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Linux Device Drivers, Second Edition by Alessandro Rubini, Jonathan Corbet

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scull’s Memory Usage

Before introducing the read and write operations, we’d better look at how and why scull performs memory allocation. “How” is needed to thoroughly understand the code, and “why” demonstrates the kind of choices a driver writer needs to make, although scull is definitely not typical as a device.

This section deals only with the memory allocation policy in scull and doesn’t show the hardware management skills you’ll need to write real drivers. Those skills are introduced in Chapter 8, and in Chapter 9. Therefore, you can skip this section if you’re not interested in understanding the inner workings of the memory-oriented scull driver.

The region of memory used by scull, also called a device here, is variable in length. The more you write, the more it grows; trimming is performed by overwriting the device with a shorter file.

The implementation chosen for scull is not a smart one. The source code for a smart implementation would be more difficult to read, and the aim of this section is to show read and write, not memory management. That’s why the code just uses kmalloc and kfree without resorting to allocation of whole pages, although that would be more efficient.

On the flip side, we didn’t want to limit the size of the “device” area, for both a philosophical reason and a practical one. Philosophically, it’s always a bad idea to put arbitrary limits on data items being managed. Practically, scull can be used to temporarily eat up your system’s memory in order to run tests under low-memory conditions. Running such tests might help you understand the system’s internals. You can use the command cp /dev/zero /dev/scull0 to eat all the real RAM with scull, and you can use the dd utility to choose how much data is copied to the scull device.

In scull, each device is a linked list of pointers, each of which points to a Scull_Dev structure. Each such structure can refer, by default, to at most four million bytes, through an array of intermediate pointers. The released source uses an array of 1000 pointers to areas of 4000 bytes. We call each memory area a quantum and the array (or its length) a quantum set. A scull device and its memory areas are shown in Figure 3-1.

The layout of a scull device

Figure 3-1. The layout of a scull device

The chosen numbers are such that writing a single byte in scull consumes eight or twelve thousand bytes of memory: four thousand for the quantum and four or eight thousand for the quantum set (according to whether a pointer is represented in 32 bits or 64 bits on the target platform). If, instead, you write a huge amount of data, the overhead of the linked list is not too bad. There is only one list element for every four megabytes of data, and the maximum size of the device is limited by the computer’s memory size.

Choosing the appropriate values for the quantum and the quantum set is a question of policy, rather than mechanism, and the optimal sizes depend on how the device is used. Thus, the scull driver should not force the use of any particular values for the quantum and quantum set sizes. In scull, the user can change the values in charge in several ways: by changing the macros SCULL_QUANTUM and SCULL_QSET in scull.h at compile time, by setting the integer values scull_quantum and scull_qset at module load time, or by changing both the current and default values using ioctl at runtime.

Using a macro and an integer value to allow both compile-time and load-time configuration is reminiscent of how the major number is selected. We use this technique for whatever value in the driver is arbitrary, or related to policy.

The only question left is how the default numbers have been chosen. In this particular case, the problem is finding the best balance between the waste of memory resulting from half-filled quanta and quantum sets and the overhead of allocation, deallocation, and pointer chaining that occurs if quanta and sets are small.

Additionally, the internal design of kmalloc should be taken into account. We won’t touch the point now, though; the innards of kmalloc are explored in Section 7.1 in Chapter 7.

The choice of default numbers comes from the assumption that massive amounts of data are likely to be written to scull while testing it, although normal use of the device will most likely transfer just a few kilobytes of data.

The data structure used to hold device information is as follows:

typedef struct Scull_Dev {
 void **data;
 struct Scull_Dev *next; /* next list item */
 int quantum;    /* the current quantum size */
 int qset;     /* the current array size */
 unsigned long size;
 devfs_handle_t handle; /* only used if devfs is there */
 unsigned int access_key; /* used by sculluid and scullpriv */
 struct semaphore sem;  /* mutual exclusion semaphore  */
} Scull_Dev;

The next code fragment shows in practice how Scull_Dev is used to hold data. The function scull_trim is in charge of freeing the whole data area and is invoked by scull_open when the file is opened for writing. It simply walks through the list and frees any quantum and quantum set it finds.

int scull_trim(Scull_Dev *dev)
{
 Scull_Dev *next, *dptr;
 int qset = dev->qset; /* "dev" is not null */
 int i;

 for (dptr = dev; dptr; dptr = next) { /* all the list items */
  if (dptr->data) {
   for (i = 0; i < qset; i++)
    if (dptr->data[i])
     kfree(dptr->data[i]);
   kfree(dptr->data);
   dptr->data=NULL;
  }
  next=dptr->next;
  if (dptr != dev) kfree(dptr); /* all of them but the first */
 }
 dev->size = 0;
 dev->quantum = scull_quantum;
 dev->qset = scull_qset;
 dev->next = NULL;
 return 0;
}

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