SchOS/kernel/drivers/audio/ac97.c

// AC'97, the audio class's first driver.
//
// Written from the Intel 82801 datasheet's audio chapter. It is the simplest
// PC audio device that exists on real machines and in QEMU both, which is why
// it is first: the point of this driver is to prove the class and /dev/dsp
// with the least chip-specific code possible. Intel HDA -- which is what a
// laptop built this century actually has -- is a much larger driver, and
// writing it against an interface nothing had ever played a sound through
// would be writing two unproven things at once.
//
// Two register files, both **I/O ports** rather than MMIO:
//
//   NAM   BAR0, the mixer. Volume, and the codec's own capabilities.
//   NABM  BAR1, bus mastering. The descriptor list, and the DMA engine that
//         walks it.
//
// The DMA engine reads a *buffer descriptor list*: 32 entries, each naming a
// physical address and a length in samples. It plays entry CIV, and stops when
// it reaches LVI -- the "last valid index". Playing continuously is therefore
// not a matter of starting it and walking away; it is a matter of keeping LVI
// ahead of CIV, which is what queue() does.

#include "types.h"
#include "defs.h"
#include "param.h"
#include "x86.h"
#include "spinlock.h"
#include "sleeplock.h"
#include "fs.h"
#include "file.h"
#include "memlayout.h"
#include "mmu.h"
#include "pci.h"
#include "device.h"
#include "audio.h"
#include "dma.h"
#include "irq.h"

// --- NAM, the mixer ---------------------------------------------------------

#define NAM_RESET          0x00
#define NAM_MASTER_VOL     0x02
#define NAM_PCM_OUT_VOL    0x18
#define NAM_EXT_ID         0x28
#define NAM_EXT_CTRL       0x2A
#define NAM_PCM_FRONT_RATE 0x2C

#define EXT_VRA            0x0001   // variable rate: the codec can do 44100

// --- NABM, the bus master ---------------------------------------------------
//
// The PCM *out* box starts at 0x10. There are three boxes (in, out, mic) and
// they have the same shape, which is why the offsets below are written as a
// base plus a field rather than as constants.

#define PO_BASE   0x10
#define PO_BDBAR  (PO_BASE + 0x00)   // uint32: physical address of the list
#define PO_CIV    (PO_BASE + 0x04)   // uint8:  entry being played now
#define PO_LVI    (PO_BASE + 0x05)   // uint8:  last entry the engine may play
#define PO_SR     (PO_BASE + 0x06)   // uint16: status
#define PO_PICB   (PO_BASE + 0x08)   // uint16: samples left in this entry
#define PO_CR     (PO_BASE + 0x0B)   // uint8:  control

#define CR_RPBM   0x01    // run
#define CR_RR     0x02    // reset the box
#define CR_IOCE   0x10    // interrupt on completion

#define SR_DCH    0x01    // DMA halted
#define SR_BCIS   0x08    // buffer completion interrupt
#define SR_LVBCI  0x04    // last valid buffer played

#define GLOB_CTRL 0x2C
#define GLOB_STA  0x30

// --- the ring ---------------------------------------------------------------
//
// 32 descriptors of 4 KB each: 128 KB, which at 48 kHz stereo is about
// two thirds of a second. Long enough that a writer can fall behind for a
// scheduling quantum without a gap, short enough that stopping is prompt.

#define NDESC     32
#define DESC_BYTES 4096

struct bdl_entry {
  uint32 addr;
  uint16 samples;     // *samples*, not bytes: 16-bit words, so bytes/2
  uint16 flags;
};

#define BDL_IOC   0x8000    // interrupt when this one finishes

struct ac97 {
  uint16 nam, nabm;         // I/O port bases
  struct bdl_entry *bdl;    // NDESC entries, DMA memory
  uint8 *buf;               // NDESC * DESC_BYTES, DMA memory
  uint32 bdl_phys, buf_phys;

  int    head;              // next descriptor to fill
  int    tail;              // oldest one still queued
  int    filled[NDESC];     // bytes in each, 0 when free
  int    running;
  struct spinlock lock;
  struct pci_dev *pdev;
};

static struct ac97 ac97dev;

static struct ac97 *
of(struct device *d)
{
  (void)d;
  return ac97dev.nabm ? &ac97dev : 0;
}

// --- mixer ------------------------------------------------------------------

// Volume is attenuation: 0 is loudest, and the mute bit is 0x8000. Writing 0
// rather than a "sensible" level on purpose -- the kernel has no business
// deciding how loud a machine is, and a device that comes up silent reads as
// broken to whoever is testing it.
static void
mixer_init(struct ac97 *a)
{
  outw(a->nam + NAM_RESET, 0);
  outw(a->nam + NAM_MASTER_VOL, 0x0000);
  outw(a->nam + NAM_PCM_OUT_VOL, 0x0000);
}

