[linux-dvb] Re: Problems with TechnoTrend dec2000-t USB

[Holger Waechtler <holger@convergence.de>]

Felix Domke wrote:
> Hi,
>
> > please use tasklets to do the real work for your interrupt handlers. 
> > Take a look in the PCI drivers for an example, the pretty simple to 
> > convert: just schedule your tasklet with old irq handler function in 
> > the new irq handler.
> > Since tasklets are strongly serialized (if the tasklet is already 
> > running while a new interrupt arrives it's not scheduled again) you 
> > have to check at the end of your tasklet function if new data arrived 
> > and if you have to run it once again. 
>
> The problem in our case is that we either had to buffer the data away 
> (in the interrupt) or rewrite the urb-rescheduling mechanism, since 
> otherwise the buffers are used again after returning from the urb-handler.
>
> I know, it's just lazyness, not a real problem.
>
> I for myself would like to have the lower dvb driver layer being able to 
> handle calls from interrupt context. tasklets always involve a higher 
> latency, and sometimes (like in our case) it's difficult to buffer the 
> data away. Or is there a strong reason not to support this? 
yes, we need to keep the overall interrupt latency low, thus the time 
with switched off interrupts must get minimized.

> (No, it's 
> not because my lazyness, but on slow platforms like the dbox2, it helped 
> a lot to move the queue-handler from a tasklet directly into the 
> interrupt handler in terms of speed. I think it was changed to a 
> bottom-half handler now, which seems to be faster (=lower latency) than 
> a tasklet (why??))
hmm, no idea. Have you asked this on the lkml?
(yes, a bottom half has a bit less overhead and all bh get serviced 
first, but is it that critical?)

Holger



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