Re: [PATCH v2 08/12] gpu: nova-core: mm: Add support to use PRAMIN windows to write to VRAM

[Alistair Popple]

On 2026-08-10 at 23:55 +1000, Eliot Courtney <ecourtney@xxxxxxxxxx> wrote...
> From: Joel Fernandes <joelagnelf@xxxxxxxxxx>
> 
> PRAMIN apertures are a crucial mechanism for direct CPU read/write to
> VRAM. Add a `Pramin` manager whose `window_at()` returns a typed MMIO
> view of VRAM through the 1 MiB PRAMIN aperture in BAR0, validating the
> view against the VRAM region and repositioning the window as needed for
> the accessed address.
> 
> A view borrows `Pramin` mutably, so the window cannot move while
> the view is in use, and it inserts an ordering point on Drop.
> 
> Signed-off-by: Joel Fernandes <joelagnelf@xxxxxxxxxx>
> [ecourtney: split the registers and HAL into the two preceding patches]
> [ecourtney: rebase w.r.t. Bar0 lifetime changes and register projections]
> [ecourtney: drop the window guard and mutex, use &mut self]
> [ecourtney: position at init to avoid reads, reposition in window_offset]
> [ecourtney: return typed MMIO views instead of read/write accessors]
> [ecourtney: insert an ordering read when a view drops]
> [ecourtney: declare the window location, drop the doc examples]
> [ecourtney: add the copyright header, doc and naming cleanups]
> [ecourtney: the pramin module is mm-internal]
> Co-developed-by: Eliot Courtney <ecourtney@xxxxxxxxxx>
> Signed-off-by: Eliot Courtney <ecourtney@xxxxxxxxxx>
> ---
>  drivers/gpu/nova-core/mm.rs        |   1 +
>  drivers/gpu/nova-core/mm/pramin.rs | 178 +++++++++++++++++++++++++++++++++++++
>  2 files changed, 179 insertions(+)
> 
> diff --git a/drivers/gpu/nova-core/mm.rs b/drivers/gpu/nova-core/mm.rs
> index 07dce4ce2473..ef5b1cad56c3 100644
> --- a/drivers/gpu/nova-core/mm.rs
> +++ b/drivers/gpu/nova-core/mm.rs
> @@ -20,6 +20,7 @@
>  };
>  
>  mod hal;
> +mod pramin;
>  mod regs;
>  
>  /// Physical VRAM address in GPU video memory.
> diff --git a/drivers/gpu/nova-core/mm/pramin.rs b/drivers/gpu/nova-core/mm/pramin.rs
> new file mode 100644
> index 000000000000..20be3fc471ba
> --- /dev/null
> +++ b/drivers/gpu/nova-core/mm/pramin.rs
> @@ -0,0 +1,178 @@
> +// SPDX-License-Identifier: GPL-2.0
> +// SPDX-FileCopyrightText: Copyright (c) 2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
> +
> +//! Utilities for accessing VRAM through the PRAMIN window.
> +
> +use core::ops::Range;
> +
> +use kernel::{
> +    io::{
> +        io_project,
> +        register,
> +        register::OffsetLoc,
> +        Io,
> +        Mmio, //
> +    },
> +    prelude::*,
> +    ptr::{
> +        Alignable,
> +        Alignment, //
> +    },
> +    sizes::{
> +        SZ_1M,
> +        SZ_64K, //
> +    },
> +};
> +
> +use crate::{
> +    driver::{
> +        Bar0,
> +        NovaRegisters, //
> +    },
> +    gpu::Chipset,
> +    mm::{
> +        hal::{
> +            self,
> +            MmHal, //
> +        },
> +        VramAddress, //
> +    },
> +    num::IntoSafeCast, //
> +};
> +
> +/// Size of the PRAMIN window (1 MiB).
> +const WINDOW_SIZE: usize = SZ_1M;
> +
> +/// The PRAMIN window, which is a 1 MiB window into VRAM at a fixed BAR0 offset.
> +#[derive(FromBytes, IntoBytes)]
> +struct PraminWindow([u8; WINDOW_SIZE]);
> +
> +register! {
> +    base: NovaRegisters;

