Technisat DVB-S2 USB: reduced buffer sizes

Hi,

I reduced the buffer size in order to make this driver work on my NAS station. It's an arm system and thus only has limited resources.

I compiled and ran with kernel 3.16.3 for some weeks and now compiled and booted kernel 3.17.2 today.
Without this patch the device doesn't work at all, in both kernel versions.

I only changed one value because this was sufficient to make it work.

I believe these sizes correspond to the output of lsusb -v But to be honest: I don't have any idea of what this all is about. USB wasn't invented yet when I studied computer science. ;) maybe someone knows better.

    Interface Descriptor:
      bAlternateSetting       0
      Endpoint Descriptor:
        bEndpointAddress     0x82  EP 2 IN
        wMaxPacketSize     0x0200  1x 512 bytes
      Endpoint Descriptor:
        bEndpointAddress     0x81  EP 1 IN
        wMaxPacketSize     0x0200  1x 512 bytes
      Endpoint Descriptor:
        bEndpointAddress     0x01  EP 1 OUT
        wMaxPacketSize     0x0200  1x 512 bytes
    Interface Descriptor:
      bAlternateSetting       1
      Endpoint Descriptor:
        bEndpointAddress     0x82  EP 2 IN
        wMaxPacketSize     0x0c00  2x 1024 bytes
          Transfer Type            Isochronous
      Endpoint Descriptor:
        bEndpointAddress     0x81  EP 1 IN
        wMaxPacketSize     0x0200  1x 512 bytes
      Endpoint Descriptor:
        bEndpointAddress     0x01  EP 1 OUT
        wMaxPacketSize     0x0200  1x 512 bytes

This is the first kernel patch I submitted. I hope everything is fine.

Jan

diff -uNr linux-3.17.2.orig/drivers/media/usb/dvb-usb/technisat-usb2.c linux-3.17.2/drivers/media/usb/dvb-usb/technisat-usb2.c
--- linux-3.17.2.orig/drivers/media/usb/dvb-usb/technisat-usb2.c

2014-10-30 17:43:25.000000000 +0100
+++ linux-3.17.2/drivers/media/usb/dvb-usb/technisat-usb2.c
2014-11-08 17:31:18.716668708 +0100
@@ -708,7 +708,7 @@
                                .endpoint = 0x2,
                                .u = {
                                        .isoc = {
-                                               .framesperurb = 32,
+                                               .framesperurb = 8,
                                                .framesize = 2048,
                                                .interval = 1,

Hello
I forget to mention, but the technique I use some error and trial to find out smallest possible buffer in a worst case. Worst case is case where stream bandwidth is ~biggest possible. Most effective modulation, less error coding, maximum channel bandwidth (symbol rate). Then add some 20% extra to that. You will see it immediately when USB buffers are too small - data starts dropping, which means audio and video are so broken you could see pixels on video and crackle on audio.

Minimum size of needed USB buffer is largely dependent use USB chip buffer size (FIFO) and data stream size which is transferred over the USB.

According to interface descriptors, that chip supports both bulk and isochronous transfers (select using different alternative setting). I would prefer always bulk over isoc due to performance reasons. Isoc guarantee some timing and bandwidth for just this device, whilst bulk is just like a best-effort delivery. For DVB streaming timing issues are not critical and you surely find it very soon if you try use too many devices on same bus, which are using all the bus bandwidth, as the stream goes garbage. I think isoc device will be just rejected if USB core cannot reserve bandwidth it requests (from interface descriptors(?)).

Also, it is very often seen that those interface descriptors are wrong. Even it says isoc payload 2x 1024, it could be something else, like 1x1024. Using maximum 3x1024 and adding debugs will print real used size.

regards
Antti

http://palosaari.fi/