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

["linux-os \(Dick Johnson\)" <linux-os@analogic.com>]

On Mon, 12 Dec 2005, [iso-8859-1] Burkhard Schölpen wrote:

> 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.
>

Don't you get an interrupt both on a completion and error?
I think you should be using interruptible_sleep_on(&write_wait_queue),
not spinning in wait_event_interruptible().

Most all my DMA transfers use as above and from the time the DMA
completion occurs until the time user-mode code gets awakened in
poll()  (Much worse latency than your code), the time is always
less than 120 us on a 400 MHz ix86 embedded machine with a 100 MHz
front-side bus.

> Kind regards,
> Burkhard

Cheers,
Dick Johnson
Penguin : Linux version 2.6.13.4 on an i686 machine (5589.56 BogoMips).
Warning : 98.36% of all statistics are fiction.

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