gpu: nova-core: add boot42 support for next-gen GPUs

NVIDIA GPUs are moving away from using NV_PMC_BOOT_0 to contain
architecture and revision details, and will instead use NV_PMC_BOOT_42
in the future. NV_PMC_BOOT_0 will contain a specific set of values
that will mean "go read NV_PMC_BOOT_42 instead".

Change the selection logic in Nova so that it will claim Turing and
later GPUs. This will work for the foreseeable future, without any
further code changes here, because all NVIDIA GPUs are considered, from
the oldest supported on Linux (NV04), through the future GPUs.

Add some comment documentation to explain, chronologically, how boot0
and boot42 change with the GPU eras, and how that affects the selection
logic.

Cc: Alexandre Courbot <[e-mail]>
Cc: Danilo Krummrich <[e-mail]>
Cc: Timur Tabi <[e-mail]>
Reviewed-by: Joel Fernandes <[e-mail]>
Signed-off-by: John Hubbard <[e-mail]>
[[e-mail]: remove unneeded `From<BOOT_0> for Revision`
implementation.]
Signed-off-by: Alexandre Courbot <[e-mail]>
Message-ID: <[e-mail]>

drivers/gpu/nova-core/gpu.rs

···
     minor: u8,
 }
 
-impl From<regs::NV_PMC_BOOT_0> for Revision {
-    fn from(boot0: regs::NV_PMC_BOOT_0) -> Self {
+impl From<regs::NV_PMC_BOOT_42> for Revision {
+    fn from(boot0: regs::NV_PMC_BOOT_42) -> Self {
         Self {
             major: boot0.major_revision(),
             minor: boot0.minor_revision(),
···
 
 impl Spec {
     fn new(bar: &Bar0) -> Result<Spec> {
+        // Some brief notes about boot0 and boot42, in chronological order:
+        //
+        // NV04 through NV50:
+        //
+        //    Not supported by Nova. boot0 is necessary and sufficient to identify these GPUs.
+        //    boot42 may not even exist on some of these GPUs.
+        //
+        // Fermi through Volta:
+        //
+        //     Not supported by Nova. boot0 is still sufficient to identify these GPUs, but boot42
+        //     is also guaranteed to be both present and accurate.
+        //
+        // Turing and later:
+        //
+        //     Supported by Nova. Identified by first checking boot0 to ensure that the GPU is not
+        //     from an earlier (pre-Fermi) era, and then using boot42 to precisely identify the GPU.
+        //     Somewhere in the Rubin timeframe, boot0 will no longer have space to add new GPU IDs.
+
         let boot0 = regs::NV_PMC_BOOT_0::read(bar);
 
-        Spec::try_from(boot0)
+        if boot0.is_older_than_fermi() {
+            return Err(ENODEV);
+        }
+
+        Spec::try_from(regs::NV_PMC_BOOT_42::read(bar))
     }
 }
 
-impl TryFrom<regs::NV_PMC_BOOT_0> for Spec {
+impl TryFrom<regs::NV_PMC_BOOT_42> for Spec {
     type Error = Error;
 
-    fn try_from(boot0: regs::NV_PMC_BOOT_0) -> Result<Self> {
+    fn try_from(boot42: regs::NV_PMC_BOOT_42) -> Result<Self> {
         Ok(Self {
-            chipset: boot0.chipset()?,
-            revision: boot0.into(),
+            chipset: boot42.chipset()?,
+            revision: boot42.into(),
         })
     }
 }

drivers/gpu/nova-core/regs.rs

···
 });
 
 impl NV_PMC_BOOT_0 {
-    /// Combines `architecture_0` and `architecture_1` to obtain the architecture of the chip.
-    pub(crate) fn architecture(self) -> Result<Architecture> {
-        Architecture::try_from(
-            self.architecture_0() | (self.architecture_1() << Self::ARCHITECTURE_0_RANGE.len()),
-        )
-    }
+    pub(crate) fn is_older_than_fermi(self) -> bool {
+        // From https://github.com/NVIDIA/open-gpu-doc/tree/master/manuals :
+        const NV_PMC_BOOT_0_ARCHITECTURE_GF100: u8 = 0xc;
 
+        // Older chips left arch1 zeroed out. That, combined with an arch0 value that is less than
+        // GF100, means "older than Fermi".
+        self.architecture_1() == 0 && self.architecture_0() < NV_PMC_BOOT_0_ARCHITECTURE_GF100
+    }
+}
+
+register!(NV_PMC_BOOT_42 @ 0x00000a00, "Extended architecture information" {
+    15:12   minor_revision as u8, "Minor revision of the chip";
+    19:16   major_revision as u8, "Major revision of the chip";
+    23:20   implementation as u8, "Implementation version of the architecture";
+    29:24   architecture as u8 ?=> Architecture, "Architecture value";
+});
+
+impl NV_PMC_BOOT_42 {
     /// Combines `architecture` and `implementation` to obtain a code unique to the chipset.
     pub(crate) fn chipset(self) -> Result<Chipset> {
         self.architecture()