cb3d28e3d6
README: rewritten around what runs (Linux software, Xorg, apk, Linux's drivers), the honest gaps, measured numbers. plan/: 1.0.md is the finish line, where each clause of the acceptance test stands, and the register (one failing clause: a root that survives a reboot); built plans (foundations, hybrid, alpine, xorg) and the decided-against one (wayland) are gone, their content lives in architecture/; roadmap.md is only what is still open. guides: building/layout/testing/adding-a-driver (Linux code may be used). history: four entries (ls -lR, TLB shootdowns, MDIO 0x02460246, netlink MSG_TRUNC). Website: cards, stack, download page, docs index. Code comments that pointed at deleted plan docs point at the architecture docs. Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01EVRhxi9zpdW2uDUN4FVcao
125 lines
6.6 KiB
Markdown
125 lines
6.6 KiB
Markdown
# The GPU, through Linux's own driver
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The laptop this is for (ASUS TUF A15 FA507NU) has a Radeon 680M in the CPU and
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an RTX 4050 beside it. SchOS draws everything with the CPU on an EFI
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framebuffer. Rendering a browser page that way keeps every core busy, and the
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machine runs hot doing it.
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Writing a Radeon driver is not on the table: amdgpu is around four million lines
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before its register headers. The plan is to **run Linux's amdgpu unmodified**,
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with its firmware, in a process under LDLS ([../architecture/ldls.md](../architecture/ldls.md)),
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and to give Mesa the `/dev/dri` it expects. Licensing is not an obstacle:
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SchOS is GPL-2.0-or-later, and the user has said that Linux code and firmware
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blobs may be shipped. That supersedes LDLS's rule 1.
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## Why LDLS and not an in-kernel linuxkpi
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FreeBSD's drm-kmod compiles Linux's DRM source into kernel modules against a
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shim (linuxkpi). SchOS has already chosen the other route, and for good reasons
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([devices.md](../architecture/devices.md)): GPU drivers live outside the kernel, a crashed driver
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falls back to the framebuffer instead of taking the machine down, and LDLS
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loads unmodified `.ko` files against the running kernel's BTF instead of
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needing a source port per Linux release. vgpud already drives a GPU from a
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process, and LDLS already runs 8139too and e1000 that way.
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## What it costs: the numbers
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Measured against Linux 6.18.54 (the host's kernel and modules), with
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`modprobe --show-depends`, `nm -u` and the export tables:
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| Driver | Modules in the closure | Symbols needed from the core kernel | LDLS provides today |
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|---|---|---|---|
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| r8169 | 12 (libphy, mdio-bus, realtek PHY, ...) | 330 | 70 |
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| amdgpu | 15 (ttm, gpu-sched, drm_buddy, drm_exec, drm_display_helper, ...) | 1,166 | 76 |
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Of amdgpu's 1,166 symbols, **314 are the DRM core** (`drm_*`). NixOS builds
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`drm.ko` and `drm_kms_helper.ko` into the kernel, so a module set that has them
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as modules is needed; see step 0. A further 45 are dma-fence, dma-buf and
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dma-resv. The rest is ordinary kernel: memory, locks, workqueues, timers, PCI,
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ACPI, firmware loading, i2c, backlight.
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## What LDLS lacks, and must grow
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LDLS today assumes one thread, no-op locks and a polled interrupt handler
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(ldls.md, "What it costs"). amdgpu cannot work under that:
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1. **Threads.** `kthread_run`, workqueues with real worker threads,
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`gpu-sched`'s scheduler threads. Map them to pthreads in the driver process.
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2. **Locks that lock.** mutex, spinlock, rwsem, `ww_mutex` (TTM and dma-resv
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depend on wound/wait semantics), completions and wait queues, on futexes.
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3. **Interrupts as interrupts.** A thread blocked in `read()` on `/dev/pci`
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calls the handler and any threaded handler, instead of the poll loop.
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4. **Timers.** `timer_list`, `hrtimer`, `delayed_work` and `jiffies`, from a
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timer thread.
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5. **Memory with physical addresses.** TTM allocates system pages for GTT and
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maps them for the GPU. Without an IOMMU, DMA address equals physical
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address, so the `PCIIOC_DMA` arena has to grow to hundreds of megabytes,
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with `struct page` (vmemmap) coverage for all of it.
