clocksource), a new compatible tag will be added instead.
> > 
> > Ok, thanks for the clarification. Can you add your answer as a comment for
> > the bits dance above ?
> 
> Are you happy with the whole quoted text above as a comment? If so I'm
> happy to include it verbatim. I would lean towards condensing or
> omitting the last 2 paragraphs (starting with "When we do...") if
> that's okay with you since they are not documenting the hw but future
> plans/policy.

Makes sense.

Agree for the verbatim minus the last 2 paragraphs.

  -- Daniel

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