SchOS/include/dev/virtio.h

#pragma once

// virtio 1.0 over PCI — the transport, and virtio-gpu's 2D protocol.
//
// It lived next to the driver while exactly one driver read it, with a note
// saying it should move when a second reader appeared. One has: cmd/vgpud is
// the same device driven from userspace (docs/architecture/devices.md's first
// eviction), and it needs the same register offsets and the same commands.
//
// Still one file rather than transport-and-protocol split in two. Both readers
// want both halves, so a seam here would be drawn for the sake of having one.
// virtio-net or virtio-blk arriving is when that changes.
//
// Written from the OASIS specification (virtio 1.1, chapters 2, 4.1 and 5.7),
// which is a public document with no licence on what you write from it. Same
// reasoning as kernel/drivers/xhci.c.
//
// Why virtio-gpu at all, when there is already a framebuffer: the VBE
// framebuffer is a flat block of memory the loader happened to be told about.
// It has no mode setting, no notion of a frame being complete, and no way to
// ask the host to present anything -- so a compositor drawing into it is
// racing the scanout with no way to synchronise. virtio-gpu has resources,
// explicit transfer and an explicit flush, which is the model every real GPU
// driver has. It is the smallest device that teaches the right shape.

// --- PCI transport ---------------------------------------------------------

#define VIRTIO_VENDOR        0x1AF4
// Modern (1.0) device IDs are 0x1040 + the virtio device type. GPU is type 16.
#define VIRTIO_DEV_GPU       0x1050

// Vendor-specific PCI capability, which is how a virtio device says where in
// its BARs each configuration structure lives.
#define PCI_CAP_ID_VNDR      0x09

#define VIRTIO_PCI_CAP_COMMON_CFG  1
#define VIRTIO_PCI_CAP_NOTIFY_CFG  2
#define VIRTIO_PCI_CAP_ISR_CFG     3
#define VIRTIO_PCI_CAP_DEVICE_CFG  4

// Offsets within the common configuration structure. Written out rather than
// declared as a struct: the fields are little-endian by specification and of
// mixed width, and a struct invites the compiler to pad or a caller to forget
// that these are MMIO and must be accessed at exactly the stated width.
#define VCOMMON_DFEAT_SEL    0x00   // le32
#define VCOMMON_DFEAT        0x04   // le32
#define VCOMMON_GFEAT_SEL    0x08   // le32
#define VCOMMON_GFEAT        0x0C   // le32
#define VCOMMON_MSIX_CFG     0x10   // le16
#define VCOMMON_NUM_QUEUES   0x12   // le16
#define VCOMMON_STATUS       0x14   // u8
#define VCOMMON_CFG_GEN      0x15   // u8
#define VCOMMON_Q_SELECT     0x16   // le16
#define VCOMMON_Q_SIZE       0x18   // le16
#define VCOMMON_Q_MSIX       0x1A   // le16
#define VCOMMON_Q_ENABLE     0x1C   // le16
#define VCOMMON_Q_NOTIFY_OFF 0x1E   // le16
#define VCOMMON_Q_DESC       0x20   // le64
#define VCOMMON_Q_DRIVER     0x28   // le64
#define VCOMMON_Q_DEVICE     0x30   // le64

// device_status bits, set in this order during bring-up.
#define VSTATUS_ACK          1
#define VSTATUS_DRIVER       2
#define VSTATUS_DRIVER_OK    4
#define VSTATUS_FEATURES_OK  8
#define VSTATUS_FAILED       0x80

#define VIRTIO_F_VERSION_1   32     // feature bit: this is a 1.0 device

// --- split virtqueue -------------------------------------------------------

#define VRING_DESC_F_NEXT    1

// Set in vring_avail.flags to tell the device not to interrupt on completion.
//
// A driver that polls the used ring wants this, and wants it before the queue
// is enabled. Without it the device raises its interrupt line on every
// completed command -- and if nothing is registered to acknowledge it, a
// level-triggered PCI line stays asserted and re-fires for ever. That is the
// failure in docs/history/kernel.md section 13, and cmd/vgpud reproduced it
// exactly on its first working run.
#define VRING_AVAIL_F_NO_INTERRUPT 1
#define VRING_DESC_F_WRITE   2      // device writes it; i.e. this is our buffer