I tried building this on drm-rust-next against the base commit
(4c9ba407018e8deb06dbc643112bac8f40404f95) but am getting some build errors:

error: no rules expected `:`
   --> /data/source/linux/drivers/gpu/nova-core/mm/pramin.rs:51:9
    |
 51 |     base: NovaRegisters;
    |         ^ no rules expected this token in macro call
    |
note: while trying to match `(`
   --> /data/source/linux/rust/kernel/io/register.rs:809:52
    |
809 |             $(#[$attr:meta])* $vis:vis $name:ident ($storage:ty)
    |                                                    ^

I'm guessing maybe because this depends on some other series that isn't in
drm-rust-next or something in drm-rust-next has changed things here.

There are a couple of others with this series applied as well:

error: no rules expected `build`
   --> /data/source/linux/drivers/gpu/nova-core/mm/pramin.rs:115:39
    |
115 |         let window = io_project!(bar, build: PRAMIN);
    |                                       ^^^^^ no rules expected this token in macro call
    |
note: while trying to match `,`
   --> /data/source/linux/rust/kernel/ptr/projection.rs:319:21
    |
319 |     (@gen $ptr:ident, ) => {};
    |                     ^

error: no rules expected reserved keyword `try`
   --> /data/source/linux/drivers/gpu/nova-core/mm/pramin.rs:174:45
    |
174 |         let view = io_project!(self.window, try: OffsetLoc::new(offset));
    |                                             ^^^ no rules expected this token in macro call
    |
note: while trying to match `,`
   --> /data/source/linux/rust/kernel/ptr/projection.rs:319:21
    |
319 |     (@gen $ptr:ident, ) => {};
    |                     ^

error: no rules expected `:`
   --> /data/source/linux/drivers/gpu/nova-core/mm/regs.rs:29:9
    |
 29 |     base: NovaRegisters;
    |         ^ no rules expected this token in macro call
    |
note: while trying to match `(`
   --> /data/source/linux/rust/kernel/io/register.rs:809:52
    |
809 |             $(#[$attr:meta])* $vis:vis $name:ident ($storage:ty)
    |                                                    ^

error: no rules expected `:`
   --> /data/source/linux/drivers/gpu/nova-core/mm/regs.rs:46:13
    |
 46 |         base: NovaRegisters;
    |             ^ no rules expected this token in macro call
    |
note: while trying to match `(`
   --> /data/source/linux/rust/kernel/io/register.rs:809:52
    |
809 |             $(#[$attr:meta])* $vis:vis $name:ident ($storage:ty)
    |                                                    ^

error: no rules expected `:`
   --> /data/source/linux/drivers/gpu/nova-core/mm/regs.rs:62:13
    |
 62 |         base: NovaRegisters;
    |             ^ no rules expected this token in macro call
    |
note: while trying to match `(`
   --> /data/source/linux/rust/kernel/io/register.rs:809:52
    |
809 |             $(#[$attr:meta])* $vis:vis $name:ident ($storage:ty)
    |                                                    ^

