[PATCH RFC 0/3] riscv: Add DMA_COHERENT support

[Guo Ren]

On Sat, Jun 5, 2021 at 12:12 AM Palmer Dabbelt <palmer at dabbelt.com> wrote:
>
> On Fri, 04 Jun 2021 07:47:22 PDT (-0700), guoren at kernel.org wrote:
> > Hi Arnd & Palmer,
> >
> > Sorry for the delayed reply, I'm working on the next version of the patch.
> >
> > On Fri, Jun 4, 2021 at 5:56 PM Arnd Bergmann <arnd at arndb.de> wrote:
> >>
> >> On Thu, Jun 3, 2021 at 5:39 PM Palmer Dabbelt <palmer at dabbelt.com> wrote:
> >> > On Wed, 02 Jun 2021 23:00:29 PDT (-0700), Anup Patel wrote:
> >> > >> This implementation, which adds some Kconfig entries that control page table
> >> > >> bits, definately isn't suitable for upstream.  Allowing users to set arbitrary
> >> > >> page table bits will eventually conflict with the standard, and is just going to
> >> > >> be a mess.  It'll also lead to kernels that are only compatible with specific
> >> > >> designs, which we're trying very hard to avoid.  At a bare minimum we'll need
> >> > >> some way to detect systems with these page table bits before setting them,
> >> > >> and some description of what the bits actually do so we can reason about
> >> > >> them.
> >> > >
> >> > > Yes, vendor specific Kconfig options are strict NO NO. We can't give-up the
> >> > > goal of unified kernel image for all platforms.
> > Okay,  Agree. Please help review the next version of the patch.
> >
> >> >
> >> > I think this is just a phrasing issue, but just to be sure:
> >> >
> >> > IMO it's not that they're vendor-specific Kconfig options, it's that
> >> > turning them on will conflict with standard systems (and other vendors).
> >> > We've already got the ability to select sets of Kconfig settings that
> >> > will only boot on one vendor's system, which is fine, as long as there
> >> > remains a set of Kconfig settings that will boot on all systems.
> >> >
> >> > An example here would be the errata: every system has errata of some
> >> > sort, so if we start flipping off various vendor's errata Kconfigs
> >> > you'll end up with kernels that only function properly on some systems.
> >> > That's fine with me, as long as it's possible to turn on all vendor's
> >> > errata Kconfigs at the same time and the resulting kernel functions
> >> > correctly on all systems.
> >>
> >> Yes, this is generally the goal, and it would be great to have that
> >> working in a way where a 'defconfig' build just turns on all the options
> >> that are needed to use any SoC specific features and drivers while
> >> still working on all hardware. There are however limits you may run
> >> into at some point, and other architectures usually only manage to span
> >> some 10 to 15 years of hardware implementations with a single
> >> kernel before it get really hard.
> > I could follow the goal in the next version of the patchset. Please
> > help review, thx.
>
> IMO we're essentially here now with the RISC-V stuff: defconfig flips on
> everything necesasry to boot normal-smelling SOCs, with everything being
> detected as the system boots.  We have some wacky configurations like
> !MMU and XIP that are coupled to the hardware, but (and sorry for
> crossing the other threads, I missed your pointer as it's early here) as
> I said in the other thread it might be time to make it explicit that
> those things are non-portable.
>
> The hope here has always been that we'd have enough in the standards
> that we could avoid a proliferation of vendor-specific code.  We've
> always put a strong "things keep working forever" stake in the ground in
> RISC-V land, but that's largely been because we were counting on the
> standards existing that make support easy.  In practice we don't have
> those standards so we're ending up with a fairly large software base
> that is required to support everything.  We don't have all that much
> hardware right now so we'll have to see how it goes, but for now I'm in
> favor of keeping defconfig as a "boots on everything" sort of setup --
> both because it makes life easier for users, and because it makes issues
> like the non-portable Kconfigs that showed up here quite explicit.
I reuse the Image header to pass vendor magic to init PTE attributes'
variable before setup_vm. Can you give me some feedback on that patch?
https://lore.kernel.org/linux-riscv/1622970249-50770-9-git-send-email-guoren@kernel.org/T/#mdc0dacba57346b5ac59a01961495c132b93cfcdb

>
> If we get to 10/15 years of hardware then I'm sure we'll be removing old
> systems from defconfig (or maybe even the kernel entirely, a lot of this
> stuff isn't in production).  I'm just hoping we make it that far ;)
>
> >> To give some common examples that make it break down:
> >>
> >> - 32-bit vs 64-bit already violates that rule on risc-v (as it does on
> >>   most other architectures)
>
> Yes, and there's no way around that on RISC-V.  They're different base
> ISAs therefor re-define the same instructions, so we're essentially at
> two kernel binaries by that point.  The platform spec says rv64gc, so we
> can kind of punt on this one for now.  If rv32 hardware shows up
> we'll probably want a standard system there too, which is why we've
> avoided coupling kernel portability to XLEN.
>
> >> - architectures that support both big-endian and little-endian kernels
> >>   tend to have platforms that require one or the other (e.g. mips,
> >>   though not arm). Not an issue for you.
>
> It is now!  We've added big-endian to RISC-V.  There's no hardware yet
> and very little software support.  IMO the right answer is to ban that
> from the platform spec, but again it'll depnd on what vendors want to
> build (though anyone is listening, please don't make my life miserable
> ;)).
>
> >> - page table formats are the main cause of incompatibility: arm32
> >>   and x86-32 require three-level tables for certain features, but those
> >>   are incompatible with older cores, arm64 supports three different
> >>   page sizes, but none of them works on all cores (4KB almost works
> >>   everywhere).
>
> We actually have some support on the works for multiple page table
> levels in a single binary, which should help with a lot of that
> incompatibility.  I don't know of any plans to couple other page table
> features to the number of levels, though.
>
> >> - SMP-enabled ARMv7 kernels can be configured to run on either
> >>   ARMv6 or ARMv8, but not both, in this case because of incompatible
> >>   barrier instructions.
>
> Our barriers aren't quite split the same way, but we do have two memory
> models (RVWMO and TSO).  IIUC we should be able to support both in the
> same kernels with some patching, but the resulting kernels would be
> biased towards one memory models over the other WRT performance.  Again,
> we'll have to see what the vendors do and I'm hoping we don't end up
> with too many headaches.
>
> >> - 32-bit Arm has a couple more remaining features that require building
> >>   a machine specific kernel if enabled because they hardcode physical
> >>   addresses: early printk (debug_ll, not the normal earlycon), NOMMU,
> >>   and XIP.
>
> We've got NOMMU and XIP as well, but we have some SBI support for early
> printk.  IMO we're not really sure if we've decoupled all the PA layout
> dependencies yet from Linux, as we really only support one vendor's
> systems, but we've had a lot of work lately on beefing up our memory
> layout so with any luck we'll be able to quickly sort out anything that
> comes up.



-- 
Best Regards
 Guo Ren

ML: https://lore.kernel.org/linux-csky/