Re: [PATCH 3/6] gpu: nova-core: only boot FRTS if it actually exists

[Timur Tabi <ttabi@nvidia.com>]

On Mon, 2026-04-13 at 23:48 +0000, Timur Tabi wrote:
> > > If FRTS is optional, WDYT about making fb_layout.frts an Option<FbRange>
> > > and having the frts_size hal method return Option<NonZeroUsize> for
> > > example? I think it will make it clearer in other locations that use
> > > FRTS that it possibly might not exist.
> > 
> > Yeah, I like that.
> 
> Actually, using Option<FbRange> won't work.  The problem is that even if frtsSize is 0, frtsStart
> (aka frtsOffset) must still be a non-zero number, which means we still need to keep the FbRange,
> but
> it would need to be FbRange<frtsStart..frtsStart>.
> 
> So I can update the HAL, but not the FbRange code.

So after working on this a bit, I don't think it's actually going to improve things.  The problem is
that even though I saw that "the FRTS region doesn't exist", it kinda does, it's just zero size. 
That's because GSP-RM actually cares (and uses) frtsStart (RM calls it frtsOffset) even if frtsSize
is zero.  So we need to keep the FbRange no matter what.  Changing frts_size() to return an
Option<NonZeroUsize> results in this code:

            let frts_size = hal.frts_size().map_or(0, |n| n.get());

We all know what's actually going on internally, but this line just obfuscates it.

I will reword the commit messages to say that the window is zero size rather than being non-
existent.