error: aborting due to 6 previous errors

 - Alistair

> +
> +    /// Location of the window inside BAR0.
> +    PRAMIN: PraminWindow @ 0x700000;
> +}
> +
> +/// Owner of the PRAMIN window state.
> +///
> +/// [`Pramin::window_at()`] repositions the window as needed and returns a typed MMIO view into
> +/// it, holding the manager borrowed for the lifetime of the view.
> +pub(super) struct Pramin<'gpu> {
> +    bar: Bar0<'gpu>,
> +    hal: &'static dyn MmHal,
> +    /// MMIO view of the PRAMIN window in BAR0.
> +    window: Mmio<'gpu, PraminWindow>,
> +    /// VRAM range to keep the PRAMIN window inside.
> +    vram_range: Range<VramAddress>,
> +    /// Cached window position.
> +    window_range: Range<VramAddress>,
> +}
> +
> +/// Typed view of VRAM through the PRAMIN window.
> +///
> +/// Inserts an ordering point after previous writes through the window on drop. Views returned
> +/// by [`PraminAccess::view()`] cannot outlive this access, so the ordering point covers every
> +/// write made through them.
> +pub(super) struct PraminAccess<'a, T>
> +where
> +    T: FromBytes + IntoBytes,
> +{
> +    view: Mmio<'a, T>,
> +}
> +
> +impl<T> PraminAccess<'_, T>
> +where
> +    T: FromBytes + IntoBytes,
> +{
> +    /// Returns the MMIO view of the accessed location.
> +    pub(super) fn view(&self) -> Mmio<'_, T> {
> +        self.view
> +    }
> +}
> +
> +impl<T> Drop for PraminAccess<'_, T>
> +where
> +    T: FromBytes + IntoBytes,
> +{
> +    fn drop(&mut self) {
> +        // Insert an ordering point after previous writes through this window.
> +        self.view.cast::<u8>().read_val();
> +    }
> +}
> +
> +impl<'gpu> Pramin<'gpu> {
> +    /// Alignment required by the PRAMIN window.
> +    const BASE_ALIGN: Alignment = Alignment::new::<SZ_64K>();
> +
> +    /// Creates the window manager for the given VRAM region.
> +    pub(super) fn new(
> +        bar: Bar0<'gpu>,
> +        chipset: Chipset,
> +        vram_range: Range<VramAddress>,
> +    ) -> Result<Self> {
> +        let hal = hal::mm_hal(chipset);
> +        let window = io_project!(bar, build: PRAMIN);
> +        let base = vram_range.start.align_down(Self::BASE_ALIGN);
> +        let window_range = Self::window_range(base)?;
> +        hal.write_pramin_window_base(bar, base)?;
> +
> +        Ok(Self {
> +            bar,
> +            hal,
> +            window,
> +            vram_range,
> +            window_range,
> +        })
> +    }
> +
> +    /// Returns the VRAM range a window based at `base` exposes.
> +    fn window_range(base: VramAddress) -> Result<Range<VramAddress>> {
> +        let end = base
> +            .checked_add(WINDOW_SIZE.into_safe_cast())
> +            .ok_or(EINVAL)?;
> +        Ok(base..end)
> +    }
> +
> +    /// Check the window covers `len` bytes at `addr`, moving it if needed.
> +    ///
> +    /// Returns the window offset at which to perform the access.
> +    fn window_offset(&mut self, addr: VramAddress, len: usize) -> Result<usize> {
> +        let end = addr.checked_add(len.into_safe_cast()).ok_or(EINVAL)?;
> +
> +        let inside = |r: &Range<VramAddress>| r.contains(&addr) && end <= r.end;
> +        if !inside(&self.vram_range) {
> +            return Err(EINVAL);
> +        }
> +
> +        // Reposition the window if the access falls outside it.
> +        if !inside(&self.window_range) {
> +            let base = addr.align_down(Self::BASE_ALIGN);
> +            let window_range = Self::window_range(base)?;
> +            if !inside(&window_range) {
> +                return Err(EINVAL);
> +            }
> +            self.hal.write_pramin_window_base(self.bar, base)?;
> +            self.window_range = window_range;
> +        }
> +
> +        Ok((addr - self.window_range.start).into_safe_cast())
> +    }
> +
> +    /// Return a typed MMIO view of a `T` at `vram_addr`.
> +    ///
> +    /// Returns an error if `vram_addr` is not aligned to `T`'s alignment, or if
> +    /// a `T` at `vram_addr` does not fit within the VRAM region.
> +    pub(super) fn window_at<'a, T>(
> +        &'a mut self,
> +        vram_addr: VramAddress,
> +    ) -> Result<PraminAccess<'a, T>>
> +    where
> +        T: FromBytes + IntoBytes,
> +    {
> +        let offset = self.window_offset(vram_addr, size_of::<T>())?;
> +        let view = io_project!(self.window, try: OffsetLoc::new(offset));
> +
> +        Ok(PraminAccess { view })
> +    }
> +}
> 
> -- 
> 2.55.0
>