Re: [PATCH v2] riscv: Fix detecting FPU support in standard extension

[Yu-Chien Peter Lin <peterlin@andestech.com>]

On Sun, Nov 13, 2022 at 03:47:38PM -0600, Samuel Holland wrote:
> On 11/5/22 01:02, Yu Chien Peter Lin wrote:
> > We should check the string until it hits underscore, in case it
> > searches for the letters in the custom extension. For example,
> > "rv64imac_xandes" will be treated as D extension support since
> > there is a "d" in "andes", resulting illegal instruction caused
> > by initializing FCSR.
> > 
> > Signed-off-by: Yu Chien Peter Lin <peterlin@andestech.com>
> > ---
> >  arch/riscv/cpu/cpu.c | 14 +++++++++++---
> >  1 file changed, 11 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/riscv/cpu/cpu.c b/arch/riscv/cpu/cpu.c
> > index 52ab02519f..d34c8efce0 100644
> > --- a/arch/riscv/cpu/cpu.c
> > +++ b/arch/riscv/cpu/cpu.c
> > @@ -36,6 +36,7 @@ static inline bool supports_extension(char ext)
> >  #ifdef CONFIG_CPU
> >  	struct udevice *dev;
> >  	char desc[32];
> > +	int i;
> >  
> >  	uclass_find_first_device(UCLASS_CPU, &dev);
> >  	if (!dev) {
> > @@ -43,9 +44,16 @@ static inline bool supports_extension(char ext)
> >  		return false;
> >  	}
> >  	if (!cpu_get_desc(dev, desc, sizeof(desc))) {
> > -		/* skip the first 4 characters (rv32|rv64) */
> > -		if (strchr(desc + 4, ext))
> > -			return true;
> > +		/*
> > +		 * skip the first 4 characters (rv32|rv64) and
> > +		 * check until underscore
> > +		 */
> 
> This probably works for the intended usage, but it is not guaranteed to
> work. Underscores are allowed even between single-letter extensions. See
> section 29.5 ("Underscores") of the unprivileged spec. Still, this is an
> improvement, so:
> 
> Reviewed-by: Samuel Holland <samuel@sholland.org>

Aah, I missed that case.
I supposed the strings always match the pattern stated in riscv cpus
binding [1], the detection logic here needs to be updated once the
case is also allowed.

[1] https://github.com/torvalds/linux/blob/v6.1-rc5/Documentation/devicetree/bindings/riscv/cpus.yaml#L83

Thanks for your review!

Best regards,
Peter Lin

> 
> > +		for (i = 4; i < sizeof(desc); i++) {
> > +			if (desc[i] == '_' || desc[i] == '\0')
> > +				break;
> > +			if (desc[i] == ext)
> > +				return true;
> > +		}
> >  	}
> >  
> >  	return false;
>