linux-kernel - Re: [PATCH 4/7] docs: gpu: nova-core: Document the PRAMIN aperture mechanism

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Message-ID: <506e84b8-1a99-4548-b2c6-b502d790f4e1@nvidia.com>
Date: Mon, 20 Oct 2025 12:36:33 -0700
From: John Hubbard <jhubbard@...dia.com>
To: Joel Fernandes <joelagnelf@...dia.com>, linux-kernel@...r.kernel.org,
 rust-for-linux@...r.kernel.org, dri-devel@...ts.freedesktop.org,
 dakr@...nel.org, acourbot@...dia.com
Cc: Alistair Popple <apopple@...dia.com>, Miguel Ojeda <ojeda@...nel.org>,
 Alex Gaynor <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...il.com>,
 Gary Guo <gary@...yguo.net>, bjorn3_gh@...tonmail.com,
 Benno Lossin <lossin@...nel.org>, Andreas Hindborg <a.hindborg@...nel.org>,
 Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 Timur Tabi <ttabi@...dia.com>, joel@...lfernandes.org,
 Elle Rhumsaa <elle@...thered-steel.dev>,
 Daniel Almeida <daniel.almeida@...labora.com>, nouveau@...ts.freedesktop.org
Subject: Re: [PATCH 4/7] docs: gpu: nova-core: Document the PRAMIN aperture
 mechanism

On 10/20/25 11:55 AM, Joel Fernandes wrote:
...> +Logically, the PRAMIN aperture mechanism is implemented by the GPU's PBUS (PCIe Bus Controller Unit)
> +and provides a CPU-accessible window into VRAM through the PCIe interface::
> +
> +    +-----------------+    PCIe     +------------------------------+
> +    |      CPU        |<----------->|           GPU                |
> +    +-----------------+             |                              |
> +                                    |  +----------------------+    |
> +                                    |  |       PBUS           |    |
> +                                    |  |  (Bus Controller)    |    |
> +                                    |  |                      |    |
> +                                    |  |  +--------------.<------------ (window always starts at
> +                                    |  |  |   PRAMIN     |    |    |     BAR0 + 0x700000)

Quick question: does "window always starts at" actually mean "windows
is always initialized to" ? Or something else?


thanks,
-- 
John Hubbard

> +                                    |  |  |   Window     |    |    |
> +                                    |  |  |   (1MB)      |    |    |
> +                                    |  |  +--------------+    |    |
> +                                    |  |         |            |    |
> +                                    |  +---------|------------+    |
> +                                    |            |                 |
> +                                    |            v                 |
> +                                    |  .----------------------.<------------ (Program PRAMIN to any
> +                                    |  |       VRAM           |    |    64KB VRAM physical boundary)
> +                                    |  |    (Several GBs)     |    |
> +                                    |  |                      |    |
> +                                    |  |  FB[0x000000000000]  |    |
> +                                    |  |          ...         |    |
> +                                    |  |  FB[0x7FFFFFFFFFF]   |    |
> +                                    |  +----------------------+    |
> +                                    +------------------------------+
> +
> +PBUS (PCIe Bus Controller) among other things is responsible in the GPU for handling MMIO
> +accesses to the BAR registers.
> +
> +PRAMIN Window Operation
> +=======================
> +
> +The PRAMIN window provides a 1MB sliding aperture that can be repositioned over
> +the entire VRAM address space using the NV_PBUS_BAR0_WINDOW register.
> +
> +Window Control Mechanism
> +-------------------------
> +
> +The window position is controlled via the PBUS BAR0_WINDOW register::
> +
> +    NV_PBUS_BAR0_WINDOW Register
> +    +-----+-----+--------------------------------------+
> +    |31-26|25-24|           23-0                       |
> +    |     |TARG |         BASE_ADDR                    |
> +    |     | ET  |        (bits 39:16 of VRAM address)  |
> +    +-----+-----+--------------------------------------+
> +
> +    TARGET field values:
> +    - 0x0: VID_MEM (Video Memory / VRAM)
> +    - 0x1: SYS_MEM_COHERENT (Coherent system memory)
> +    - 0x2: SYS_MEM_NONCOHERENT (Non-coherent system memory)
> +
> +64KB Alignment Requirement
> +---------------------------
> +
> +The PRAMIN window must be aligned to 64KB boundaries in VRAM. This is enforced
> +by the BASE_ADDR field representing bits [39:16] of the target address::
> +
> +    VRAM Address Calculation:
> +    actual_vram_addr = (BASE_ADDR << 16) + pramin_offset
> +    Where:
> +    - BASE_ADDR: 24-bit value from NV_PBUS_BAR0_WINDOW[23:0]
> +    - pramin_offset: 20-bit offset within PRAMIN window [0x00000-0xFFFFF]
> +    Example Window Positioning:
> +    +---------------------------------------------------------+
> +    |                    VRAM Space                           |
> +    |                                                         |
> +    |  0x000000000  +-----------------+ <-- 64KB aligned      |
> +    |               | PRAMIN Window   |                       |
> +    |               |    (1MB)        |                       |
> +    |  0x0000FFFFF  +-----------------+                       |
> +    |                                                         |
> +    |       |              ^                                  |
> +    |       |              | Window can slide                 |
> +    |       v              | to any 64KB boundary             |
> +    |                                                         |
> +    |  0x123400000  +-----------------+ <-- 64KB aligned      |
> +    |               | PRAMIN Window   |                       |
> +    |               |    (1MB)        |                       |
> +    |  0x1234FFFFF  +-----------------+                       |
> +    |                                                         |
> +    |                       ...                               |
> +    |                                                         |
> +    |  0x7FFFF0000  +-----------------+ <-- 64KB aligned      |
> +    |               | PRAMIN Window   |                       |
> +    |               |    (1MB)        |                       |
> +    |  0x7FFFFFFFF  +-----------------+                       |
> +    +---------------------------------------------------------+
> diff --git a/Documentation/gpu/nova/index.rst b/Documentation/gpu/nova/index.rst
> index 46302daace34..e77d3ee336a4 100644
> --- a/Documentation/gpu/nova/index.rst
> +++ b/Documentation/gpu/nova/index.rst
> @@ -33,3 +33,4 @@ vGPU manager VFIO driver and the nova-drm driver.
>     core/fwsec
>     core/falcon
>     core/msgq
> +   core/pramin