Devin Heitmueller wrote:
On Thu, Sep 10, 2009 at 11:55 AM, Antti Palosaari<cr...@iki.fi> wrote:
Devin Heitmueller wrote:
The URB size is something that varies on a device-by-device basis,
depending on the bridge chipset. There really is no
"one-size-fits-all" value you can assume.
I doubt no. I tested last week rather many USB chips and all I tested
allowed to set it as x188 or x512 bytes. If it is set something than chip
does not like it will give errors or packets that are not as large as
requested. You can test that easily, look dvb-usb module debug uxfer and use
powertop.
I usually take a look at a USB trace of the device under Windows, and
then use the same value.
I have seen logs where different sizes of urbs used even same chip.
Yes, the URB size can change depending on who wrote the driver, or
what the required throughput is. For example, the em28xx has a
different URB size depending on whether the target application is
19Mbps ATSC or 38Mbps QAM. That just reinforces what I'm saying - the
size selected in many cases is determined by the requirements of the
chipset.
Yes thats just what I tried to say for. Look my previous thread where
all currently sizes are listed. We need to define suitable values that
are used. For example USB2.0 DVB-C, DVB-T, ATSC and same values for
USB1.1 too. And stream size can vary much depending used transmission
parameters too but I think such kind resolution logic is not needed.
Currently there is almost everything between 512 to 65k used for DVB-T
that makes huge difference to load device causing.
Does anyone know if there is some table which says what are good USB
transmission parameters for each bandwidth needed?
Making it some multiple of 188 for DVB is logical since that's the
MPEG packet size. That seems pretty common in the bridges I have
worked with.
Devin
Antti
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