Linux Device Drivers, Second Edition

Boot-Time Allocation

If you really need a huge buffer of physically contiguous memory, you need to allocate it by requesting memory at boot time. This technique is inelegant and inflexible, but it is also the least prone to failure. Needless to say, a module can’t allocate memory at boot time; only drivers directly linked to the kernel can do that.

Allocation at boot time is the only way to retrieve consecutive memory pages while bypassing the limits imposed by get_free_pages on the buffer size, both in terms of maximum allowed size and limited choice of sizes. Allocating memory at boot time is a “dirty” technique, because it bypasses all memory management policies by reserving a private memory pool.

One noticeable problem with boot-time allocation is that it is not a feasible option for the average user: being only available for code linked in the kernel image, a device driver using this kind of allocation can only be installed or replaced by rebuilding the kernel and rebooting the computer. Fortunately, there are a pair of workarounds to this problem, which we introduce soon.

Even though we won’t suggest allocating memory at boot time, it’s something worth mentioning because it used to be the only way to allocate a DMA-capable buffer in the first Linux versions, before __GFP_DMA was introduced.

Acquiring a Dedicated Buffer at Boot Time

When the kernel is booted, it gains access to all the physical memory available in the system. It then initializes each of its subsystems ...