[PATCH 1/2] Documentation: riscv: Add early boot document

[Conor Dooley]

On Mon, Jun 19, 2023 at 04:04:52PM +0200, Alexandre Ghiti wrote:
> On Mon, Jun 19, 2023 at 2:26 PM Conor Dooley <conor.dooley at microchip.com> wrote:
> > On Mon, Jun 19, 2023 at 11:47:04AM +0200, Alexandre Ghiti wrote:

> > > diff --git a/Documentation/riscv/boot.rst b/Documentation/riscv/boot.rst
> > > new file mode 100644
> > > index 000000000000..b02230818b79
> > > --- /dev/null
> > > +++ b/Documentation/riscv/boot.rst
> > > @@ -0,0 +1,181 @@
> > > +.. SPDX-License-Identifier: GPL-2.0
> > > +
> > > +=============================================
> > > +Early boot requirements/constraints on RISC-V
> > > +=============================================
> >
> > Please use "title case", here and elsewhere in the doc.
> 
> You mean using "title: " instead of "===="? Or using uppercase for the
> first letter of each word? FYI I followed
> https://docs.kernel.org/doc-guide/sphinx.html?highlight=title#specific-guidelines-for-the-kernel-documentation

The latter. That's weird, I guess it would be nice to see what Jon
thinks about that.

> > > +Reserved memory for resident firmware
> > > +-------------------------------------
> > > +
> > > +The RISC-V kernel expects the firmware to mark any resident memory with the
> >
> > Should this be
> > "...resident memory, or memory it has protected with PMPs, with..."
> > ?
> 
> I used "resident" memory instead of "PMP" memory because it was more
> general. I mean you can have a region that is resident but not
> protected by PMP, and I don't think the kernel should ask for this
> resident memory to be protected with PMP right?

Nah, I was thinking about the opposite. PMP protected regions that do
not have memory-resident programs in them.

> > > +`no-map` flag, thus the kernel won't map those regions in the direct mapping
> >
> > "no-map" is a DT specific term, should this section be moved down under
> > DT, as a sub-section of that?
> 
> Maybe I can rephrase with something like that:
> 
> "The RISC-V kernel must not map any resident memory in the direct
> mapping, so the firmware must correctly mark those regions as follows:
> - when using a devicetree, using the `no-map` flag,
> - when booting with UEFI without devicetree, either as
> `EfiRuntimeServicesData/Code` or `EfiReserved`."

I'm not sure you need to have a list to be honest. Could do it in
free-form text if you like. But you've got 3 options there for Efi
stuff, isn't only one of them a valid equivalent for "no-map"?
> 
> > > +(avoiding issues with hibernation, speculative accesses and probably other
> > > +subsystems).
> >
> > I'm not sure that this () section is beneficial. To be honest, recent
> > issues aside, this section here seems like a statement of the obvious...
> 
> But I made the mistake, so it was not that straightforward to
> me...I'll remove the ().

I know, hence the "recent issues aside" ;)

> > > +that's not the case.
> > > +
> > > +Device-tree
> 
> Damn, missed this one, thanks!
> 
> >
> > s/Device-tree/Devicetree/ and...
> >
> > > +-----------
> > > +
> > > +The RISC-V kernel always expects a device tree, it is:
> >
> > ...s/device tree/devicetree/ to match elsewhere in the kernel docs.
> > Same applies to the other instances of "device tree" in this patch,
> > please.
> 
> Ok I'll do that (but I'm happy to say that I thought about that and it
> was intentional since "git grep "device tree" | wc -l" returns a
> significant number of instances :)).

Yeah, I had the same thoughts recently too. It's completely mixed
unfortunately, but I suppose I was going off of the headings in the DT
docs that are in rst form. It's not a big deal obviously.

> > > +- either passed directly to the kernel from the previous stage using the `$a1`
> > > +  register,
> > > +- or when booting with UEFI, the device tree will be retrieved by the EFI stub
> > > +  using the EFI configuration table or it will be created.
> >
> > Can I suggest changing this around a little, pulling the "either" &
> > dropping some boilerplate so that it reads (to me!) a little more
> > naturally:
> >         The RISC-V kernel always expects a devicetree, it is either:
> >
> >         - passed directly to the kernel from the previous stage using the `$a1`
> >           register,
> >         - retrieved by the EFI stub when booting with UEFI, using the EFI
> >           configuration table or it will be created by ____.
> >
> > Also, please elaborate on what it will be created by.
> 
> Is it better this way:
> 
> "The RISC-V kernel always expects a devicetree, it is either:
> 
> - passed directly to the kernel from the previous stage using the
> `$a1`
>   register,
> - retrieved by the EFI stub if present in the EFI configuration table,
> - created by the EFI stub otherwise."

Nah, I think the 2 bullet structure was better. This 3 bullet mode
implies that if not passed in a1, then the EFI stub will create it.
Which is obviously not true
> 
> >
> > > +
> > > +Bootflow
> >
> > "Boot Flow", no?
> > I am not sure that this is the "correct" heading for the content it
> > describes, but I have nothing better to offer :/
> 
> Yes you're right, what about simply "Kernel Entrance"? Not sure this
> is easily understandable though.

