vn_ring.c

/*
 * Copyright 2021 Google LLC
 * SPDX-License-Identifier: MIT
 */

#include "vn_ring.h"

#include "vn_renderer.h"

/* must be power-of-two */
#define VN_RING_BUFFER_SIZE (1u << 11)
#define VN_RING_BUFFER_MASK (VN_RING_BUFFER_SIZE - 1)

enum vn_ring_status_flag {
   VN_RING_STATUS_IDLE = 1u << 0,
};

static uint32_t
vn_ring_load_head(const struct vn_ring *ring)
{
   /* the renderer is expected to store the head with memory_order_release,
    * forming a release-acquire ordering
    */
   return atomic_load_explicit(ring->shared.head, memory_order_acquire);
}

static void
vn_ring_store_tail(struct vn_ring *ring)
{
   /* the renderer is expected to load the tail with memory_order_acquire,
    * forming a release-acquire ordering
    */
   return atomic_store_explicit(ring->shared.tail, ring->cur,
                                memory_order_release);
}

static uint32_t
vn_ring_load_status(const struct vn_ring *ring)
{
   /* this must be called and ordered after vn_ring_store_tail */
   return atomic_load_explicit(ring->shared.status, memory_order_seq_cst);
}

static void
vn_ring_write_buffer(struct vn_ring *ring, const void *data, size_t size)
{
   assert(ring->cur + size - vn_ring_load_head(ring) <= VN_RING_BUFFER_SIZE);

   const size_t offset = ring->cur & VN_RING_BUFFER_MASK;
   if (offset + size <= VN_RING_BUFFER_SIZE) {
      memcpy(ring->shared.buffer + offset, data, size);
   } else {
      const size_t s = VN_RING_BUFFER_SIZE - offset;
      memcpy(ring->shared.buffer + offset, data, s);
      memcpy(ring->shared.buffer, data + s, size - s);
   }

   ring->cur += size;
}

static bool
vn_ring_ge_seqno(const struct vn_ring *ring, uint32_t a, uint32_t b)
{
   /* this can return false negative when not called fast enough (e.g., when
    * called once every couple hours), but following calls with larger a's
    * will correct itself
    *
    * TODO use real seqnos?
    */
   if (a >= b)
      return ring->cur >= a || ring->cur < b;
   else
      return ring->cur >= a && ring->cur < b;
}

static void
vn_ring_retire_submits(struct vn_ring *ring, uint32_t seqno)
{
   list_for_each_entry_safe(struct vn_ring_submit, submit, &ring->submits,
                            head) {
      if (!vn_ring_ge_seqno(ring, seqno, submit->seqno))
         break;

      for (uint32_t i = 0; i < submit->shmem_count; i++)
         vn_renderer_shmem_unref(ring->renderer, submit->shmems[i]);

      list_del(&submit->head);
      list_add(&submit->head, &ring->free_submits);
   }
}

static uint32_t
vn_ring_wait_seqno(const struct vn_ring *ring, uint32_t seqno)
{
   /* A renderer wait incurs several hops and the renderer might poll
    * repeatedly anyway.  Let's just poll here.
    */
   uint32_t iter = 0;
   do {
      const uint32_t head = vn_ring_load_head(ring);
      if (vn_ring_ge_seqno(ring, head, seqno))
         return head;
      vn_relax(&iter);
   } while (true);
}

static uint32_t
vn_ring_wait_space(const struct vn_ring *ring, uint32_t size)
{
   assert(size <= VN_RING_BUFFER_SIZE);

   /* see the reasoning in vn_ring_wait_seqno */
   uint32_t iter = 0;
   do {
      const uint32_t head = vn_ring_load_head(ring);
      if (ring->cur + size - head <= VN_RING_BUFFER_SIZE)
         return head;
      vn_relax(&iter);
   } while (true);
}

void
vn_ring_get_layout(size_t extra_size, struct vn_ring_layout *layout)
{
   /* this can be changed/extended quite freely */
   struct layout {
      uint32_t head __attribute__((aligned(64)));
      uint32_t tail __attribute__((aligned(64)));
      uint32_t status __attribute__((aligned(64)));

      uint8_t buffer[] __attribute__((aligned(64)));
   };
   const size_t buf_size = VN_RING_BUFFER_SIZE;

   assert(buf_size && util_is_power_of_two_or_zero(buf_size));

   layout->head_offset = offsetof(struct layout, head);
   layout->tail_offset = offsetof(struct layout, tail);
   layout->status_offset = offsetof(struct layout, status);

   layout->buffer_offset = offsetof(struct layout, buffer);
   layout->buffer_size = buf_size;

   layout->extra_offset = layout->buffer_offset + layout->buffer_size;
   layout->extra_size = extra_size;

   layout->shmem_size = layout->extra_offset + layout->extra_size;
}

void
vn_ring_init(struct vn_ring *ring,
             struct vn_renderer *renderer,
             const struct vn_ring_layout *layout,
             void *shared)
{
   memset(ring, 0, sizeof(*ring));
   memset(shared, 0, layout->shmem_size);

   ring->renderer = renderer;

   ring->shared.head = shared + layout->head_offset;
   ring->shared.tail = shared + layout->tail_offset;
   ring->shared.status = shared + layout->status_offset;
   ring->shared.buffer = shared + layout->buffer_offset;
   ring->shared.extra = shared + layout->extra_offset;

   list_inithead(&ring->submits);
   list_inithead(&ring->free_submits);
}

void
vn_ring_fini(struct vn_ring *ring)
{
   vn_ring_retire_submits(ring, ring->cur);
   assert(list_is_empty(&ring->submits));

   list_for_each_entry_safe(struct vn_ring_submit, submit,
                            &ring->free_submits, head)
      free(submit);
}

struct vn_ring_submit *
vn_ring_get_submit(struct vn_ring *ring, uint32_t shmem_count)
{
   const uint32_t min_shmem_count = 2;
   struct vn_ring_submit *submit;

   /* TODO this could be simplified if we could omit shmem_count */
   if (shmem_count <= min_shmem_count &&
       !list_is_empty(&ring->free_submits)) {
      submit =
         list_first_entry(&ring->free_submits, struct vn_ring_submit, head);
      list_del(&submit->head);
   } else {
      shmem_count = MAX2(shmem_count, min_shmem_count);
      submit =
         malloc(sizeof(*submit) + sizeof(submit->shmems[0]) * shmem_count);
   }

   return submit;
}

bool
vn_ring_submit(struct vn_ring *ring,
               struct vn_ring_submit *submit,
               const void *cs_data,
               size_t cs_size,
               uint32_t *seqno)
{
   const uint32_t cur_seqno = vn_ring_wait_space(ring, cs_size);
   vn_ring_write_buffer(ring, cs_data, cs_size);
   vn_ring_store_tail(ring);
   const bool notify = vn_ring_load_status(ring) & VN_RING_STATUS_IDLE;

   vn_ring_retire_submits(ring, cur_seqno);

   submit->seqno = ring->cur;
   list_addtail(&submit->head, &ring->submits);

   *seqno = submit->seqno;
   return notify;
}

/**
 * This is thread-safe.
 */
void
vn_ring_wait(const struct vn_ring *ring, uint32_t seqno)
{
   vn_ring_wait_seqno(ring, seqno);
}