gpu: nova-core: Clarify sysmembar operations

sysmembar is a critical operation that the GSP falcon needs to perform
in the reset sequence. Add some code comments to clarify.

[acourbot@nvdidia.com: move relevant documentation to SysmemFlush type]

Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Link: https://lore.kernel.org/r/20250708-nova-docs-v4-2-9d188772c4c7@nvidia.com
[ Minor grammar fix in the PFB register documentation. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

drivers/gpu/nova-core/fb.rs

@@ -17,6 +17,16 @@ mod hal;
 /// Type holding the sysmem flush memory page, a page of memory to be written into the
 /// `NV_PFB_NISO_FLUSH_SYSMEM_ADDR*` registers and used to maintain memory coherency.
 ///
+/// A system memory page is required for `sysmembar`, which is a GPU-initiated hardware
+/// memory-barrier operation that flushes all pending GPU-side memory writes that were done through
+/// PCIE to system memory. It is required for falcons to be reset as the reset operation involves a
+/// reset handshake. When the falcon acknowledges a reset, it writes into system memory. To ensure
+/// this write is visible to the host and prevent driver timeouts, the falcon must perform a
+/// sysmembar operation to flush its writes.
+///
+/// Because of this, the sysmem flush memory page must be registered as early as possible during
+/// driver initialization, and before any falcon is reset.
+///
 /// Users are responsible for manually calling [`Self::unregister`] before dropping this object,
 /// otherwise the GPU might still use it even after it has been freed.
 pub(crate) struct SysmemFlush {

drivers/gpu/nova-core/gpu.rs

@@ -170,7 +170,7 @@ pub(crate) struct Gpu {
     bar: Devres<Bar0>,
     fw: Firmware,
     /// System memory page required for flushing all pending GPU-side memory writes done through
-    /// PCIE into system memory.
+    /// PCIE into system memory, via sysmembar (A GPU-initiated HW memory-barrier operation).
     sysmem_flush: SysmemFlush,
 }
 
@@ -283,7 +283,6 @@ impl Gpu {
         gfw::wait_gfw_boot_completion(bar)
             .inspect_err(|_| dev_err!(pdev.as_ref(), "GFW boot did not complete"))?;
 
-        // System memory page required for sysmembar to properly flush into system memory.
         let sysmem_flush = SysmemFlush::register(pdev.as_ref(), bar, spec.chipset)?;
 
         let gsp_falcon = Falcon::<Gsp>::new(

drivers/gpu/nova-core/regs.rs

@@ -51,6 +51,9 @@ register!(NV_PBUS_SW_SCRATCH_0E@0x00001438  {
 
 /* PFB */
 
+// The following two registers together hold the physical system memory address that is used by the
+// GPU to perform sysmembar operations (see `fb::SysmemFlush`).
+
 register!(NV_PFB_NISO_FLUSH_SYSMEM_ADDR @ 0x00100c10 {
     31:0    adr_39_08 as u32;
 });