1============================ 2DMA with ISA and LPC devices 3============================ 4 5:Author: Pierre Ossman <drzeus@drzeus.cx> 6 7This document describes how to do DMA transfers using the old ISA DMA 8controller. Even though ISA is more or less dead today the LPC bus 9uses the same DMA system so it will be around for quite some time. 10 11Headers and dependencies 12------------------------ 13 14To do ISA style DMA you need to include two headers:: 15 16 #include <linux/dma-mapping.h> 17 #include <asm/dma.h> 18 19The first is the generic DMA API used to convert virtual addresses to 20bus addresses (see Documentation/core-api/dma-api.rst for details). 21 22The second contains the routines specific to ISA DMA transfers. Since 23this is not present on all platforms make sure you construct your 24Kconfig to be dependent on ISA_DMA_API (not ISA) so that nobody tries 25to build your driver on unsupported platforms. 26 27Buffer allocation 28----------------- 29 30The ISA DMA controller has some very strict requirements on which 31memory it can access so extra care must be taken when allocating 32buffers. 33 34(You usually need a special buffer for DMA transfers instead of 35transferring directly to and from your normal data structures.) 36 37The DMA-able address space is the lowest 16 MB of _physical_ memory. 38Also the transfer block may not cross page boundaries (which are 64 39or 128 KiB depending on which channel you use). 40 41In order to allocate a piece of memory that satisfies all these 42requirements you pass the flag GFP_DMA to kmalloc. 43 44Unfortunately the memory available for ISA DMA is scarce so unless you 45allocate the memory during boot-up it's a good idea to also pass 46__GFP_RETRY_MAYFAIL and __GFP_NOWARN to make the allocator try a bit harder. 47 48(This scarcity also means that you should allocate the buffer as 49early as possible and not release it until the driver is unloaded.) 50 51Address translation 52------------------- 53 54To translate the virtual address to a bus address, use the normal DMA 55API. Do _not_ use isa_virt_to_bus() even though it does the same 56thing. The reason for this is that the function isa_virt_to_bus() 57will require a Kconfig dependency to ISA, not just ISA_DMA_API which 58is really all you need. Remember that even though the DMA controller 59has its origins in ISA it is used elsewhere. 60 61Note: x86_64 had a broken DMA API when it came to ISA but has since 62been fixed. If your arch has problems then fix the DMA API instead of 63reverting to the ISA functions. 64 65Channels 66-------- 67 68A normal ISA DMA controller has 8 channels. The lower four are for 698-bit transfers and the upper four are for 16-bit transfers. 70 71(Actually the DMA controller is really two separate controllers where 72channel 4 is used to give DMA access for the second controller (0-3). 73This means that of the four 16-bits channels only three are usable.) 74 75You allocate these in a similar fashion as all basic resources: 76 77extern int request_dma(unsigned int dmanr, const char * device_id); 78extern void free_dma(unsigned int dmanr); 79 80The ability to use 16-bit or 8-bit transfers is _not_ up to you as a 81driver author but depends on what the hardware supports. Check your 82specs or test different channels. 83 84Transfer data 85------------- 86 87Now for the good stuff, the actual DMA transfer. :) 88 89Before you use any ISA DMA routines you need to claim the DMA lock 90using claim_dma_lock(). The reason is that some DMA operations are 91not atomic so only one driver may fiddle with the registers at a 92time. 93 94The first time you use the DMA controller you should call 95clear_dma_ff(). This clears an internal register in the DMA 96controller that is used for the non-atomic operations. As long as you 97(and everyone else) uses the locking functions then you only need to 98reset this once. 99 100Next, you tell the controller in which direction you intend to do the 101transfer using set_dma_mode(). Currently you have the options 102DMA_MODE_READ and DMA_MODE_WRITE. 103 104Set the address from where the transfer should start (this needs to 105be 16-bit aligned for 16-bit transfers) and how many bytes to 106transfer. Note that it's _bytes_. The DMA routines will do all the 107required translation to values that the DMA controller understands. 108 109The final step is enabling the DMA channel and releasing the DMA 110lock. 111 112Once the DMA transfer is finished (or timed out) you should disable 113the channel again. You should also check get_dma_residue() to make 114sure that all data has been transferred. 115 116Example:: 117 118 int flags, residue; 119 120 flags = claim_dma_lock(); 121 122 clear_dma_ff(); 123 124 set_dma_mode(channel, DMA_MODE_WRITE); 125 set_dma_addr(channel, phys_addr); 126 set_dma_count(channel, num_bytes); 127 128 dma_enable(channel); 129 130 release_dma_lock(flags); 131 132 while (!device_done()); 133 134 flags = claim_dma_lock(); 135 136 dma_disable(channel); 137 138 residue = dma_get_residue(channel); 139 if (residue != 0) 140 printk(KERN_ERR "driver: Incomplete DMA transfer!" 141 " %d bytes left!\n", residue); 142 143 release_dma_lock(flags); 144 145Suspend/resume 146-------------- 147 148It is the driver's responsibility to make sure that the machine isn't 149suspended while a DMA transfer is in progress. Also, all DMA settings 150are lost when the system suspends so if your driver relies on the DMA 151controller being in a certain state then you have to restore these 152registers upon resume. 153