gpu: nova-core: clean up FSP FRTS comments

Two comments in the FSP Chain of Trust message setup had drifted from
the code. One referred to a variable name that no longer exists, and
another described the unused sysmem FRTS fields as future work rather
than explaining why they are zero. Update both to describe the code as
it stands.

Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Eliot Courtney <ecourtney@nvidia.com>
Link: https://patch.msgid.link/20260603235034.131354-3-jhubbard@nvidia.com
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>

drivers/gpu/nova-core/fsp.rs

@@ -132,7 +132,8 @@ fn new<'a>(
         fsp_fw: &'a FspFirmware,
         args: &'a FmcBootArgs,
     ) -> Result<impl Init<Self> + 'a> {
-        // frts_offset is relative to FB end: FRTS_location = FB_END - frts_offset
+        // frts_vidmem_offset is measured from the end of FB, so FRTS sits at
+        // (end of FB) - frts_vidmem_offset.
         let frts_vidmem_offset = if !args.resume {
             let frts_reserved_size = fb_layout.heap.len() + u64::from(fb_layout.pmu_reserved_size);
 
@@ -164,8 +165,8 @@ fn new<'a>(
             msg.cot.gsp_fmc_sysmem_offset = fsp_fw.fmc_image.dma_handle();
             msg.cot.frts_vidmem_offset = frts_vidmem_offset;
             msg.cot.frts_vidmem_size = frts_size;
-            // frts_sysmem_* intentionally left at zero for now, but will be needed for e.g.
-            // systems without VRAM.
+            // frts_sysmem_* are left at zero because this path places FRTS in vidmem. The sysmem
+            // fields point to an FRTS buffer in sysmem instead, for systems without VRAM.
             msg.cot.gsp_boot_args_sysmem_offset = args.fmc_boot_params.dma_handle();
             msg.cot.sigs = *fsp_fw.fmc_sigs;