[linux-dvb] [BUG] changeset 9029 (http://linuxtv.org/hg/v4l-dvb/rev/aa3e5cc1d833)

Steven Toth stoth at linuxtv.org
Tue Feb 17 16:16:55 CET 2009

Trent Piepho wrote:
> On Mon, 16 Feb 2009, Steven Toth wrote:
>>> Hartmut, Oliver and Trent: Thanks for helping with this issue. I've just
>>> reverted the changeset. We still need a fix at dm1105, au0828-dvb and maybe
>>> other drivers that call the filtering routines inside IRQ's.
>> Fix the demux, add a worker thread and allow drivers to call it directly.
>> I'm not a big fan of videobuf_dvb or having each driver do it's own thing as an
>> alternative.
>> Fixing the demux... Would this require and extra buffer copy? probably, but it's
>> a trade-off between  the amount of spent during code management on a driver by
>> driver basis vs wrestling with videobuf_dvb and all of problems highlighted on
>> the ML over the last 2 years.
> Have the driver copy the data into the demuxer from the interrupt handler
> with irqs disabled?  That's still too much.
> The right way to do it is to have a queue of DMA buffers.  In the interrupt
> handler the driver takes the completed DMA buffer off the "to DMA" queue
> and puts it in the "to process" queue.  The driver should not copy and
> cetainly not demux the data from the interrupt handler.

I know what the right way is Trent (see cx23885) although thank you for 
reminding me. videobuf_dvb hasn't convinced people like me to bury myself in its 
mess or complexity during retro fits cases. Retro fitting videobuf_dvb into cx18 
(at the time) was way too much effort.

Retro fitting it into existing drivers can be painful and I haven't seen any 
volunteers stand up over the last 24 months to get this done.

My suggestion? For the most part we're talking about very low data rates for 
DVB, coupled with fast memory buses for memcpy's. If the group doesn't want 
calls to the sw_filter methods then implement a half-way-house replacement for 
those drivers - as I mentioned above - with the memcpy. Either this approach, or 
make videobuf_dvb mandatory and deprecate sw_filter_...n() to intensionally 
break drivers and force maintainers to comply. This will upset people.

Realistically, this thread will probably go on for a couple of days then trail 
off into nothing. The subject will rear it's head again in a few months, like 
it's done in the past.

A general question to the group: Who wants to volunteer to retro fit 
videobuf_dvb into the current drivers so we can avoid calls to sw_filter_...n() 

I'm willing to loan any hardware I have to the volunteer assuming we can work 
out something on shipping.

- Steve

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