Hello,
I'm writing a driver for a custom pci card with an FPGA
(Xilinx Spartan 2 (XC2S150-6) with PCI 32 LogiCore),
which can act as a pci bus master. The device is designed
to do DMA transfers with high bandwidth. One task is to
send image data to a printer which already works quite
well, but sometimes there are randomly occuring
problems concerning the timing between two DMA
transfers. The issue seems to be something like interrupt
latency in hardware. Measuring some signals with an
oscilloscope shows, that the delay from generating the
interrupt, which signals a finished transfer, to the time
when the interrupt register on the card is reset (i.e. the
beginning of the ISR) sometimes increases to more
than 500 microseconds, which is dimensions too high.
I already tried with other hardware deactivated, which
could cause traffic on the pci bus or generate many interrupts
(except hard disk). I also increased the priority of the IRQ
used by the pci board (with a tool called irqtune) to the
maximum possible value. Another consideration is, that
another driver could lock all interrupts for too long (but for
500 us??).
As my experience on DMA stuff is not yet too
great, I would be very glad if somebody could give me some
advice to solve this problem. Below there is some further
information about my environment and how I set up the DMA
transfers.
Kind regards,
Burkhard Schölpen
Here is how I set up dma transfers from RAM to the pci device:
while (down_interruptible(my_device->write_semaphore));
my_device->dma_write_complete = 0;
my_device->dma_direction = PCI_DMA_TODEVICE;
writel (cpu_to_le32 (virt_to_phys(dma_buffer)), 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:
my_device->dma_write_complete = 1;
wake_up_interruptible(&write_wait_queue);
return IRQ_HANDLED;
Here is some information about the PC:
- Gigabyte GA-8I945GMF mainboard with Pentium D processor
- custom pci board with Xilinx FPGA Spartan 2 (XC2S150-6) with PCI 32 LogiCore
- Debian Linux with 2.6.13.4 SMP kernel
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