Re: scull device driver query !!

["arshad hussain" <arshad.super@xxxxxxxxx>]

On Jan 6, 2008 6:58 PM, Onkar <onkar.n.m@xxxxxxxxx> wrote:

Hi all,

i am implementing scull (character device driver in LDD : chapter 3 ) , in that I found ::

Please refer to Linux Device Drivers Rubunu et al  , 3/e.pg 57

ssize_t scull_read(struct file *filp, char __user *buf, size_t count,loff_t *f_pos)
 
Scull_Dev *scull_follow(Scull_Dev *dev, int n)
{
    while (n--) {
        if (!dev->next) {
            dev->next = kmalloc(sizeof(Scull_Dev), GFP_KERNEL); <<<============== why memory allocations in read operations??
            memset(dev->next, 0, sizeof(Scull_Dev));
        }
        dev = dev->next;
        continue;
    }
    return dev;
}

IMHO this implementation is WRONG ?? Correct me if I am wrong.

other implementation is :

/*
 scull_follow
Take a walk in the linked list until node 'n' has been reached.
*/
struct scull_qset* scull_follow(struct scull_dev *dev, int n){
  struct scull_qset *qs = dev->data;

  // Allocate first node if needed

 

 Only if qs == NULL then allocate memory. (In case of write)
else skip this statement block altogether. For read and if qs has
some persistent data already, this statement block will be skipped.

  if(!qs) {
    // Reserve memory
    qs = dev->data = "" scull_qset), GFP_KERNEL);
    // Failure :(
    if(qs == NULL){
      return NULL;
    }
    // Zero out the region
    memset(qs, 0, sizeof(struct scull_qset));
  }

Same  below.

  while(n--){
    // If the next node does not exist, allocate it.
    if(!qs->next){
      qs->next = kmalloc(sizeof(struct scull_qset), GFP_KERNEL);
      if(qs->next== NULL){
    return NULL;
      }
      memset(qs->next, 0, sizeof(struct scull_qset));
    }
    qs = qs->next;
    continue;
  }
  return qs;
}

But here too i am not convinced with memory allocation ?? Why memory allocations in read when we only
want to reach the first partially filled node ??

Thanks.