dma hardware underrun

dma hardware underrun

[treul]

Hi all,


I'm currently experiencing a tough DMA underrun problem.

The system is like:

                 global bus
                      ||
data-compressor ------||------//---- memory
IDE-PCI harddisk -----||
data-decompressor ----||
data-consumer --------||
                      ||

1. harddisk has the compressed data, read to compressed buffer by DMA1;
2. data-decompressor gets data from compressed buffer by DMA2, and puts 
decompressed data to decompressed buffers in ISR;
3. data-consumer eats the decompressed data at an uncompromisable speed;

The data-consumer encounters DMA underrun...

My debugging result shows the underrun goes away if I put 2 printf's in the 
harddisk ISR. The printf is at milli-seconds whereas the duration of the 
original ISR is at lower magnitude. In addition, I find a SATA-PCI harddisk 
does not have the underrun problem. IDE-PCI device gets the trouble.

In addition, all interrupts are at the same priority and ISR's can not be 
preempted by others.

So, do you have any idea about where the root cause can be hiding? Or what 
do you think of adding printf's in ISR? (performance? incurring other 
problems by disabling interrupt too long?)

Thanks!!


-treul

Re: dma hardware underrun

[Alan Cox]

> My debugging result shows the underrun goes away if I put 2 printf's in the 
> harddisk ISR. The printf is at milli-seconds whereas the duration of the 
> original ISR is at lower magnitude. In addition, I find a SATA-PCI harddisk 
> does not have the underrun problem. IDE-PCI device gets the trouble.

Is the SATA controller on the motherboard and the PATA one on the PCI
bus ?

> So, do you have any idea about where the root cause can be hiding? Or what 
> do you think of adding printf's in ISR? (performance? incurring other 
> problems by disabling interrupt too long?

On a standard PCI bus an IDE DMA disk can saturate the bus. That may be
your underlying problem ?