Re: em28xx_isoc_dvb_max_packetsize for EM2884 (Terratec Cinergy HTC Stick)

[Mauro Carvalho Chehab <mchehab@redhat.com>]

On 24-12-2011 19:58, Dennis Sperlich wrote:
> Hi,
> 
> I have a Terratec Cinergy HTC Stick an tried the new support for the DVB-C part. It works for SD material (at least for free receivable stations, I tried afair only QAM64), but did not for HD stations (QAM256). I have only access to unencrypted ARD HD, ZDF HD and arte HD (via KabelDeutschland). The HD material was just digital artefacts, as far as mplayer could decode it. When I did a dumpstream and looked at the resulting file size I got something about 1MB/s which seems a little too low, because SD was already about 870kB/s. After looking around I found a solution in increasing the isoc_dvb_max_packetsize from 752 to 940 (multiple of 188). Then an HD stream was about 1.4MB/s and looked good. I'm not sure, whether this is the correct fix, but it works for me.
> 
> If you need more testing pleas tell.
> 
> Regards,
> Dennis
> 
> 
> 
> index 804a4ab..c518d13 100644
> --- a/drivers/media/video/em28xx/em28xx-core.c
> +++ b/drivers/media/video/em28xx/em28xx-core.c
> @@ -1157,7 +1157,7 @@ int em28xx_isoc_dvb_max_packetsize(struct em28xx *dev)
>                  * FIXME: same as em2874. 564 was enough for 22 Mbit DVB-T
>                  * but not enough for 44 Mbit DVB-C.
>                  */
> -               packet_size = 752;
> +               packet_size = 940;
>         }
> 
>         return packet_size;

As you can see there at the code, the packet size depends on the chipset, as
not all will support 940 for packet size.

Could you please provide us what was the chip detected id?

It should be a message on your dmesg like:

	chip ID is em2870
or
	em28xx chip ID = 38

The patch should change it only for your specific chipset model, in order to
avoid regressions to other supported chipsets, like:

int em28xx_isoc_dvb_max_packetsize(struct em28xx *dev)
{
        unsigned int chip_cfg2;
        unsigned int packet_size;

        switch (dev->chip_id) {
        case CHIP_ID_EM2710:
        case CHIP_ID_EM2750:
        case CHIP_ID_EM2800:
        case CHIP_ID_EM2820:
...
	case CHIP_ID_foo:
		packet_size = 940;
...
        }

        return packet_size;
}

The case you're touching seems to be for em2884, but the switch covers both
em2884 and em28174 (plus the default for newer chipsets).

Maybe Michael/Devin may have something to say, with regards to a way to detect
the maximum supported packet size by each specific em28xx model.

Regards,
Mauro