Re: [PATCH] gpu: nova-core: fb: make sure to unregister SysmemFlush on boot failure

["Gary Guo" <gary@garyguo.net>]

On Thu Apr 9, 2026 at 1:15 PM BST, Eliot Courtney wrote:
> Current `Gpu::new` will not unregister SysmemFlush if something fails
> after it is created, since it needs manual unregistering. Add a `Drop`
> implementation which will clean it up in that case. Maintain the manual
> unregister path because it can stay infallible, unlike the Drop path
> which depends on revocable access. In the case that `Gpu::new` fails the
> access is guaranteed to succeed, however.
>
> Fixes: 6554ad65b589 ("gpu: nova-core: register sysmem flush page")
> Signed-off-by: Eliot Courtney <ecourtney@nvidia.com>
> ---
>  drivers/gpu/nova-core/fb.rs  | 29 ++++++++++++++++++++---------
>  drivers/gpu/nova-core/gpu.rs |  7 ++++++-
>  2 files changed, 26 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/gpu/nova-core/fb.rs b/drivers/gpu/nova-core/fb.rs
> index bdd5eed760e1..edfbdc9a2512 100644
> --- a/drivers/gpu/nova-core/fb.rs
> +++ b/drivers/gpu/nova-core/fb.rs
> @@ -7,6 +7,7 @@
>  
>  use kernel::{
>      device,
> +    devres::Devres,
>      dma::CoherentHandle,
>      fmt,
>      io::Io,
> @@ -16,7 +17,10 @@
>          Alignment, //
>      },
>      sizes::*,
> -    sync::aref::ARef, //
> +    sync::{
> +        aref::ARef,
> +        Arc, //
> +    },
>  };
>  
>  use crate::{
> @@ -46,12 +50,14 @@
>  /// 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.
> +/// Users should call [`Self::unregister`] before unloading to ensure unregistering is infallible.
> +/// [`Drop`] performs a best-effort fallback using revocable BAR access.
>  pub(crate) struct SysmemFlush {
>      /// Chipset we are operating on.
>      chipset: Chipset,
>      device: ARef<device::Device>,
> +    /// MMIO mapping of PCI BAR 0.
> +    bar: Arc<Devres<Bar0>>,
>      /// Keep the page alive as long as we need it.
>      page: CoherentHandle,
>  }
> @@ -60,6 +66,7 @@ impl SysmemFlush {
>      /// Allocate a memory page and register it as the sysmem flush page.
>      pub(crate) fn register(
>          dev: &device::Device<device::Bound>,
> +        devres_bar: Arc<Devres<Bar0>>,
>          bar: &Bar0,
>          chipset: Chipset,
>      ) -> Result<Self> {
> @@ -70,18 +77,17 @@ pub(crate) fn register(
>          Ok(Self {
>              chipset,
>              device: dev.into(),
> +            bar: devres_bar,
>              page,
>          })
>      }
>  
>      /// Unregister the managed sysmem flush page.
> -    ///
> -    /// In order to gracefully tear down the GPU, users must make sure to call this method before
> -    /// dropping the object.
>      pub(crate) fn unregister(&self, bar: &Bar0) {
>          let hal = hal::fb_hal(self.chipset);
> +        let registered_dma_handle = hal.read_sysmem_flush_page(bar);
>  
> -        if hal.read_sysmem_flush_page(bar) == self.page.dma_handle() {
> +        if registered_dma_handle == self.page.dma_handle() {
>              let _ = hal.write_sysmem_flush_page(bar, 0).inspect_err(|e| {
>                  dev_warn!(
>                      &self.device,
> @@ -89,8 +95,7 @@ pub(crate) fn unregister(&self, bar: &Bar0) {
>                      e
>                  )
>              });
> -        } else {
> -            // Another page has been registered after us for some reason - warn as this is a bug.
> +        } else if registered_dma_handle != 0 {
>              dev_warn!(
>                  &self.device,
>                  "attempt to unregister a sysmem flush page that is not active\n"
> @@ -99,6 +104,12 @@ pub(crate) fn unregister(&self, bar: &Bar0) {
>      }
>  }
>  
> +impl Drop for SysmemFlush {
> +    fn drop(&mut self) {
> +        let _ = self.bar.try_access_with(|bar| self.unregister(bar));

I feel that this is the wrong solution to the problem.

The thing we want is to *ensure* that `SysmemFlush` Drop is called with device
still being bound.

It's not yet fully clear to me how we'd want to guarantee that, but one API that
might make sense is to create a DevRes API that allows you to reference an
existing `DevRes` and have driver-core making sure that the tear down happens in
reverse order. So inside the `Drop` the `bar` can still be unconditionally
access.

Best,
Gary

> +    }
> +}
> +
>  pub(crate) struct FbRange(Range<u64>);
>  
>  impl FbRange {
> diff --git a/drivers/gpu/nova-core/gpu.rs b/drivers/gpu/nova-core/gpu.rs
> index 0f6fe9a1b955..5bad5a055b3b 100644
> --- a/drivers/gpu/nova-core/gpu.rs
> +++ b/drivers/gpu/nova-core/gpu.rs
> @@ -257,7 +257,12 @@ pub(crate) fn new<'a>(
>                      .inspect_err(|_| dev_err!(pdev, "GFW boot did not complete\n"))?;
>              },
>  
> -            sysmem_flush: SysmemFlush::register(pdev.as_ref(), bar, spec.chipset)?,
> +            sysmem_flush: SysmemFlush::register(
> +                pdev.as_ref(),
> +                devres_bar.clone(),
> +                bar,
> +                spec.chipset,
> +            )?,
>  
>              gsp_falcon: Falcon::new(
>                  pdev.as_ref(),
>
> ---
> base-commit: a7a080bb4236ebe577b6776d940d1717912ff6dd
> change-id: 20260409-fix-systemflush-de66dc90378a
>
> Best regards,
> --  
> Eliot Courtney <ecourtney@nvidia.com>