Re: [PATCH] riscv: add BUILTIN_DTB support for MMU-enabled targets

[Anup Patel]

On Thu, Dec 31, 2020 at 3:23 PM Vitaly Wool <vitaly.wool@xxxxxxxxxxxx> wrote:
>
> On Tue, Dec 29, 2020 at 6:05 AM Anup Patel <anup@xxxxxxxxxxxxxx> wrote:
> >
> > On Mon, Dec 28, 2020 at 10:08 PM Vitaly Wool <vitaly.wool@xxxxxxxxxxxx> wrote:
> > >
> > > On Mon, Dec 28, 2020 at 3:10 PM Anup Patel <anup@xxxxxxxxxxxxxx> wrote:
> > > >
> > > > On Mon, Dec 28, 2020 at 7:05 PM Vitaly Wool <vitaly.wool@xxxxxxxxxxxx> wrote:
> > > > >
> > > > > On Mon, Dec 28, 2020 at 12:59 PM Anup Patel <anup@xxxxxxxxxxxxxx> wrote:
> > > > > >
> > > > > > On Sat, Dec 26, 2020 at 10:03 PM Vitaly Wool <vitaly.wool@xxxxxxxxxxxx> wrote:
> > > > > > >
> > > > > > > Sometimes, especially in a production system we may not want to
> > > > > > > use a "smart bootloader" like u-boot to load kernel, ramdisk and
> > > > > > > device tree from a filesystem on eMMC, but rather load the kernel
> > > > > > > from a NAND partition and just run it as soon as we can, and in
> > > > > > > this case it is convenient to have device tree compiled into the
> > > > > > > kernel binary. Since this case is not limited to MMU-less systems,
> > > > > > > let's support it for these which have MMU enabled too.
> > > > > > >
> > > > > > > Signed-off-by: Vitaly Wool <vitaly.wool@xxxxxxxxxxxx>
> > > > > > > ---
> > > > > > >  arch/riscv/Kconfig   |  1 -
> > > > > > >  arch/riscv/mm/init.c | 12 ++++++++++--
> > > > > > >  2 files changed, 10 insertions(+), 3 deletions(-)
> > > > > > >
> > > > > > > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> > > > > > > index 2b41f6d8e458..9464b4e3a71a 100644
> > > > > > > --- a/arch/riscv/Kconfig
> > > > > > > +++ b/arch/riscv/Kconfig
> > > > > > > @@ -419,7 +419,6 @@ endmenu
> > > > > > >
> > > > > > >  config BUILTIN_DTB
> > > > > > >         def_bool n
> > > > > > > -       depends on RISCV_M_MODE
> > > > > > >         depends on OF
> > > > > > >
> > > > > > >  menu "Power management options"
> > > > > > > diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> > > > > > > index 87c305c566ac..5d1c7a3ec01c 100644
> > > > > > > --- a/arch/riscv/mm/init.c
> > > > > > > +++ b/arch/riscv/mm/init.c
> > > > > > > @@ -194,12 +194,20 @@ void __init setup_bootmem(void)
> > > > > > >         setup_initrd();
> > > > > > >  #endif /* CONFIG_BLK_DEV_INITRD */
> > > > > > >
> > > > > > > +       /*
> > > > > > > +        * If DTB is built in, no need to reserve its memblock.
> > > > > > > +        * OTOH, initial_boot_params has to be set to properly copy DTB
> > > > > > > +        * before unflattening later on.
> > > > > > > +        */
> > > > > > > +       if (IS_ENABLED(CONFIG_BUILTIN_DTB))
> > > > > > > +               initial_boot_params = __va(dtb_early_pa);
> > > > > >
> > > > > > Don't assign initial_boot_params directly here because the
> > > > > > early_init_dt_scan() will do it.
> > > > >
> > > > > early_init_dt_scan will set initial_boot_params to dtb_early_va from
> > > > > the early mapping which will be gone by the time
> > > > > unflatten_and_copy_device_tree() is called.
> > > >
> > > > That's why we are doing early_init_dt_verify() again for the MMU-enabled
> > > > case which already takes care of your concern.
> > >
> > > I might be out in the woods here but... Do you mean the call to
> > > early_init_dt_verify() in setup_arch() which is compiled out
> > > completely in the CONFIG_BUILTIN_DTB case?
> > > Or is there any other call that I'm overlooking?
> >
> > Sorry for the confusion, what I meant was that we are calling
> > early_init_dt_verify() from setup_arch() for the MMU-enabled
> > with built-in DTB disabled case to update "initial_boot_params"
> > after the boot CPU has switched from early_pg_dir to swapper_pg_dir.
> >
> > For MMU-enabled with built-in DTB case, if setup_vm() sets the
> > dtb_early_va and dtb_early_pa correctly then early_init_dt_scan()
> > called from setup_arch() will automatically set correct value for
> > "initial_boot_params".
>
> Oh I think I get it now. You are suggesting to skip the temporary
> mapping for DT altogether since it is anyway in the kernel mapping
> range, aren't you?
> That does make sense indeed, thanks :)

Yes, that's what I am suggesting.

>
> > It is strange that early_init_dt_verify() is being compiled-out for you
> > because the early_init_dt_scan() called from setup_arch() also uses
> > early_init_dt_verify(). I quickly compiled the NoMMU kernel for K210
> > which also uses built-in DTB and I see that early_init_dt_verify()
> > is not being compiled-out when built-in DTB is enabled.
>
> I did not say that early_init_dt_verify() is compiled out, it's the
> call to it in setup_arch() that is compiled out if BUILTIN_DTB is
> selected. However, if I understand you correctly now, it should not
> matter if we don't set dtb_early_va to point to the temporary mapping.

Yes, dtb_early_va can point to some other location such as
built-in DTB.

For MMU-enabled with built-in DTB case, the setup_vm() should
skip the temporary mapping to DT and instead set dtb_early_va
and dtb_early_pa to point to the built-in DTB.

Regards,
Anup