gpu: nvgpu: implement lockless allocator

Implement a lockless allocator for fixed-size data
structures.

Bug 1795076

Change-Id: I70a5f52cbdb4452cc0fd9a8edf26735be29ede57
Signed-off-by: Sachit Kadle <skadle@nvidia.com>
Reviewed-on: http://git-master/r/1213211
(cherry picked from commit e4bff7da0f39c8f4b5691169c02e482bc9d4166e)
Reviewed-on: http://git-master/r/1223246
GVS: Gerrit_Virtual_Submit
Reviewed-by: Terje Bergstrom <tbergstrom@nvidia.com>
Tested-by: Terje Bergstrom <tbergstrom@nvidia.com>

drivers/gpu/nvgpu/gk20a/lockless_allocator_priv.h

@@ -0,0 +1,121 @@
+/*
+ * Copyright (c) 2016, NVIDIA CORPORATION.  All rights reserved.
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms and conditions of the GNU General Public License,
+ * version 2, as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/*
+ * Basics:
+ *
+ *    - Lockless memory allocator for fixed-size structures, whose
+ *      size is defined up front at init time.
+ *    - Memory footprint scales linearly w/ the number of structures in
+ *      the pool. It is ~= sizeof(int) * N.
+ *    - Memory is pre-allocated by the client. The allocator itself
+ *      only computes the addresses for allocations.
+ *    - Limit of MAX_INT nodes that the allocator can be responsible for.
+ *
+ * Implementation details:
+ *
+ *    The allocator maintains a single list of free nodes. We allocate &
+ *    free nodes from the head of the list. We rely on the cmpxchg() operator
+ *    to maintain atomicity on the head.
+ *
+ *    So, both allocs & frees are O(1)!!
+ *
+ *    -- Definitions --
+ *    Block Size - size of a single structure that this allocator will
+ *                 allocate.
+ *    Node       - one of the elements of size blk_size in the
+ *                 client-allocated buffer.
+ *    Node Index - zero-based index of a node in the client-allocated
+ *                 contiguous buffer.
+ *
+ *    -- Initial State --
+ *    We maintain the following to track the state of the free list:
+ *
+ *    1) A "head" index to track the index of the first free node in the list
+ *    2) A "next" array to track the index of the next free node in the list
+ *       for every node. So next[head], will give the index to the 2nd free
+ *       element in the list.
+ *
+ *    So, to begin with, the free list consists of all node indices, and each
+ *    position in the next array contains index N + 1:
+ *
+ *    head = 0
+ *    next = [1, 2, 3, 4, -1] : Example for a user-allocated buffer of 5 nodes
+ *    free_list = 0->1->2->3->4->-1
+ *
+ *    -- Allocations --
+ *    1) Read the current head (aka acq_head)
+ *    2) Read next[acq_head], to get the 2nd free element (aka new_head)
+ *    3) cmp_xchg(&head, acq_head, new_head)
+ *    4) If it succeeds, compute the address of the node, based on
+ *       base address, blk_size, & acq_head.
+ *
+ *    head = 1;
+ *    next = [1, 2, 3, 4, -1] : Example after allocating Node #0
+ *    free_list = 1->2->3->4->-1
+ *
+ *    head = 2;
+ *    next = [1, 2, 3, 4, -1] : Example after allocating Node #1
+ *    free_list = 2->3->4->-1
+ *
+ *    -- Frees --
+ *    1) Based on the address to be freed, calculate the index of the node
+ *       being freed (cur_idx)
+ *    2) Read the current head (old_head)
+ *    3) So the freed node is going to go at the head of the list, and we
+ *       want to put the old_head after it. So next[cur_idx] = old_head
+ *    4) cmpxchg(head, old_head, cur_idx)
+ *
+ *    head = 0
+ *    next = [2, 2, 3, 4, -1]
+ *    free_list = 0->2->3->4->-1 : Example after freeing Node #0
+ *
+ *    head = 1
+ *    next = [2, 0, 3, 4, -1]
+ *    free_list = 1->0->2->3->4->-1 : Example after freeing Node #1
+ */
+
+#ifndef LOCKLESS_ALLOCATOR_PRIV_H
+#define LOCKLESS_ALLOCATOR_PRIV_H
+
+struct gk20a_allocator;
+
+struct gk20a_lockless_allocator {
+	struct gk20a_allocator *owner;
+
+	u64 base;		/* Base address of the space. */
+	u64 length;		/* Length of the space. */
+	u64 blk_size;		/* Size of the structure being allocated */
+	int nr_nodes;		/* Number of nodes available for allocation */
+
+	int *next;		/* An array holding the next indices per node */
+	int head;		/* Current node at the top of the stack */
+
+	u64 flags;
+
+	bool inited;
+
+	/* Statistics */
+	atomic_t nr_allocs;
+};
+
+static inline struct gk20a_lockless_allocator *lockless_allocator(
+	struct gk20a_allocator *a)
+{
+	return (struct gk20a_lockless_allocator *)(a)->priv;
+}
+
+#endif