[PATCH v6 03/17] riscv: vector: Use vlenb from DT

[Charlie Jenkins]

On Thu, May 16, 2024 at 05:24:25PM +0100, Conor Dooley wrote:
> On Thu, May 16, 2024 at 10:00:12PM +0800, Andy Chiu wrote:
> > On Sat, May 4, 2024 at 2:21 AM Charlie Jenkins <charlie at rivosinc.com> wrote:
> 
> > > +               if (elf_hwcap & COMPAT_HWCAP_ISA_V && has_riscv_homogeneous_vlenb() < 0) {
> > > +                       pr_warn("Unsupported heterogeneous vlen detected, vector extension disabled.\
> > > +                       elf_hwcap &= ~COMPAT_HWCAP_ISA_V;
> > > +               }
> > 
> > We only touch COMPAT_HWCAP_ISA_V and the failed case only turns off the
> > rectified V. So here we have nothing to do with the Xtheadvector.
> 
> There's nothing t-head related in the tree at this point, so doing
> anything with it would cause build issues.
> 
> > However, I am still confused because I think Xtheadvector would also
> > need to call into this check, so as to setup vlenb.
> 
> 
> > Apart from that, it seems like some vendor stating Xtheadvector is
> > actually vector-0.7.
> 
> The T-Head implementation is 0.7.x, but I am not really sure what you
> mean by this comment.

Andy, the idea of this patch was to be able to support this binding on
more than just xtheadvector.

You are correct though Andy, this is a problem that a later patch in
this series doesn't disable xtheadvector when vlenb is not homogeneous.
I am going to wait to send out any more versions until after this merge
window but I will fix this in the next version. Thank you! 

> 
> > Please correct me if I speak anything wrong. One
> > thing I noticed is that Xtheadvector wouldn't trap on reading
> > th.vlenb but vector-0.7 would. If that is the case, should we require
> > Xtheadvector to specify `riscv,vlenb` on the device tree?
> 
> In the world of Linux, "vector-0.7" isn't a thing. There's only 1.0, and
> after this patchset, "xtheadvector". My understanding, from discussion
> on earlier versions of this series the trap is actually accessing
> th.vlenb register, despite the documentation stating that it is
> unprivileged:
> https://github.com/T-head-Semi/thead-extension-spec/blob/master/xtheadvector.adoc
> I assume Charlie tried it but was trapping, as v1 had a comment:
> +		 * Although xtheadvector states that th.vlenb exists and
> +		 * overlaps with the vector 1.0 extension overlaps, an illegal
> +		 * instruction is raised if read. These systems all currently
> +		 * have a fixed vector length of 128, so hardcode that value.

On my board with a c906 attempting to read th.vlenb (which is supposed
to have the same encoding as vlenb) raises an illegal instruction
exception from S-mode even though the documentation states that it
shouldn't. Because the documentation states that vlenb is available, I
haven't made it required for xtheadvector, I am not sure the proper
solution for that.

- Charlie

> 
> Cheers,
> Conor.