请教帮忙分析关于位图的一个问题?

alphaxiang 2011-04-16 03:37:59

大家好,最近我看到一段代码,没怎么明白其中的原理,大家有空的请帮我看看并解释下其中的原理.
(代码来至:Linux ATM driver for Motorola MPC8260 下载地址:http://sourceforge.net/projects/atm8260/)

这段代码的功能我觉得是通过位图来记录0~N之间的数的使用情况,并提供分配(返回一个尚未被使用的数),释放(释放一个在使用中的数). 但如果功能是我分析的这样,我觉得代码可以写的更简明一些.不知道是不是我想的不对,还请大家帮我看看.

代码如下(其中intpool_init中的第二个参数log2_size二进制位宽,比如要管理的范围为0~127,则intpool_init=7):



typedef struct

{

    ushort *bitmask;

    ulong *chksums;

    int bitmask_sz;

    int chksums_sz;

    int log2_hwords_in_chksum;

    int inuse;

} intpool_t;



void intpool_init(intpool_t *ip, int log2_size)

{

    int log2_bitmask_sz;

    int log2_chksums_sz;

    int log2_hwords_in_chksum;

    int i;



    // how many ushorts do we need to store 1<<log2_size bitflags?

    log2_bitmask_sz = log2_size - 4;

    if (log2_bitmask_sz < 0)

        log2_bitmask_sz = 0;



    // how many checksums do we need and how many ushorts should each checksum cover

    log2_chksums_sz = log2_bitmask_sz/2;

    log2_hwords_in_chksum = log2_bitmask_sz - (log2_bitmask_sz/2);



    // Save the results in object

    ip->bitmask_sz =  1<<log2_bitmask_sz;

    ip->chksums_sz =  1<<log2_chksums_sz;

    ip->log2_hwords_in_chksum =  log2_hwords_in_chksum;



    // Allocate space for the bitmasks and the checksums

    ip->bitmask = kmalloc(ip->bitmask_sz * sizeof(ushort), GFP_KERNEL);

    ip->chksums = kmalloc(ip->chksums_sz * sizeof(ulong), GFP_KERNEL);



    // Clear the bitmasks and the checksums

    memset(ip->bitmask, 0, ip->bitmask_sz * sizeof(ushort));

    memset(ip->chksums, 0, ip->chksums_sz * sizeof(ushort));



    // Keep track of number allocated

    ip->inuse = 0;



    // Allocate all the bits we don't use (to stop them being allocated again!)

    for (i = 1<<log2_size; i < ip->bitmask_sz<<4 ; i++)

    {

        ip->bitmask[i>>4] |= 1<<(i&0xf);

        ip->chksums[(i>>4)>>(ip->log2_hwords_in_chksum)] += 1<<(i&0xf);

    }

    

}

/*

#########################################################

#

#  Function : intpool_destroy

#              

#  Purpose  : destroys a intpool_t object freeing memory

#              

#  Args     : ip = pointer to object 

#              

########################################################## 

#  Edit history: 

#  Who     When    What 

#  AJZ     27Jul01 Created 

########################################################## 

*/ 

void intpool_destroy(intpool_t *ip)

{

    kfree(ip->bitmask);

    kfree(ip->chksums);

}



/*

#########################################################

#

#  Function : intpool_alloc

#              

#  Purpose  : allocated a number that has never

#             been allocated before, or has been recently freed

#              

#  Args     : ip = pointer to object 

#

#  Returns  : smallest free value or -1 for failure              

########################################################## 

#  Edit history: 

#  Who     When    What 

#  AJZ     27Jul01 Created 

########################################################## 

*/ 

int intpool_alloc(intpool_t *ip)

{

    int i;

    int j;

    int k;

    

    // Go through the checksums till we find one that isn't

    // full

    for (i = 0; i < ip->chksums_sz; i++)

    {

        if ((ip->chksums[i]) != (0xffff<<ip->log2_hwords_in_chksum))

            break;

    }



    // did we find one?

    if (i == ip->chksums_sz)

        return -1; // No!



    // Yes

    // There must be a clear bit in ip->bitmask[i<<ip->log2_hwords_in_chksum] to 

    //  ip->bitmask[((i+1)<<ip->log2_hwords_in_chksum)-1]

    for (j = i<<(ip->log2_hwords_in_chksum); 

         j < (i+1)<<(ip->log2_hwords_in_chksum);

         j++)

    {

        if (ip->bitmask[j] != 0xffff)

            break;

    }



    // Note: We must have found j in the range!

    // Find the least significant unset bit

    for (k = 0; k < 16; k++)

    {

        if ((ip->bitmask[j] & (1<<k)) == 0)

        {

            // set bit

            ip->bitmask[j] |= (1<<k);



            // update chksum

            ip->chksums[i] += (1<<k);



            // update alloc count

            ip->inuse++;



            // return the number

            return k + (j<<4);

        }

    }



    // This is impossible

    printk(KERN_ERR "Error: (" __FUNCTION__ ") something impossible happened\n");

    return -1;

}



/*

#########################################################

#

#  Function : intpool_free

#              

#  Purpose  : free a currently allocated number

#              

#  Args     : ip = pointer to objeip; val = number to free

#

########################################################## 

#  Edit history: 

#  Who     When    What 

#  AJZ     27Jul01 Created 

########################################################## 

*/ 

void intpool_free(intpool_t *ip, int val)

{

    // Don't do anything if val is out of range

    if ((val>>4) >= ip->bitmask_sz)

        return;



    // Don't do anything if not already allocated

    if (ip->bitmask[val>>4] & (1<<(val&0xf)))

    {

        ip->bitmask[val>>4] ^= 1<<(val&0xf);

        ip->chksums[(val>>4)>>(ip->log2_hwords_in_chksum)] -= 1<<(val&0xf);

    }



    // update alloc count

    ip->inuse--;

}



/*

#########################################################

#

#  Function : intpool_inuse

#             

#  Purpose  : return the number of values allocated

#             

#  Args     : ip = pointer to intpool

#             

##########################################################

#  Edit history:

#  Who     When    What

#  AJZ     19Nov01 Created

##########################################################

*/

int intpool_inuse(intpool_t *ip)

{

    return ip->inuse;

}