/* SPDX-License-Identifier: GPL-2.0-only */
/*
 * Copyright 2024 Google LLC
 *
 * dbitmap - dynamically sized bitmap library.
 *
 * Used by the binder driver to optimize the allocation of the smallest
 * available descriptor ID. Each bit in the bitmap represents the state
 * of an ID.
 *
 * A dbitmap can grow or shrink as needed. This part has been designed
 * considering that users might need to briefly release their locks in
 * order to allocate memory for the new bitmap. These operations then,
 * are verified to determine if the grow or shrink is sill valid.
 *
 * This library does not provide protection against concurrent access
 * by itself. Binder uses the proc->outer_lock for this purpose.
 */

#ifndef _LINUX_DBITMAP_H
#define _LINUX_DBITMAP_H
#include <linux/bitmap.h>

#define NBITS_MIN       BITS_PER_TYPE(unsigned long)

struct dbitmap {
        unsigned int nbits;
        unsigned long *map;
};

static inline int dbitmap_enabled(struct dbitmap *dmap)
{
        return !!dmap->nbits;
}

static inline void dbitmap_free(struct dbitmap *dmap)
{
        dmap->nbits = 0;
        kfree(dmap->map);
        dmap->map = NULL;
}

/* Returns the nbits that a dbitmap can shrink to, 0 if not possible. */
static inline unsigned int dbitmap_shrink_nbits(struct dbitmap *dmap)
{
        unsigned int bit;

        if (dmap->nbits <= NBITS_MIN)
                return 0;

        /*
         * Determine if the bitmap can shrink based on the position of
         * its last set bit. If the bit is within the first quarter of
         * the bitmap then shrinking is possible. In this case, the
         * bitmap should shrink to half its current size.
         */
        bit = find_last_bit(dmap->map, dmap->nbits);
        if (bit < (dmap->nbits >> 2))
                return dmap->nbits >> 1;

        /* find_last_bit() returns dmap->nbits when no bits are set. */
        if (bit == dmap->nbits)
                return NBITS_MIN;

        return 0;
}

/* Replace the internal bitmap with a new one of different size */
static inline void
dbitmap_replace(struct dbitmap *dmap, unsigned long *new, unsigned int nbits)
{
        bitmap_copy(new, dmap->map, min(dmap->nbits, nbits));
        kfree(dmap->map);
        dmap->map = new;
        dmap->nbits = nbits;
}

static inline void
dbitmap_shrink(struct dbitmap *dmap, unsigned long *new, unsigned int nbits)
{
        if (!new)
                return;

        /*
         * Verify that shrinking to @nbits is still possible. The @new
         * bitmap might have been allocated without locks, so this call
         * could now be outdated. In this case, free @new and move on.
         */
        if (!dbitmap_enabled(dmap) || dbitmap_shrink_nbits(dmap) != nbits) {
                kfree(new);
                return;
        }

        dbitmap_replace(dmap, new, nbits);
}

/* Returns the nbits that a dbitmap can grow to. */
static inline unsigned int dbitmap_grow_nbits(struct dbitmap *dmap)
{
        return dmap->nbits << 1;
}

static inline void
dbitmap_grow(struct dbitmap *dmap, unsigned long *new, unsigned int nbits)
{
        /*
         * Verify that growing to @nbits is still possible. The @new
         * bitmap might have been allocated without locks, so this call
         * could now be outdated. In this case, free @new and move on.
         */
        if (!dbitmap_enabled(dmap) || nbits <= dmap->nbits) {
                kfree(new);
                return;
        }

        /*
         * Check for ENOMEM after confirming the grow operation is still
         * required. This ensures we only disable the dbitmap when it's
         * necessary. Once the dbitmap is disabled, binder will fallback
         * to slow_desc_lookup_olocked().
         */
        if (!new) {
                dbitmap_free(dmap);
                return;
        }

        dbitmap_replace(dmap, new, nbits);
}

/*
 * Finds and sets the next zero bit in the bitmap. Upon success @bit
 * is populated with the index and 0 is returned. Otherwise, -ENOSPC
 * is returned to indicate that a dbitmap_grow() is needed.
 */
static inline int
dbitmap_acquire_next_zero_bit(struct dbitmap *dmap, unsigned long offset,
                              unsigned long *bit)
{
        unsigned long n;

        n = find_next_zero_bit(dmap->map, dmap->nbits, offset);
        if (n == dmap->nbits)
                return -ENOSPC;

        *bit = n;
        set_bit(n, dmap->map);

        return 0;
}

static inline void
dbitmap_clear_bit(struct dbitmap *dmap, unsigned long bit)
{
        clear_bit(bit, dmap->map);
}

static inline int dbitmap_init(struct dbitmap *dmap)
{
        dmap->map = bitmap_zalloc(NBITS_MIN, GFP_KERNEL);
        if (!dmap->map) {
                dmap->nbits = 0;
                return -ENOMEM;
        }

        dmap->nbits = NBITS_MIN;

        return 0;
}
#endif