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