// --- the class ops ----------------------------------------------------------

static int
ac97_configure(struct device *d, struct audio_format *f)
{
  struct ac97 *a = of(d);
  uint16 ext;

  if (!a)
    return -1;

  // Two channels always. AC'97 plays stereo; a mono stream would have to be
  // doubled, and doing that in the driver would hide it from the caller that
  // asked -- which is what the write-back in the ioctl is for.
  f->channels = 2;
  f->bits = AUDIO_FMT_S16LE;

  ext = inw(a->nam + NAM_EXT_ID);
  if (ext & EXT_VRA) {
    outw(a->nam + NAM_EXT_CTRL, inw(a->nam + NAM_EXT_CTRL) | EXT_VRA);
    outw(a->nam + NAM_PCM_FRONT_RATE, (uint16)f->rate);
    // What the codec settled on, which is not always what was asked: it is
    // entitled to round to something it can generate.
    f->rate = inw(a->nam + NAM_PCM_FRONT_RATE);
  } else {
    // No variable rate. 48 kHz is the only thing this codec plays, and saying
    // so is more useful than accepting 44100 and playing it 9% fast.
    f->rate = 48000;
  }

  return 0;
}

// How many descriptors the engine has finished with.
// Descriptors with nothing in them, counted forward from where the next write
// would go. Stops at the first one still holding audio, which is what makes
// this the number of bytes a write can take without waiting.
static int
free_descs(struct ac97 *a)
{
  int n = 0;

  for (int i = 0; i < NDESC; i++) {
    if (a->filled[(a->head + i) % NDESC])
      break;
    n++;
  }
  return n;
}

static int
ac97_space(struct device *d)
{
  struct ac97 *a = of(d);
  int n;

  if (!a)
    return 0;
  acquire(&a->lock);
  n = free_descs(a) * DESC_BYTES;
  release(&a->lock);
  return n;
}

static int
ac97_queue(struct device *d, const void *pcm, int bytes)
{
  struct ac97 *a = of(d);
  int civ, idx, take;

  if (!a)
    return -1;

  acquire(&a->lock);

  civ = inb(a->nabm + PO_CIV) % NDESC;
  idx = a->head;

  // Retire what the engine has finished with.
  //
  // `tail` is the oldest descriptor we still believe is queued; CIV is the one
  // being played. Everything between them is done. Doing this here as well as
  // in the interrupt handler is deliberate -- the handler is an optimisation,
  // and audio that only works when its interrupt arrives is audio that stops
  // dead on a machine whose line is misrouted.
  while (a->tail != civ && a->filled[a->tail]) {
    a->filled[a->tail] = 0;
    a->tail = (a->tail + 1) % NDESC;
  }

  if (a->filled[idx]) {
    release(&a->lock);
    return 0;                     // ring full
  }

  take = bytes > DESC_BYTES ? DESC_BYTES : bytes;
  memmove(a->buf + (uint64)idx * DESC_BYTES, pcm, take);

  a->bdl[idx].addr    = a->buf_phys + (uint32)idx * DESC_BYTES;
  a->bdl[idx].samples = (uint16)(take / 2);      // 16-bit words
  a->bdl[idx].flags   = BDL_IOC;
  a->filled[idx]      = take;
  a->head             = (idx + 1) % NDESC;

  // LVI is the last entry the engine may play. Keeping it at the descriptor
  // just written is what makes this a queue rather than a one-shot.
  outb(a->nabm + PO_LVI, (uint8)idx);

  release(&a->lock);
  return take;
}

static void
ac97_start(struct device *d)
{
  struct ac97 *a = of(d);

  if (!a)
    return;
  acquire(&a->lock);
  if (!a->running) {
    outb(a->nabm + PO_CR, CR_RPBM | CR_IOCE);
    a->running = 1;
  }
  release(&a->lock);
}

static void
ac97_stop(struct device *d)
{
  struct ac97 *a = of(d);

  if (!a)
    return;
  acquire(&a->lock);
  outb(a->nabm + PO_CR, 0);
  outb(a->nabm + PO_CR, CR_RR);       // reset the box: CIV, LVI, PICB to 0
  for (int i = 0; i < NDESC; i++)
    a->filled[i] = 0;
  a->head = 0;
  a->tail = 0;
  a->running = 0;
  release(&a->lock);
}

static int
ac97_pending(struct device *d)
{
  struct ac97 *a = of(d);
  int n = 0;

  if (!a)
    return 0;
  acquire(&a->lock);
  if (a->running && !(inw(a->nabm + PO_SR) & SR_DCH)) {
    for (int i = 0; i < NDESC; i++)
      if (a->filled[i])
        n += a->filled[i];
  }
  release(&a->lock);
  return n;
}

static const struct audio_ops ac97_ops = {
  .configure = ac97_configure,
  .space     = ac97_space,
  .queue     = ac97_queue,
  .start     = ac97_start,
  .stop      = ac97_stop,
  .pending   = ac97_pending,
};