struct vring_desc {
  uint64 addr;
  uint32 len;
  uint16 flags;
  uint16 next;
} __attribute__((packed));

struct vring_avail {
  uint16 flags;
  uint16 idx;
  uint16 ring[];
} __attribute__((packed));

struct vring_used_elem {
  uint32 id;
  uint32 len;
} __attribute__((packed));

struct vring_used {
  uint16 flags;
  uint16 idx;
  struct vring_used_elem ring[];
} __attribute__((packed));

// --- virtio-gpu, 2D --------------------------------------------------------

#define VIRTIO_GPU_CMD_GET_DISPLAY_INFO      0x0100
#define VIRTIO_GPU_CMD_RESOURCE_CREATE_2D    0x0101
#define VIRTIO_GPU_CMD_RESOURCE_UNREF        0x0102
#define VIRTIO_GPU_CMD_SET_SCANOUT           0x0103
#define VIRTIO_GPU_CMD_RESOURCE_FLUSH        0x0104
#define VIRTIO_GPU_CMD_TRANSFER_TO_HOST_2D   0x0105
#define VIRTIO_GPU_CMD_RESOURCE_ATTACH_BACKING 0x0106

#define VIRTIO_GPU_RESP_OK_NODATA            0x1100
#define VIRTIO_GPU_RESP_OK_DISPLAY_INFO      0x1101

#define VIRTIO_GPU_MAX_SCANOUTS 16

// B8G8R8X8, which is what a 32-bit little-endian 0x00RRGGBB word looks like in
// memory -- the same layout vga.c already draws into, so a framebuffer can be
// handed over without a conversion pass.
#define VIRTIO_GPU_FORMAT_B8G8R8X8_UNORM 2

struct virtio_gpu_ctrl_hdr {
  uint32 type;
  uint32 flags;
  uint64 fence_id;
  uint32 ctx_id;
  uint32 padding;
} __attribute__((packed));

struct virtio_gpu_rect {
  uint32 x, y, width, height;
} __attribute__((packed));

struct virtio_gpu_display_one {
  struct virtio_gpu_rect r;
  uint32 enabled;
  uint32 flags;
} __attribute__((packed));

struct virtio_gpu_resp_display_info {
  struct virtio_gpu_ctrl_hdr hdr;
  struct virtio_gpu_display_one pmodes[VIRTIO_GPU_MAX_SCANOUTS];
} __attribute__((packed));

struct virtio_gpu_resource_create_2d {
  struct virtio_gpu_ctrl_hdr hdr;
  uint32 resource_id;
  uint32 format;
  uint32 width;
  uint32 height;
} __attribute__((packed));

// Releasing a resource, which a modeset has to do before it can create one of
// a different size. The backing pages are detached with it; they belong to the
// driver and are not freed by this.
struct virtio_gpu_resource_unref {
  struct virtio_gpu_ctrl_hdr hdr;
  uint32 resource_id;
  uint32 padding;
} __attribute__((packed));

struct virtio_gpu_mem_entry {
  uint64 addr;
  uint32 length;
  uint32 padding;
} __attribute__((packed));

struct virtio_gpu_resource_attach_backing {
  struct virtio_gpu_ctrl_hdr hdr;
  uint32 resource_id;
  uint32 nr_entries;
} __attribute__((packed));

struct virtio_gpu_set_scanout {
  struct virtio_gpu_ctrl_hdr hdr;
  struct virtio_gpu_rect r;
  uint32 scanout_id;
  uint32 resource_id;
} __attribute__((packed));

struct virtio_gpu_transfer_to_host_2d {
  struct virtio_gpu_ctrl_hdr hdr;
  struct virtio_gpu_rect r;
  uint64 offset;
  uint32 resource_id;
  uint32 padding;
} __attribute__((packed));

struct virtio_gpu_resource_flush {
  struct virtio_gpu_ctrl_hdr hdr;
  struct virtio_gpu_rect r;
  uint32 resource_id;
  uint32 padding;
} __attribute__((packed));