Re: [PATCH v1 3/3] arm64: dts: add Hi6220 mailbox node

From: Mark Rutland
Date: Mon Aug 24 2015 - 08:48:15 EST

> > > > I don't see why you need reserved-memory here, given you're not referring to
> > > > these regions by phandle anyway.
> > >
> > > - Now we have enabled EFI_STUB, so the memory node will be removed in
> > > kernel:
> > > efi_entry()
> > > \-> allocate_new_fdt_and_exit_boot()
> > > \-> update_fdt();
> > >
> > > Finally in kernel it cannot use memory node to carve out reseved
> > > memory regions.
> > >
> > > - On the other hand, DTS's the memory node is to "describes the
> > > physical memory layout for the system"; so it's better to use it only
> > > to describe the hardware info for memory. We can use reserved-memory
> > > to help manage the memory regions which are reserved from software
> > > perspective.
> >
> > The fact that you have no-map means that the memory should not be
> > described to the kernel as mappable in the first place. It's wrong to
> > place such memory in the memory node, even if listed in reserved-memory.
> >
> > If your EFI memory map describes the memory as mappable, it is wrong.
> When kernel is working, kernel will create its own page table based on
> UEFI memory map. Since it's reserved in DTS file as Leo's patch, it'll
> be moved to reserved memblock. Why is it wrong?

That is a _Linux_ detail, not a _UEFI_ detail.

Anything which only handles UEFI and knows nothing of reserved-memory
(e.g. GRUB) will be broken and/or break the Linux use of the region, as
it will happily try to allocate memory in the region (and could even
decide to reserve it permanently for its own usage).

If the memory is truly specific to the mailbox, then UEFI needs to know
that it is reserved as such. If it is not, then it need not be described
in the DT and can be allocated dynamically by the kernel.

> In the second, UEFI is firmware. When it's stable, nobody should change
> it without any reason. These reserved memory are used in mailbox driver.
> Look. It's driver, so it could be changed at any time. Why do you want
> to UEFI knowing this memory range? Do you hope UEFI to change when
> mailbox driver is changed?

It shouldn't need to change if that memory is truly reserved for the
sole use of the mailbox. If that's not the case then we have a different

If the memory range to use can be allocated by the driver, then I don't
see why it should be described in reserved-memory. It certainly should
not require a no-map attribute.

Additionally, the driver needs to ensure that the requisite cache
maintenance takes place prior to the use of the memory region given
prior agents may have ampped it as cacheable, leaving stale (perhaps
dirty) lines in the caches.

> > > According to upper info, we still need to use reserved-memory node to
> > > depict the reserved memory regions. i have no knowledge about EFI_STUB,
> > > so please confirm or correct as needed.
> >
> > If the memory shouldn't be mapped, it should neither be in the memory
> > node nor EFI memory map (with attributes allowing it to be mapped) to
> > begin with.
> As I said above, kernel will create its own page table. When kernel's
> page table is working, UEFI's page table is destroying. So the memory
> won't be mapped twice at the same time. What's wrong?
> >
> > As far as I can see you do not need to use reserved-memory.
> 1. Are we talking on the same thing? Leo already mentioned that all
> memory node in DTB will be destroyed by kernel when EFI_STUB is enabled
> on arm. Did you read the source code after his reply?
> And you suggested that Leo to use discrete memory region in DTB. It is
> really wrong. Kernel only gets memory map information from UEFI, not
> DTB.

I did _not_ suggest that Leo use discrete memory. I suggested that
reserved regions should not be described in the memory node (the same
premise applying to the UEFI memory map).

w.r.t. UEFI, please see my comments above. If you're using the UEFI
memory map, you have to use the UEFI memory map, not the UEFI memory map
with additional (non-UEFI) caveats applied atop.

> 2. The working flow is in below.
> a. Kernel gets memory map information from UEFI.
> b. Kernel loads the memory reserved information from DTB.

This relies on Linux, and ignores other UEFI clients.

> 3. Do you mean the reserved-memory is totally wrong? If it's wrong,
> please submit patches to remove all reserved-memory in linux kernel
> first.

I did not say that.

I said that describing some memory in a memory node, then also
describing that in reserved-memory with a no-map property was wrong. If
it's never meant to be mapped then there's no reason for it to be in the
memory node.

> 4. Again and again. Memory node should be only used to describe the
> RAM information.

The memory node describes the memory available to the OS. There are some
caveats w.r.t. /memreserve/, regions which may be mapped but remain
unused and so on, but the memory node does generally encode a policy
that the memory may be used.

Describing unusable memory in a memory node is pointless, and has an
adverse effect on clients which don't support reserved-memory. It's
doubly harmful when that memory should never be mapped.

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