[linux-dvb] question in em28xx-core.c

kevin liu lwtbenben at gmail.com
Wed Nov 21 07:32:51 CET 2007


Hi,
    I am confused by the number "172" in em28xx-core.c, function
em28xx_isoc_video(),
    When we acquire the bottom field, we first see if everything is correct
using these states:
+++++++++++++++++++++++++++++++++++++++++++++++++++
             if (!(*f)->top_field) {
                 (*f)->state = F_ERROR;
                 em28xx_isocdbg ("unexpected begin of bottom field;
discarding it");
             } else if ((*f)-> fieldbytesused < dev->field_size - 172) {
                 (*f)->state = F_ERROR;
                 em28xx_isocdbg ("dropping incomplete top field (%i missing
bytes)",
                      dev->field_size-(*f)->fieldbytesused);
             } else {
                 (*f)->top_field = 0;
                 (*f)->fieldbytesused = 0;
             }
++++++++++++++++++++++++++++++++++++++++++++++++++++
but why do we need minus 172 in the second branch??????
I think when we finish the top field grabbing, (*f)->fieldbytesused
=dev->field_size,
but the code seems not, (*f)->fieldbytesused = dev->field_size, why???
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