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6. **Firmware.** `request_firmware` reads `/lib/firmware/amdgpu/*.bin`, from
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Alpine's `linux-firmware-amdgpu` package.
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7. **ACPI.** On an APU the VBIOS comes from the VFCT table, and amdgpu calls
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ATIF/ATCS. A small `/dev/acpi` (read a table, evaluate a method through
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uACPI) covers both.
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Each of these is general LDLS work, not amdgpu work. Every later driver needs
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the same pieces: e1000e, iwlwifi, nouveau.
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## The new kernel piece: `/dev/dri` served by a process
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Mesa and Xorg open `/dev/dri/card0` and `/dev/dri/renderD128`, then use
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`ioctl`, `mmap` and `poll`, and pass buffers to each other as dma-buf file
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descriptors. The kernel has to forward each of those to the driver process:
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- **open, ioctl, poll, close** go to the server process with the client's
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identity attached, the same pattern `/dev/netdev` uses for NICs, extended to
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ioctls with their argument buffers.
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- **mmap** is the part the kernel must do itself. The server replies with
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physical ranges (VRAM through the BAR, or GTT pages from the arena), and the
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kernel maps those into the client's address space.
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- **dma-buf** becomes a kernel file object naming a buffer the server owns.
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`SCM_RIGHTS` passes it between processes (already built; see
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[networking.md](../architecture/networking.md#descriptor-passing)), and X's DRI3 needs exactly that.
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- **sysfs**: `/sys/class/drm/card0`, `/sys/dev/char/226:*`, and the
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vendor/device files that libdrm and Mesa read to pick a driver.
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## Order
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Each step ends with something you can run.
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0. **One module set.** Build Linux 6.18 from source, once, with `DRM=m`,
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`DRM_AMDGPU=m` and BTF on. That gives `drm.ko`, `drm_kms_helper.ko` and the
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rest, plus a matching `vmlinux` BTF. `make ldls-modules` does the build, and
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packaging it as an apk makes it installable.
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1. **LDLS concurrency and memory** (lacks 1–5 above), proven on **r8169**.
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The real Linux r8169 brings PHY tuning, the firmware patch, ASPM handling
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and every RTL8168 variant. It is the laptop NIC, so this step also fixes the
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network properly. Done when the laptop's ethernet runs at gigabit speed
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through `ldls0`.
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2. **amdgpu probes.** The driver process loads firmware, brings up the IP
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blocks and takes the display from the EFI framebuffer. The kernel console
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must hand over the screen and get it back if the process dies. Done when
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`dmesg` shows amdgpu's "SMU is initialized" and the panel stays lit.
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3. **KMS.** `/dev/dri/card0` with ioctl forwarding and the mmap path. Done
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when `modetest` (Alpine `libdrm-tests`) sets a mode and draws a frame.
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4. **Rendering.** `renderD128`, GEM mmap, dma-buf, syncobj. Done when
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`eglinfo` names radeonsi and `glmark2-es2-drm` and `vkcube` (RADV) run.
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5. **The desktop.** Xorg's modesetting driver with glamor, then Epiphany with
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GL compositing. Done when the fans stay quiet while a page scrolls.
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## The RTX 4050
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Later, and in two parts:
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- **Powered off, now.** Without a driver the dGPU sits in D0 and draws power.
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Putting it in D3cold through its ACPI power resources (`_PR3` / `_OFF`) is
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a few dozen lines in the kernel and helps the heat and the battery today.
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- **Driven, after amdgpu.** nouveau with GSP firmware (Ada needs it), through
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the same LDLS and `/dev/dri` machinery, for PRIME render offload.
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## Not GPU, but also why the laptop runs hot
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Until now, idle CPUs halted at C1 and never reached the deep states listed in
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ACPI `_CST`. That kept every core powered at an idle shell prompt. It is fixed
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alongside this plan: the scheduler's idle path now enters `_CST`'s C2 (CC6 on
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Zen). P-state control (AMD CPPC) is still missing, so the cores run at the
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firmware's default clock under load.
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