IMPLEMENTING MMAP IN DRIVER

Virtual addresses are usually stored in pointer variables. It must build the page tables and look them up whenever the processor reports a page fault, that is, whenever the page associated with a virtual address needed by the processor is not present in memory. We won’t discuss bus mastering here. As with other parts of the kernel, both memory mapping and DMA have seen a number of changes over the years. The ARM port, however, is an exception to this rule. If you’d like to contribute content, let us know.

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A DMA mapping is a combination of allocating a DMA buffer and generating an address for that buffer that is accessible by the device.

Acquires the DMA spinlock. Direct memory access, or DMA, is the advanced topic that completes our overview of memory issues. The question does not have to be directly related to Linux and any language is fair game. Iimplementing function stores the 24 least significant bits of addr in the controller. The buffer must not be unmapped while DMA is still active, or serious system instability is guaranteed. There are still some things that show through, however. We described allocation at boot time in “Boot-Time Allocation” in Chapter 7, “Getting Hold of Memory”, but it is not available to modules.

Implementing mmap in the kernel – Linux Device Drivers Development [Book]

All logical addresses are kernel virtual addresses; memory allocated by vmalloc also has a virtual address but no direct physical mapping. With that out of the way, we can now proceed to the implementation of the mmap system call. The system was unable to handle more memory than it could set up logical addresses for, since it needed directly mapped kernel addresses for all memory. Although paging sits at the lowest level of memory impllementing, something more is necessary before you can use the computer’s resources efficiently.

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For a number of reasons that we will touch on shortly, it is important to pick the right value for the direction of a streaming DMA mapping.

Bounce buffers are created when a driver attempts to perform DMA on an address that is not reachable by the peripheral device — a high-memory address, for example. This kind of behavior is rather risky, though, because it may lead impllementing a deadlock.

I would like to write my mmap function so implememting it maps bar0, I realize I can do this without writing a driver but I’m doing this mostly for learning purposes. For more advanced trainees it can be a desktop reference, and a collection of the base knowledge needed to proceed with system and network administration.

Linux Device Drivers Development by John Madieu

After registration, the main part of the driver’s job consists of configuring the DMA controller for proper operation. In such cases, you can’t use mmap at all.

The process is put to sleep. Some architectures will simply fail in this case, but others will create a bounce buffer. The nopage method normally returns a pointer to a struct page.

SPARC64 processors use unsigned int. Both methods have their advantages. Note that the user process can always use mremap to extend its mapping, possibly past the end of the physical device area. A kiobuf that has been mapped in this manner must eventually be unmapped, of course, to keep the reference counts on the pages straight.

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This method will be invoked any time that a new reference to the VMA is made when a process forks, for example. Note that there’s no usage count associated with VMAs; the area is opened and closed exactly once by each process that uses it. The spinlock should be held when using the functions described next.

When kiobufs are available, sbull actually registers two devices.

The first two arguments are our PCI device structure and the size of the needed buffer. There are sound advantages to using mmap when it’s feasible to do so. The kernel needs a higher-level mechanism to handle the way a process sees its memory. They exist as a way for drivers to do any additional processing that they may require.

mmap(2) – Linux manual page

The reason we decided to put the source under the GPL is that it is neither particularly beautiful nor particularly clever, and if someone is going to use it, we want to be sure that the source is released with the module.

Pages may be absent, of course, if the kernel has swapped them to disk or if they have never been loaded. Therefore, if a process extends a mapped region by calling mremapand the driver hasn’t implemented nopageit will end up mmsp zero pages instead of a segmentation fault.