"Non-boot Hart Initialisation"?

> > > +--------
> > > +
> > > +There exist 2 methods to enter the kernel:
> > > +
> > > +- `RISCV_BOOT_SPINWAIT`: the firmware releases all harts in the kernel, one hart
> > > +  wins a lottery and executes the early boot code while the other harts are
> > > +  parked waiting for the initialization to finish. This method is now
> >
> > nit: s/now//
> 
> Ok
> 
> >
> > What do you mean by deprecated? There's no requirement to implement the
> > HSM extension, right?
> 
> I would say yes, you have to use a recent version of openSBI that
> supports the HSM extension. @Atish Kumar Patra WDYT?

Uh, you don't need to use OpenSBI in the first place, let alone use a
recent version of it, right? What am I missing?
Also, what about !SMP systems? Although my suggested new section title
kinda addresses that!

> > > +  **deprecated**.
> > > +- Ordered booting: the firmware releases only one hart that will execute the
> > > +  initialization phase and then will start all other harts using the SBI HSM
> > > +  extension.
> > > +
> > > +UEFI
> > > +----
> > > +
> > > +UEFI memory map
> > > +~~~~~~~~~~~~~~~
> > > +
> > > +When booting with UEFI, the RISC-V kernel will use only the EFI memory map to
> > > +populate the system memory.
> > > +
> > > +The UEFI firmware must parse the subnodes of the `/reserved-memory` device tree
> > > +node and abide by the device tree specification to convert the attributes of
> > > +those subnodes (`no-map` and `reusable`) into their correct EFI equivalent
> > > +(refer to section "3.5.4 /reserved-memory and UEFI" of the device tree
> > > +specification).
> > > +
> > > +RISCV_EFI_BOOT_PROTOCOL
> > > +~~~~~~~~~~~~~~~~~~~~~~~
> > > +
> > > +When booting with UEFI, the EFI stub requires the boot hartid in order to pass
> > > +it to the RISC-V kernel in `$a1`. The EFI stub retrieves the boot hartid using
> > > +one of the following methods:
> > > +
> > > +- `RISCV_EFI_BOOT_PROTOCOL` (**preferred**).
> > > +- `boot-hartid` device tree subnode (**deprecated**).
> > > +
> > > +Any new firmware must implement `RISCV_EFI_BOOT_PROTOCOL` as the device tree
> > > +based approach is deprecated now.
> > > +
> > > +During kernel boot: (Kernel internals)
> >
> > With the other section titles changed, this could be:
> >         Early Boot Requirements and Constraints
> >         =======================================
> >
> >         The RISC-V kernel's early boot process operates under the
> >         following constraints:
> >
> > Thoughts?
> 
> I think it's better as you propose, I changed it, thanks.
> 
> >
> > > +======================================
> > > +
> > > +EFI stub and device tree
> >
> > Same comments about "device tree" here etc.
> >
> > > +------------------------
> > > +
> > > +When booting with UEFI, the device tree is supplemented by the EFI stub with the
> > > +following parameters (largely shared with arm64 in Documentation/arm/uefi.rst):
> > > +
> > > +==========================  ======   ===========================================
> > > +Name                        Size     Description
> > > +==========================  ======   ===========================================
> > > +linux,uefi-system-table     64-bit   Physical address of the UEFI System Table.
> >
> > nit: Hmm, I think for all of these sizes s/-bit/ bits/.
> 
> That's a copy-paste from the link just above the table.
> 
> But maybe I should have pointed to the doc and added only the
> "bootargs" stuff (maybe that's also present for arm64 actually).

If it is identical, sounds like a good idea. It's common code that does
that stuff, right?

> > > +---------------------
> > > +
> > > +The installation of the virtual mapping is done in 2 steps in the RISC-V kernel:
> > > +
> > > +1. :c:func:`setup_vm` installs a temporary kernel mapping in
> > > +   :c:var:`early_pg_dir` which allows to discover the system memory: only the
> >
> > s/to discover/discovery of/
> 
> You mean "the discovery of" right?

No? The "the" there would not be required.

> > > +For :c:func:`virt_to_phys` and :c:func:`phys_to_virt` to be able to correctly
> > > +convert direct mapping addresses to physical addresses, it needs to know the
> >
> > nit: s/it/they/
> 
> Ok
> 
> >
> > > +start of the DRAM: this happens after 1, right before 2 installs the direct
> >
> > s/:/./
> 
> Ahah, you really don't like long sentences :)

I dunno if it is long sentences per se, I just think it is easier to
follow.

> But of course ok :)
> 
> > Also how about s/1/step 1/ & s/2/step 2/?
> 
> Way better, thanks

> > > +Pre-MMU execution
> > > +-----------------
> > > +
> > > +Any code that executes before even the first virtual mapping is established
> > > +must be very carefully compiled as:
> >
> > Could you point out what the non-obvious examples of this code are?
> 
> I can do a list, yes

Not even a list, just something like "...established, for example early
alternatives and foo, must be very..."

> Thanks for your thorough review!

NW chief.

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