// --- interrupt --------------------------------------------------------------

static void
ac97_intr(void *arg)
{
  struct ac97 *a = arg;
  uint16 sr;

  if (!a || !a->nabm)
    return;

  sr = inw(a->nabm + PO_SR);

  // Acknowledge by writing the bits back. A status bit left set on a
  // level-triggered line is an interrupt that re-fires for ever -- the same
  // shape as the e1000 bug in docs/history/kernel.md section 13.
  if (sr & (SR_BCIS | SR_LVBCI | SR_DCH))
    outw(a->nabm + PO_SR, sr & (SR_BCIS | SR_LVBCI | SR_DCH));

  {
    int civ = inb(a->nabm + PO_CIV) % NDESC;

    // Everything the engine has passed is free again. Through `tail` rather
    // than by clearing one entry, so this and ac97_queue() cannot disagree
    // about which descriptors are in flight.
    while (a->tail != civ && a->filled[a->tail]) {
      a->filled[a->tail] = 0;
      a->tail = (a->tail + 1) % NDESC;
    }
  }

  // The writer is asleep on the tick rather than on this device, so there is
  // nothing to wake -- see audio_wait(). Keeping the handler to "acknowledge
  // and account" is deliberate: an interrupt handler that can be deleted
  // without the audio stopping is one that cannot deadlock the audio either.
}

// --- probe ------------------------------------------------------------------

static int
ac97_probe(struct pci_dev *d)
{
  struct ac97 *a = &ac97dev;

  if (a->nabm)
    return -1;                   // one is enough

  // Both BARs are I/O ports on this device, which is why nothing here maps
  // anything: `is_io` is what says so, and a driver that ioremapped these
  // would be mapping a port number as if it were an address.
  if (!d->bar[0].is_io || !d->bar[1].is_io ||
      !d->bar[0].base || !d->bar[1].base) {
    cprintf("ac97: expected two I/O BARs; not driving this device\n");
    return -1;
  }

  a->nam  = (uint16)d->bar[0].base;
  a->nabm = (uint16)d->bar[1].base;
  a->pdev = d;
  initlock(&a->lock, "ac97");

  pci_enable(d, PCI_CMD_IO | PCI_CMD_BUS_MASTER);

  // The descriptor list and the audio itself, both from the DMA arena: the
  // engine reads them by physical address, and kalloc's pages are adjacent
  // only by chance.
  a->bdl = dma_alloc(NDESC * sizeof(struct bdl_entry), 4096);
  a->buf = dma_alloc(NDESC * DESC_BYTES, 4096);
  if (!a->bdl || !a->buf) {
    cprintf("ac97: no DMA memory for a %d KB ring\n",
            (NDESC * DESC_BYTES) / 1024);
    a->nabm = 0;
    return -1;
  }
  a->bdl_phys = (uint32)dma_phys(a->bdl);
  a->buf_phys = (uint32)dma_phys(a->buf);
  memset(a->bdl, 0, NDESC * sizeof(struct bdl_entry));
  memset(a->buf, 0, NDESC * DESC_BYTES);

  // Cold reset of the whole controller, then the PCM-out box.
  outl(a->nabm + GLOB_CTRL, 0x00000002);
  mixer_init(a);
  outb(a->nabm + PO_CR, CR_RR);
  for (int spin = 0; spin < 100000 && (inb(a->nabm + PO_CR) & CR_RR); spin++)
    ;

  outl(a->nabm + PO_BDBAR, a->bdl_phys);

  if (d->irq != 0xFF && d->irq < 24)
    irq_register(d->irq, ac97_intr, a, "ac97", 1);

  kinfo("ac97: nam %x nabm %x, %d KB ring, irq %d\n",
        a->nam, a->nabm, (NDESC * DESC_BYTES) / 1024, d->irq);
  return 0;
}

static const struct pci_match ac97_match[] = {
  { 0x8086, 0x2415, 0, 0 },     // 82801AA, which is what QEMU's -device AC97 is
  { 0x8086, 0x2425, 0, 0 },     // 82801AB
  { 0x8086, 0x2445, 0, 0 },     // 82801BA
  { 0x8086, 0x24C5, 0, 0 },     // ICH4
  { 0x8086, 0x24D5, 0, 0 },     // ICH5
  { 0x8086, 0x266E, 0, 0 },     // ICH6
  { 0, 0, 0, 0 },
};

const struct pci_driver ac97_driver = {
  .name  = "ac97",
  .devname = "audio",
  .match = ac97_match,
  .ops   = &ac97_ops,
  .probe = ac97_probe,
};