[PATCH v2] gpu: nova-core: falcon: pad firmware DMA object size to required block alignment

[Alexandre Courbot]

Commit a88831502c8f ("gpu: nova-core: falcon: use dma::Coherent")
dropped the nova-local `DmaObject` device memory type for the
kernel-global `Coherent` one.

This switch had a side-effect: `DmaObject` always aligned the requested
size to `PAGE_SIZE`, and also reported that adjusted size when queried.
`Coherent`, on the other hand, does page-align allocation sizes but only
allows CPU access on the exact size provided by the caller.

This change runs into a limitation of falcon DMA copies, namely that DMA
accesses are done on blocks of exactly 256 bytes. If the provided data
does not have a length that is a multiple of 256, `dma_wr` returns
an error.

It was expected that all firmwares would present the proper adjusted
size, but this is not the case at least on my GA107:

    NovaCore 0000:08:00.0: DMA transfer goes beyond range of DMA object
    NovaCore 0000:08:00.0: Failed to load FWSEC firmware: EINVAL
    NovaCore 0000:08:00.0: probe with driver NovaCore failed with error -22

Fix this by padding the `Coherent`'s size to `MEM_BLOCK_ALIGNMENT` (i.e.
256) when allocating it and filling it with zeroes, before copying the
firmware on top of it.

Fixes: a88831502c8f ("gpu: nova-core: falcon: use dma::Coherent")
Reviewed-by: John Hubbard <jhubbard@xxxxxxxxxx>
Reviewed-by: Gary Guo <gary@xxxxxxxxxxx>
Signed-off-by: Alexandre Courbot <acourbot@xxxxxxxxxx>
---
Changes in v2:
- Replace incorrect use of "alignment" with "padding" (thanks John!).
- Use more concise range syntax (thanks Gary!).
- Collect tags.
- Link to v1: https://patch.msgid.link/20260404-falcon-dma-roundup-v1-1-1f56d808b091@xxxxxxxxxx
---
 drivers/gpu/nova-core/falcon.rs | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/nova-core/falcon.rs b/drivers/gpu/nova-core/falcon.rs
index e0315fda576b..33927af4134c 100644
--- a/drivers/gpu/nova-core/falcon.rs
+++ b/drivers/gpu/nova-core/falcon.rs
@@ -11,6 +11,7 @@
     },
     dma::{
         Coherent,
+        CoherentBox,
         DmaAddress,
         DmaMask, //
     },
@@ -613,8 +614,24 @@ fn dma_load<F: FalconFirmware<Target = E> + FalconDmaLoadable>(
         bar: &Bar0,
         fw: &F,
     ) -> Result {
-        // Create DMA object with firmware content as the source of the DMA engine.
-        let dma_obj = Coherent::from_slice(dev, fw.as_slice(), GFP_KERNEL)?;
+        // DMA object with firmware content as the source of the DMA engine.
+        let dma_obj = {
+            let fw_slice = fw.as_slice();
+
+            // DMA copies are done in chunks of `MEM_BLOCK_ALIGNMENT`, so pad the length
+            // accordingly and fill with `0`.
+            let mut dma_obj = CoherentBox::zeroed_slice(
+                dev,
+                fw_slice.len().next_multiple_of(MEM_BLOCK_ALIGNMENT),
+                GFP_KERNEL,
+            )?;
+
+            // PANIC: `dma_obj` has been created with a length equal to or larger than
+            // `fw_slice.len()`, so the range `..fw_slice.len()` is valid.
+            dma_obj[..fw_slice.len()].copy_from_slice(fw_slice);
+
+            dma_obj.into()
+        };
 
         self.dma_reset(bar);
         bar.update(regs::NV_PFALCON_FBIF_TRANSCFG::of::<E>().at(0), |v| {

---
base-commit: 7c50d748b4a635bc39802ea3f6b120e66b1b9067
change-id: 20260404-falcon-dma-roundup-0edd764a9840

Best regards,
--  
Alexandre Courbot <acourbot@xxxxxxxxxx>