Strange delay on PCI-DMA-transfer completion by wait_event_interruptible()

[Burkhard Schölpen <bschoelpen@web.de>]

Hi all,

I'm trying to write a driver for a custom PCI-Board which is DMA-Busmaster capable (kernel 2.6.13 with SMP). Unfortunately I get some strange delay between the start of the transfer until the interrupt appears, which signals its completion.

Concerning a dma transfer from RAM to the pci device, my code does the following:

while (down_interruptible(my_device->write_semaphore));
my_device->dma_write_complete = 0;
my_device->dma_direction  = PCI_DMA_TODEVICE;
my_device->bus_addr = pci_map_single(my_device->pci_device, pointer_to_buffer, my_device->dma_size, my_device->dma_direction);

writel (cpu_to_le32 (bus_addr), MY_DMA_ADDR_REGISTER);
writel (cpu_to_le32 (my_device->dma_size/4), MY_DMA_COUNT_REGISTER);	    //triggers dma transfer	

if (wait_event_interruptible(write_wait_queue, my_device->dma_write_complete))
{
      //handle error...
}
//test, if MY_DMA_COUNT_REGISTER contains 0
up(my_device->write_semaphore);

Inside the Interrupt-handler I do the following:

pci_unmap_single (my_device->pci_device, my_device->bus_addr, my_device->dma_size, my_device->dma_direction);
my_device->dma_write_complete = 1;
wake_up_interruptible(&write_wait_queue);
return IRQ_HANDLED;

Actually the dma transfer works but I get a strange timing issue, which seems to be caused by wait_event_interruptible(). I measured the clock ticks elapsing from the start of the transfer until the interrupt appears. Converted to microseconds I get more than 600 us for less than 3 kB buffers. If I try out busy waiting using "while (!my_device->dma_write_complete)" instead of wait_event_interruptible() the transfer already completes successfully after about 80 us. The device has to transport very large amounts of data, so I have to get the transfer rate as high as possible.

I'm sorry if I made a very simple mistake, because I'm quite unexperienced in driver development, so hints would be very appreciated.

Kind regards,
Burkhard
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