Re: [PATCH v2 00/12] arm64: Kconfig: Update ARCH_EXYNOS select configs

[Tomasz Figa <tomasz.figa@xxxxxxxxx>]

2021年10月2日(土) 4:26 Saravana Kannan <saravanak@xxxxxxxxxx>:
>
> On Fri, Oct 1, 2021 at 8:27 AM Olof Johansson <olof@xxxxxxxxx> wrote:
> >
> > On Fri, Oct 1, 2021 at 2:01 AM Arnd Bergmann <arnd@xxxxxxxx> wrote:
> > >
> > > On Fri, Oct 1, 2021 at 10:19 AM Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> wrote:
> > > > On Fri, Oct 1, 2021 at 7:24 AM Saravana Kannan <saravanak@xxxxxxxxxx> wrote:
> > > > > GIC and arch timer. Basically the minimal kernel would need a timer
> > > > > for the scheduler tick and IRQ controller to get the timer IRQ and the
> > > > > fixed clock driver if the archtimer uses one to get its frequency and
> > > > > the early UART console is pointless as a module (so build it in to
> > > > > allow debugging/development).
> > > > >
> > > > > And then all new drivers, we should make sure are implemented as
> > > > > tristate drivers. And we can go back and slowly work on converting
> > > > > existing drivers to modules (community effort -- not one person or
> > > > > entity) -- at least the ones where the author has hardware or ones
> > > > > where the change is very likely to be correct and someone else is
> > > > > willing to test it. We'll never be able to support some/all ARM32 (do
> > > > > they even have a GIC/arch timer standard?), but at least for ARM64,
> > > > > this seems like a viable goal.
> > > >
> > > > Cortex-A7/A15 and later have GIC and architectured timer, so it should
> > > > work for contemporary systems.
> > > > Cortex-A9 systems may have GIC, and TWD and/or Global Timer (but I've
> > > > seen SoCs where the interrupt for the latter was not wired :-(.
> > >
> > > There are a number of well-known examples even with 64-bit chips or
> > > Cortex-A7/A15 based SoCs that can't use the architected timer,
> > > irqchip or iommu.
> > >
> > > Apple M1, Broadcom BCM283x, Samsung Exynos5 and
> > > some Hisilicon server parts come to mind, I'm sure there
> > > are more.
> >
> > There's also more and more movement towards having coprocessors with
> > standardized interfaces dealing with this functionality. We're
> > currently at the point where they have coprocessors with
> > non-standardized interfaces, and it's useful to keep encouraging
> > convergence in this area to everybody's benefit. I don't find it
> > particularly useful to make life easier for the custom solutions at
> > the expense of others like this patchset does, when that's (just
> > beyond? on?) the horizon.
> >
> > > > What are the plans for other architectures?
> > > > I've seen similar patches being applied for e.g. MIPS.
> > >
> > > There is some work in the more actively maintained MIPS
> > > platforms to make those behave more like Arm/powerpc/riscv/m68k
> > > platforms, using a single image and moving drivers into modules.
> > > Most MIPS platforms seem unlikely to get updated to this,
> > > and will continue to require a SoC specific kernel binary forever,
> > > similar to the renesas superh platforms. Most of the less
> > > common architectures (arc, csky, hexagon, nios2, xtensa,
> > > microblaze, nds32, openrisc, sparc/leon) are way behind that
> > > though, and generally don't work at all without out-of-tree
> > > code.
> >
> > One of the arguments for needing some of these core drivers in-kernel
> > is that some platforms boot at very conservative DVFS operating
> > points, to a degree that you really want to turn up the CPU clocks
> > fairly early during boot.
> >
> > If you don't have the drivers built-in, you can't do that and/or you
> > create possible fragile or awkward inter-module dependencies with
> > deferred probing, etc. We do care about boot time enough to prefer to
> > just build them in for this reason.
>
> Go look at a Pixel 5, we got this working just fine with all these
> drivers as modules and we definitely care about boot time. You just
> need to load your CPU freq driver and the other ones it needs early
> on. And with fw_devlink=on (default in upstream), there's hardly any
> deferred probing.
>
> > If vmlinux binary size is a concern, maybe it's time to consider
> > splitting the drivers into a bare-minimum piece that's not a module
> > for early setup, and the rest that can be loaded post-boot.
>
> Isn't this literally what I was suggesting with my
> ARM64_MINIMAL_GENERIC_KERNEL suggestion? Build in all the bare minimum
> drivers that are needed before module loading can happen? You'd just
> select them all under that config. And any existing platform that
> wants to use it would break up their drivers into modules and switch
> to it.

I think the point here is that pinctrl and clk are considered a part
of "bare minimum" for Exynos.