Re: [PATCH 04/10] riscv: Add BUILTIN_DTB support

[Palmer Dabbelt <palmer@dabbelt.com>]

On Wed, 12 Feb 2020 02:34:26 PST (-0800), Damien Le Moal wrote:
> Enable a kernel builtin dtb for boards not capable of providing a
> device tree to the kernel.

I'd prefer if we picked a mechanism that allows a single kernel binary to be
run on multiple systems.  I think there's two use cases here:

* Bootloaders that provide no DTB at all.
* Bootloaders that provied a DTB that, for some reason, isn't usable.

Given those constraints, could we do something similar to the early fixups?
I'm thinking we could build a mapping between a hardware identifier and a DTB,
then look up the DTB we want to use.  Users that want a kernel that only runs
on a single device can do so by configuring only a single DTB, users that want
a more portable kernel can select a bunch -- that's essentially the same as how
we're treating everything else (for example, the CONFIG_SOC_* stuff).

For the hardware ID, could we do something like:

* Check for the top-level DT compatible string, on systems where we have a
  provided DTB.
* Check for a matching mimpid/marchid/mvendorid tuple, maybe with some sort of
  masking functionality if we later need one.  These are availiable via SBI
  calls, but I'd be inclined to restrict them to M-mode boot and just say the
  SBI must provide a device tree with at least a suitable compatible string.

While I suppose we could put together a tool for generating these tables, for
now we could probably just stick the mappings in a table for now given that
there's only one of them.

That said, I'm not sure what to do about the different Kendryte boards -- is
there any way to poke the hardware to see which is which?

> Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
> ---
>  arch/riscv/Kbuild            |  1 +
>  arch/riscv/Kconfig           | 18 ++++++++++++++++++
>  arch/riscv/boot/dts/Makefile |  4 ++++
>  arch/riscv/kernel/setup.c    |  6 ++++++
>  arch/riscv/mm/init.c         |  4 ++++
>  5 files changed, 33 insertions(+)
>
> diff --git a/arch/riscv/Kbuild b/arch/riscv/Kbuild
> index d1d0aa70fdf1..988804e430e4 100644
> --- a/arch/riscv/Kbuild
> +++ b/arch/riscv/Kbuild
> @@ -1,3 +1,4 @@
>  # SPDX-License-Identifier: GPL-2.0-only
>
>  obj-y += kernel/ mm/ net/
> +obj-$(CONFIG_USE_BUILTIN_DTB)	+= boot/dts/
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 1a3b5a5276be..28899e15f548 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -355,6 +355,24 @@ config CMDLINE_FORCE
>
>  endchoice
>
> +config USE_BUILTIN_DTB
> +	bool "Use builtin DTB"
> +	help
> +	  Link a device tree blob for particular hardware into the kernel,
> +	  suppressing use of the DTB pointer provided by the bootloader.
> +	  This option should only be used with hardware or bootloaders that
> +	  are not capable of providing a DTB to the kernel, or for
> +	  experimental hardware without stable device tree bindings.
> +
> +config BUILTIN_DTB_SOURCE
> +	string "Source file for builtin DTB"
> +	default ""
> +	depends on USE_BUILTIN_DTB
> +	help
> +	  Base name (without suffix, relative to arch/riscv/boot/dts) for
> +	  the a DTS file that will be used to produce the DTB linked into
> +	  the kernel.
> +
>  endmenu
>
>  menu "Power management options"
> diff --git a/arch/riscv/boot/dts/Makefile b/arch/riscv/boot/dts/Makefile
> index dcc3ada78455..0bf2669aa12d 100644
> --- a/arch/riscv/boot/dts/Makefile
> +++ b/arch/riscv/boot/dts/Makefile
> @@ -1,2 +1,6 @@
>  # SPDX-License-Identifier: GPL-2.0
> +ifneq ($(CONFIG_BUILTIN_DTB_SOURCE),"")
> +obj-$(CONFIG_USE_BUILTIN_DTB) += $(patsubst "%",%,$(CONFIG_BUILTIN_DTB_SOURCE)).dtb.o
> +else
>  subdir-y += sifive
> +endif
> diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
> index 0a6d415b0a5a..3e89be9d888c 100644
> --- a/arch/riscv/kernel/setup.c
> +++ b/arch/riscv/kernel/setup.c
> @@ -68,7 +68,13 @@ void __init setup_arch(char **cmdline_p)
>
>  	setup_bootmem();
>  	paging_init();
> +
> +#if IS_ENABLED(CONFIG_USE_BUILTIN_DTB)
> +	unflatten_and_copy_device_tree();
> +#else
>  	unflatten_device_tree();
> +#endif
> +
>  	clint_init_boot_cpu();
>
>  #ifdef CONFIG_SWIOTLB
> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> index 965a8cf4829c..1274e889d008 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -480,7 +480,11 @@ static void __init setup_vm_final(void)
>  #else
>  asmlinkage void __init setup_vm(uintptr_t dtb_pa)
>  {
> +#if IS_ENABLED(CONFIG_USE_BUILTIN_DTB)
> +	dtb_early_va = __dtb_start;
> +#else
>  	dtb_early_va = (void *)dtb_pa;
> +#endif
>  }
>
>  static inline void setup_vm_final(void)