Re: [RFC PATCH 2/3] riscv: Add early_param to decrease firmware region

[Alexandre Ghiti]

On Tue, Nov 23, 2021 at 12:53 PM Jessica Clarke <jrtc27@xxxxxxxxxx> wrote:
>
> On 23 Nov 2021, at 03:44, Anup Patel <anup@xxxxxxxxxxxxxx> wrote:
> >
> > +Alex
> >
> > On Tue, Nov 23, 2021 at 7:27 AM <guoren@xxxxxxxxxx> wrote:
> >>
> >> From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> >>
> >> Using riscv.fw_size in cmdline to tell the kernel what the
> >> firmware (opensbi) size is. Then reserve the proper size of
> >> firmware to save memory instead of the whole 2MB. It's helpful
> >> to satisfy a small memory system (D1s/F133 from Allwinner).
> >>
> >> Signed-off-by: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> >> Cc: Palmer Dabbelt <palmer@xxxxxxxxxxx>
> >> Cc: Anup Patel <anup.patel@xxxxxxx>
> >> Cc: Atish Patra <atishp@xxxxxxxxxxxx>
> >> ---
> >> arch/riscv/mm/init.c | 14 +++++++++++++-
> >> 1 file changed, 13 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> >> index 920e78f8c3e4..f7db6d40213d 100644
> >> --- a/arch/riscv/mm/init.c
> >> +++ b/arch/riscv/mm/init.c
> >> @@ -159,6 +159,15 @@ static int __init early_mem(char *p)
> >> }
> >> early_param("mem", early_mem);
> >>
> >> +static phys_addr_t firmware_size __initdata;
> >> +static int __init early_get_firmware_size(char *arg)
> >> +{
> >> +       firmware_size = memparse(arg, &arg);
> >> +
> >> +       return 0;
> >> +}
> >> +early_param("riscv.fwsz", early_get_firmware_size);
> >> +
> >
> > We have avoided any RISC-V specific kernel parameter till now
> > and I don't think adding "riscv.fwsz" is the right approach.
> >
> > OpenSBI adds a reserved memory node (mmode_resv@8000000)
> > to mark the memory where it is running as reserved. In fact, all
> > M-mode runtime firmware should be adding a reserved memory
> > node just like OpenSBI.

Yes I agree that this should be in the device tree, IMO there is no
need to introduce a new kernel parameter.

>
> BBL does not do this and, even if it’s modified today, older versions
> will still need to be supported for quite a while longer.

It's fair to expect the firmware to advertise its existence: we
briefly discussed that last year with Atish [1] and he proposed to
introduce a document that describes what the kernel expects from the
'platform' when it boots, that would be a way to drop those old legacy
bootloaders.

[1] https://lkml.org/lkml/2020/6/3/696



>
> In FreeBSD[1] we only reserve the first 2 MiB of DRAM (we don’t care
> about RV32) if there is no reserved memory node covering the DRAM base
> address, which avoids this issue. The only downside with that approach
> is that if firmware occupies a different region than the beginning of
> DRAM (or there is no firmware resident in the supervisor’s physical
> address space, as is the case for a virtualised guest) then it
> unnecessarily reserves that first 2 MiB, but that’s not a huge deal,
> and can’t be avoided so long as BBL continues to exist (well, I guess
> you could probe the SBI implementation ID if you really cared about
> that, but I’ve yet to hear of a platform where the SBI implementation,
> if it exists, isn’t at the start of DRAM, and if you’re virtualising
> then you probably have enough DRAM that you don’t notice 2 MiB going
> missing).
>
> Jess
>
> [1] https://github.com/freebsd/freebsd-src/blob/main/sys/riscv/riscv/machdep.c#L554-L568
>