| /* $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 <linux/kernel.h> |
| #include <linux/errno.h> |
| #include <asm/dma.h> |
| |
| |
| /* 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 */ |
| |