/* $Id: dma.c,v 1.5 1992/11/18 02:49:05 root Exp root $ * linux/kernel/dma.c: A DMA channel allocator. Inspired by linux/kernel/irq.c. * Written by Hennus Bergman, 1992. */ #include #include #include /* A note on resource allocation: * * All drivers needing DMA channels, should allocate and release them * through the public routines `request_dma()' and `free_dma()'. * * In order to avoid problems, all processes should allocate resources in * the same sequence and release them in the reverse order. * * So, when allocating DMAs and IRQs, first allocate the IRQ, then the DMA. * When releasing them, first release the DMA, then release the IRQ. * If you don't, you may cause allocation requests to fail unnecessarily. * This doesn't really matter now, but it will once we get real semaphores * in the kernel. */ /* Channel n is busy iff dma_chan_busy[n] != 0. * DMA0 is reserved for DRAM refresh, I think. * DMA4 is reserved for cascading (?). */ static volatile unsigned int dma_chan_busy[MAX_DMA_CHANNELS] = { 1, 0, 0, 0, 1, 0, 0, 0 }; /* Atomically swap memory location [32 bits] with `newval'. * This avoid the cli()/sti() junk and related problems. * [And it's faster too :-)] * Maybe this should be in include/asm/mutex.h and be used for * implementing kernel-semaphores as well. */ static __inline__ unsigned int mutex_atomic_swap(volatile unsigned int * p, unsigned int newval) { unsigned int semval = newval; /* If one of the operands for the XCHG instructions is a memory ref, * it makes the swap an uninterruptible RMW cycle. * * One operand must be in memory, the other in a register, otherwise * the swap may not be atomic. */ asm __volatile__ ("xchgl %2, %0\n" : /* outputs: semval */ "=r" (semval) : /* inputs: newval, p */ "0" (semval), "m" (*p) ); /* p is a var, containing an address */ return semval; } /* mutex_atomic_swap */ int request_dma(unsigned int dmanr) { if (dmanr >= MAX_DMA_CHANNELS) return -EINVAL; if (mutex_atomic_swap(&dma_chan_busy[dmanr], 1) != 0) return -EBUSY; else /* old flag was 0, now contains 1 to indicate busy */ return 0; } /* request_dma */ void free_dma(unsigned int dmanr) { if (dmanr >= MAX_DMA_CHANNELS) { printk("Trying to free DMA%d\n", dmanr); return; } if (mutex_atomic_swap(&dma_chan_busy[dmanr], 0) == 0) printk("Trying to free free DMA%d\n", dmanr); } /* free_dma */