[linux-dvb] Re: CRC32 errors in sections

[Johannes Stezenbach <js@convergence.de>]

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...


Regards,
Johannes


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