Read and Write

Reading and writing a scull device means transferring data between the kernel address space and the user address space. The operation cannot be carried out through pointers in the usual way, or through memcpy, because pointers operate in the current address space, and the driver’s code is executing in kernel space, while the data buffers are in user space.

If the target device is an expansion board instead of RAM, the same problem arises, because the driver must nonetheless copy data between user buffers and kernel space. In fact, the role of a device driver is mainly managing data transfers between devices (kernel space) and applications (user space).

Cross-space copy is performed in Linux by special functions, which are defined in <asm/segment.h>. The functions devoted to performing such a copy are optimized for different data sizes (char, short, int, long); most of them will be introduced in Section 5.1.4 in Chapter 5.

Driver code for read and write in scull needs to copy a whole segment of data to or from the user address space. This capability is offered by the following functions, which copy an arbitrary array of bytes:

void memcpy_fromfs(void *to, const void *from, unsigned long count);
void memcpy_tofs(void *to, const void *from, unsigned long count);

The names of the functions date back to the first Linux versions, when the only supported architecture was the i386 and there was a lot of assembler code peeking through the C. On Intel platforms, Linux ...

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