linux-kernel - Re: [PATCH v4] riscv: Use PUD/P4D/PGD pages for the linear mapping

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Date:   Fri, 27 Jan 2023 09:58:03 +0100
From:   Andrew Jones <ajones@...tanamicro.com>
To:     Alexandre Ghiti <alexghiti@...osinc.com>
Cc:     Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>, Guo Ren <guoren@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org, linux-arch@...r.kernel.org,
        Rob Herring <robh@...nel.org>
Subject: Re: [PATCH v4] riscv: Use PUD/P4D/PGD pages for the linear mapping

On Fri, Jan 27, 2023 at 09:45:21AM +0100, Alexandre Ghiti wrote:
...
> > > > > > diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c
> > > > > > index f08b25195ae7..58107bd56f8f 100644
> > > > > > --- a/drivers/of/fdt.c
> > > > > > +++ b/drivers/of/fdt.c
> > > > > > @@ -891,12 +891,13 @@ const void * __init of_flat_dt_match_machine(const void *default_match,
> > > > > >  static void __early_init_dt_declare_initrd(unsigned long start,
> > > > > >                                        unsigned long end)
> > > > > >  {
> > > > > > -   /* ARM64 would cause a BUG to occur here when CONFIG_DEBUG_VM is
> > > > > > -    * enabled since __va() is called too early. ARM64 does make use
> > > > > > -    * of phys_initrd_start/phys_initrd_size so we can skip this
> > > > > > -    * conversion.
> > > > > > +   /*
> > > > > > +    * __va() is not yet available this early on some platforms. In that
> > > > > > +    * case, the platform uses phys_initrd_start/phys_initrd_size instead
> > > > > > +    * and does the VA conversion itself.
> > > > > >      */
> > > > > > -   if (!IS_ENABLED(CONFIG_ARM64)) {
> > > > > > +   if (!IS_ENABLED(CONFIG_ARM64) &&
> > > > > > +       !(IS_ENABLED(CONFIG_RISCV) && IS_ENABLED(CONFIG_64BIT))) {
> > > > >
> > > > > There are now two architectures, so maybe it's time for a new config
> > > > > symbol which would be selected by arm64 and riscv64 and then used here,
> > > > > e.g.
> > > > >
> > > > >   if (!IS_ENABLED(CONFIG_NO_EARLY_LINEAR_MAP)) {
> > > >
> > > > I see v5 left this as it was. Any comment on this suggestion?
> > >
> > > Introducing a config for this only use case sounds excessive to me,
> > > but I'll let Rob decide what he wants to see here.
> >
> > To me, the suggestion is less about trying to tidy up DT code and more
> > about bringing this comment about arm64 and riscv64 not being able to
> > use the linear map as early as other architectures up out of the
> > depths of DT code. Seeing an architecture select something like
> > NO_EARLY_LINEAR_MAP, which has a paragraph explaining what that
> > means, may help avoid other early uses of __va() which may or may
> > not fail quickly and cleanly with a BUG.
> >
> 
> You're right, do you have some bandwidth for doing that?
>

Sure, I'll post something today.

Thanks,
drew