Other kinds of devices, such as tape drives, may not have the same constraints. The peripheral device The device must activate the DMA request signal when it’s ready to transfer data. Memory returned from kmalloc has a logical address. Most of the scullv implementation is like the one we’ve just seen for scullp , except that there is no need to check the order parameter that controls memory allocation. We also suggest that you request the DMA channel after you’ve requested the interrupt line and that you release it before the interrupt.

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So now you’re convinced that mapping of scatterlists is worthwhile in some situations. Their API was already described in the 4. Before then, DMA was handled in a more direct — and system-dependent — implementint.

It’s interesting to note how software memory management exploits the same page tables that are used by the CPU itself. The process is put to sleep.

Linux Device Drivers Development by John Madieu

This chapter introduced the following symbols related to memory handling. The contents are initialized to zero.

Does MMAP provide a mapping of a file and returns a pointer of that location in physical memory or it returns with an address of the mapping table? The mmap method is disabled for a scullp device if the allocation order is greater than zero, because nopage deals with single kn rather than clusters of pages. Join our community today! This chapter delves into the area of Linux memory management, with an emphasis on techniques that are useful to the device driver writer.


Actually, there is another way to ipmlementing DMA space: The flip-flop automatically toggles when 8 bits have been transferred; the programmer must clear the flip-flop to set it to a known state before accessing the DMA registers.

Registration is quick, simple and absolutely free. When a VMA is unmapped, the kernel will decrement the usage count for every page in the area.

The only addition was the DMA spinlock in 2. Therefore you won’t be able to use these functions in a module running on the SPARC, but you won’t usually need to.

Linux Device Drivers, 2nd Edition: Chapter mmap and DMA

Therefore, if a process extends a mapped region by calling mremapand the driver hasn’t implemented nopageit will end up with zero pages instead of a implemennting fault. For those who need to write portable PCI drivers, mmqp.

Occasionally a driver will need to access the contents of a streaming DMA buffer without un it. When a process calls readthe driver method allocates a DMA buffer and instructs the hardware to transfer its data. The nopage method takes care of “remapping” one page at a time and returning the address of its struct page structure. The value returned by the function is the usual 0 or a negative error code. If your driver has no nopage method, it will never be notified of this extension, and the additional area will map to the zero page.


Making code work properly with bounce buffers requires adherence to some rules, as we will see shortly. It is worth noting implementlng in Linux systems, there is no need for block drivers to provide this sort of interface.

Not exactly, this ties into the next answer though, so I’ll cover it there. Data transfer can be triggered in two ways: Some architectures allow for significant optimizations when streaming mappings are used, as we will see, but imppementing mappings also are subject to a stricter set of rules in how they may be accessed. As of version 2.

These functions change the mmp of the DMA channel. Some functions and macros are defined for translating between struct page pointers and virtual addresses:. Post as a guest Name. This means that writes to a mapped area in one process are immediately visible in all related parent, child or sibling processes.

This function handles both the allocation and the mapping of the buffer. The time now is