Now I activated the debug messages in em28xx. From the messages I
see no correlation of the pool exhaustion and lost sync. Also I
cannot see any error messages from the em28xx driver.
I see a lot of init_isoc/stop_urbs (maybe EPG scan?) without
draining the coherent pool (checked with 'cat
/debug/dma-api/num_free_entries', which gave stable numbers), but
after half an hour there are only init_isoc messages without
corresponding stop_urbs messages and num_free_entries decreased
until coherent pool exhaustion.

Hi Soeren

em28xx_stop_urbs() is only called by em28xx_stop_streaming().

em28xx_stop_streaming() is only called by em28xx_stop_feed()
when 0 == dvb->nfeeds.

em28xx_stop_feed()and em28xx_start_feed() look O.K, dvb->nfeeds is
protected by a mutex etc.

Now, em28xx_init_isoc() is also called by buffer_prepare(). This uses
em28xx_alloc_isoc() to do the actual allocation, and that function
sets up the urb such that on completion the function
em28xx_irq_callback() is called.

It looks like there might be issues here:

Once the data has been copied out, it resubmits the urb:

       urb->status = usb_submit_urb(urb, GFP_ATOMIC);
        if (urb->status) {
                em28xx_isocdbg("urb resubmit failed (error=%i)\n",
                               urb->status);
        }
  
However, if the ubs_submit_urb fails, it looks like the urb is lost.

If you look at other code submitting urbs you have this pattern:

               rc = usb_submit_urb(isoc_bufs->urb[i], GFP_ATOMIC);
                if (rc) {
                        em28xx_err("submit of urb %i failed (error=%i)\n", i,
                                   rc);
                        em28xx_uninit_isoc(dev, mode);
                        return rc;
                }
 
Do you have your build such that you would see "urb resubmit failed"
in your logs? Are there any?

     Andrew