On the other hand, sometimes there is no other way available.
In practice, the code invokes until the call fails; it then waits until the kernel frees some pages, and then allocates everything once again.
The kernel can’t offer a unified interface, either, because a driver can’t abstract too much from the underlying hardware mechanisms.
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But the request is prone to fail even when the allocated buffer is less than 128 KB, because system memory becomes fragmented over time.
is to allocate memory at boot time or reserve the top of physical RAM for your buffer.
Note that many drivers allocate their buffers at initialization time and use them until shutdown—the word This section covers the allocation of DMA buffers at a low level; we will introduce a higher-level interface shortly, but it is still a good idea to understand the material presented here.
The main problem with the DMA buffer is that when it is bigger than one page, it must occupy contiguous pages in physical memory because the device transfers data using the ISA or PCI system bus, both of which carry physical addresses.
Another relevant item introduced here is the DMA buffer.