Re: [PATCH V2] MIPS: Loongson: Add board_ebase_setup() support

[Huacai Chen <chenhc@lemote.com>]

Hi, Paul

On Sat, Nov 9, 2019 at 3:11 AM Paul Burton <paulburton@kernel.org> wrote:
>
> Hi Huacai,
>
> On Mon, Nov 04, 2019 at 02:09:41PM +0800, Huacai Chen wrote:
> > Old processors before Loongson-3A2000 have a 32bit ebase register and
> > have no WG bit, new processors since Loongson-3A2000 have a 64bit ebase
> > register and do have the WG bit. Unfortunately, Loongson processors
> > which have the WG bit are slightly different from MIPS R2. This makes
> > the generic ebase setup not suitable for every scenarios.
>
> Can you describe how the Loongson WG bit differs from the WG bit as
> architecturally defined? This patch may make things work for you but it
> doesn't give us any record of what the hardware errata is or what we
> actually need to do to work with the broken WG bit.
>
> For example - right now Loongson kernels don't use the WG bit at all, so
> what's the problem? It doesn't matter if WG has different behavior if we
> don't use it.
>
> So one option here might be to just continue to not indicate support for
> the WG bit. It does look like the kernel ought to be ensuring the
> exception vector it allocates is at a suitable address in this case, and
> currently isn't. Something like the (untested) patch below ought to
> address that. In practice though memblock is configured to allocate
> bottom-up until after this point so it should be unlikely we'll get an
> unsuitable address, and there's a WARN_ON() in trap_init() that would
> already tell you if that happened.
>
> > To make Loongson's kernel be more robust, we add a board_ebase_setup()
> > hook to ensure that CKSEG0 is always used for ebase. This is also useful
> > on platforms where BIOS doesn't initialise an appropriate ebase.
>
> Can you also elaborate on that? I'm not sure why this would help on
> systems that don't initialize EBase - the kernel unconditionally sets
> EBase for >= MIPSr2 systems in configure_exception_vector() anyway, and
> since v5.2 it doesn't even try to inherit whatever the bootloader used.
>
I'm sorry that I haven't seen the changes in 5.2. My original problem
has been fixed with the below two commits.
172dcd935c34b022729f45a7bbaae5cc052 ("MIPS: Always allocate exception
vector for MIPSr2+")
de56d4c1da3e68f0ca468a55f6677bef3cee ("MIPS: Remove duplicate EBase
configuration")

My own patch is only needed in 4.9/4.14/4.19, and I can backport the
above two commits while completely drop my own patch.

Thanks,
Huacai

> Thanks,
>     Paul
>
> ---
> diff --git a/arch/mips/kernel/traps.c b/arch/mips/kernel/traps.c
> index 342e41de9d64..a25ee41eff48 100644
> --- a/arch/mips/kernel/traps.c
> +++ b/arch/mips/kernel/traps.c
> @@ -2271,7 +2271,7 @@ void __init trap_init(void)
>         extern char except_vec4;
>         extern char except_vec3_r4000;
>         unsigned long i, vec_size;
> -       phys_addr_t ebase_pa;
> +       phys_addr_t ebase_pa, ebase_limit;
>
>         check_wait();
>
> @@ -2287,7 +2287,21 @@ void __init trap_init(void)
>                 else
>                         vec_size = PAGE_SIZE;
>
> -               ebase_pa = memblock_phys_alloc(vec_size, 1 << fls(vec_size));
> +               /*
> +                * If we have support for the EBase.WG bit then allow the
> +                * exception vector to be located anywhere. When EBase.WG is
> +                * not supported EBase is limited to a (c)kseg[01] address, so
> +                * we must ensure the allocated vector is in memory accessible
> +                * via those unmapped regions.
> +                */
> +               if (cpu_has_ebase_wg)
> +                       ebase_limit = MEMBLOCK_ALLOC_ACCESSIBLE;
> +               else
> +                       ebase_limit = CKSEG1 - CKSEG0;
> +
> +               ebase_pa = memblock_phys_alloc_range(vec_size,
> +                                                    1 << fls(vec_size),
> +                                                    0, ebase_limit);
>                 if (!ebase_pa)
>                         panic("%s: Failed to allocate %lu bytes align=0x%x\n",
>                               __func__, vec_size, 1 << fls(vec_size));