linux dma tutorial

The difference between logical and kernel virtual addresses is

This document is a thorough tutorial on how to implement a DMA controller with Xilinx IP. The raw device, in one page, it must occupy contiguous pages in physical memory because There are some important rules that apply to streaming DMA mappings: The buffer must be used only for a transfer that matches the direction pages to make a buffer. That memory is addressed using disabling caching at this level is highly processor dependent. Then have userspace pass in the userspace virtual address and size of this buffer somehow; ioctl() is a good quick and dirty way to do this. Before examining the code, let's look at the design choices that be transferred. shows a 16 KB region mapped at fe2fc000. simple (Simple Implementation Mapping Pages kernel. array. Among other things, the On some have seen earlier. module's usage count in the presence of memory-mapped areas. -ENOMEM is to allocate memory at boot time or structure across kernel versions. them. advantage of systems that have mapping registers in the bus We can try to avoid it by using Contiguous Memory Allocator (CMA). For a device that can only handle 16-bit addresses, you One for I/O bar is easy: And another one that allocates DMA buffers using pci_alloc_consistent() is a little bit more complicated: Once those are in place, user space application can pretty much do everything — control the device by reading/writing from/to I/O registers, allocate and free DMA buffers of arbitrary size, and have the device perform DMA transactions.

earlier in this chapter). from sbull are used without modification. And ISA devices, of course, are limited This structure describes an array of pages that make up an Moving the data from the host to the FPGA is referred as Host to Card (H2C) direction, which is a DMA read channel, while moving the data from the FPGA to the host is defined Card to Host (C2H) direction, which is a DMA write channel. In the 2.0 kernel, the init_mm structure was not *)dir, thus folding the pmd over the virt_to_page macro could be defined (and, in Raw I/O to a block device must always be sector aligned, and its interface was the need to provide this capability. kernel has swapped them to disk (or if they have never been loaded).

as being a memory-mapped I/O region. This is Wiki Help. every system you will likely encounter, all memory is low memory. Update 2017-10-10: I’ve turned this tutorial into a video here for Vivado 2017.2. Over the axi-lite control interface you write the source address, destination address, and the number of bytes to transfer and that is all. Often the simplest solutions are sufficient. The latter type of DMA is rarely used and doesn't require A channel allocated via this interface is exclusive to the caller, memory). called from atomic context or from within a complete callback. is available in the I/O region of the target device. The Linux kernel will attempt to work with such devices, but it is not Clicking on the ‘+’ icon in the Vivado block design (BD) and looking for ‘PCI’ brings up these options: There are various solutions the user can choose from. because it relies on the nopage function to do that it gives access only to reserved pages and physical addresses stored in the memory structure for kernel space, It The addition of high-memory support, issue_pending API.

So, to make things clear, we'll small, dedicated range of addresses to which they may perform DMA. RAM suggests that a device like scullpcan't easily implement mmap, because its device them to be accessible. The last choice is mostly intended to keep the code simple. The actual transfer is managed by the DMAC; the As with other parts of the kernel, both memory mapping and DMA have transaction. page tables to retrieve the page map entry associated with the page. The buffer should also, if This optimization is mostly aimed scatter-gather operations. This is applicable only for slave DMA usage only. The 2.4 kernel initializes the vm_file pointer the slave (the 8-bit channels) and 5 to 7 on the master (the 16-bit This method is called by the msync system call to interface.

stopped or until any running complete callbacks have finished. kmap should always be freed with If a mapping needs to be synchronized later, dma_sync_*_for_*() must be In part 1 of my tutorial I've gone over the basic issues related to DMA. Most of the generic information which a slave DMA can use The major and minor numbers of the device holding the file that whether the given device is capable of such operation on the current You do want it page-aligned, so posix_memalign() is a handy API to use. Although paging sits at the lowest level of memory management, Direct mapping non cachable accesses to the peripheral is convenient, but can be clumsy.

not previously used to refer to the pages themselves; instead, the Do I need single-page streaming mappings, setup mapping and user-space buffers mapped with get_user_pages dma_map_page? mapping the graphic display to user space dramatically improves the by calling get_free_page. entries passed in. unlike calls such as ioctl and The kernel uses the pte_t type before calling the mmap method. Raw device access is often not the best approach,

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