Re: [v4 PATCH 8/8] RISC-V: Assign hwcap as per comman capabilities.

[Atish Patra]

On 2/13/19 4:38 PM, Palmer Dabbelt wrote:
> On Wed, 13 Feb 2019 00:44:42 PST (-0800), johan@xxxxxxxxxx wrote:
> > On Tue, Feb 12, 2019 at 11:58:10AM -0800, Atish Patra wrote:
> > > On 2/12/19 3:25 AM, Johan Hovold wrote:
> > > > On Tue, Feb 12, 2019 at 03:10:12AM -0800, Atish Patra wrote:
> > > > > Currently, we set hwcap based on first valid hart from DT. This may not
> > > > > be correct always as that hart might not be current booting cpu or may
> > > > > have a different capability.
> > > > >
> > > > > Set hwcap as the capabilities supported by all possible harts with "okay"
> > > > > status.
> > > > >
> > > > > Signed-off-by: Atish Patra <atish.patra@xxxxxxx>
> > > > > ---
> > > > >    arch/riscv/kernel/cpufeature.c | 41 ++++++++++++++++++++++-------------------
> > > > >    1 file changed, 22 insertions(+), 19 deletions(-)
> > > > >
> > > > > diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
> > > > > index e7a4701f..a1e4fb34 100644
> > > > > --- a/arch/riscv/kernel/cpufeature.c
> > > > > +++ b/arch/riscv/kernel/cpufeature.c
> > > > > @@ -20,6 +20,7 @@
> > > > >    #include <linux/of.h>
> > > > >    #include <asm/processor.h>
> > > > >    #include <asm/hwcap.h>
> > > > > +#include <asm/smp.h>
> > > > >
> > > > >    unsigned long elf_hwcap __read_mostly;
> > > > >    #ifdef CONFIG_FPU
> > > > > @@ -42,28 +43,30 @@ void riscv_fill_hwcap(void)
> > > > >
> > > > >    	elf_hwcap = 0;
> > > > >
> > > > > -	/*
> > > > > -	 * We don't support running Linux on hertergenous ISA systems.  For
> > > > > -	 * now, we just check the ISA of the first "okay" processor.
> > > > > -	 */
> > > > >    	for_each_of_cpu_node(node) {
> > > > > -		if (riscv_of_processor_hartid(node) >= 0)
> > > > > -			break;
> > > > > -	}
> > > > > -	if (!node) {
> > > > > -		pr_warn("Unable to find \"cpu\" devicetree entry\n");
> > > > > -		return;
> > > > > -	}
> > > > > +		unsigned long this_hwcap = 0;
> > > > >
> > > > > -	if (of_property_read_string(node, "riscv,isa", &isa)) {
> > > > > -		pr_warn("Unable to find \"riscv,isa\" devicetree entry\n");
> > > > > -		of_node_put(node);
> > > > > -		return;
> > > > > -	}
> > > > > -	of_node_put(node);
> > > > > +		if (riscv_of_processor_hartid(node) < 0)
> > > > > +			continue;
> > >
> > > > > -	for (i = 0; i < strlen(isa); ++i)
> > > > > -		elf_hwcap |= isa2hwcap[(unsigned char)(isa[i])];
> > > > > +		if (of_property_read_string(node, "riscv,isa", &isa)) {
> > > > > +			pr_warn("Unable to find \"riscv,isa\" devicetree entry\n");
> > > > > +			return;
> > > >
> > > > Did you want "continue" here to continue processing the other harts?
> > >
> > > Hmm. If a cpu node doesn't have isa in DT, that means DT is wrong. A
> > > "continue" here will let user space use other harts just with a warning
> > > message?
> > >
> > > Returning here will not set elf_hwcap which forces the user to fix the
> > > DT. I am not sure what should be the defined behavior in this case.
> > >
> > > Any thoughts ?
> >
> > The problem is that the proposed code might still set elf_hwcap -- it
> > all depends on the order of the hart nodes in dt (i.e. it will only be
> > left unset if the first node is malformed).
> >
> > For that reason, I'd say it's better to either bail out (hard or at
> > least with elf_hwcap unset) or to continue processing the other nodes.
> >
> > The former might break current systems with malformed dt, though.
> >
> > And since the harts are expected to have the same ISA, continuing the
> > processing while warning and ignoring the malformed node might be
> > acceptable.
>
> Handling malformed device trees by providing a warning and an empty HWCAP seems
> like the right way to go to me.

If I understand you correctly, you prefer following things to be done in 
case of malformed DT.

1. Print a warning message
2. Unset the entire HWCAP
3. Return without processing other harts. This will most likely result 
in panic when user space starts.

Is this correct ?

Regards,
Atish
> > Johan