[linux-dvb] Re: CRC32 errors in sections

[Jeremy Hall <jhall@maoz.com>]

In the new year, Johannes Stezenbach wrote:
> 
> On Fri, Mar 14, 2003 at 12:40:17PM -0500, Jeremy Hall wrote:
> > Well, to put it another way, when the userspace program requests a section 
> > filter, and starts it, what exactly does the driver prepare for it? The 
> > user gets the section length from the second and third bytes of the 
> > section.  If those are corrupt, and they are still less than the buffer 
> > size, the userspace application will read however much data it was told to 
> > read, we can't recover from that.  Then when we've read part way into 
> > another section, how do we get resynched with section boundaries?
> > 
> > If they are corrupt and larger than the userspace buffer size, how does 
> > the userspace program know how far to read ahead to find the next section? 
> > If CRC failure is detected, is it best to close and reopen the filter?
> 
> The section length field has 12 bits, so if you use a buffer of
> 4096 bytes length for your read, you will alwas get one full section
> per read() call (the API guarantees that). If you find the section to be
> broken, throw it away and read the next one.
> 
> OTOH the API also guarantees that you never get broken sections
> if you use DMX_CHECK_CRC (provided the section_syntax_indicator is 1).
> The av7110 implementation must be fixed.
> 
> The reason that it's broken now is that the AV7110 has hardware for the
> CRC check, but no one got around implementing the check in the
> firmware. Or maybe the Metzlers tried and it didn't work. Anyway, it's
> simpler to implement it on the PC now. If someone sometime implements
> it in the firmware, the check on the PC can be eleminated...
> 
I don't mind doing it in the PC, my goal right now is to figure out why 
the arm is crashing.

One thing I am looking at now is

line 1011 of av7110.c, 
if (av7110->handle2filter[handle])

Could this create a race condition with its counterpart 3 lines down? or 
could this be optimized to avoid the same call twice?

_J

> 
> Regards,
> Johannes
> 
> 
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