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. 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 -- Devin J. Heitmueller - Kernel Labs http://www.kernellabs.com -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
