I agree. I think that the line should be removed altogether. Everything seems to work without it. I am testing on the CP2105. The device's buffers are cleared based upon the settings configured in the device's memory (some devices have a one-time programmable EPROM; others have an EEPROM). They can be configured to flush on open, close, etc. The Linux kernel buffers are a different story; I don't know what happens there, but I'd presume that performing a USB reset on a serial device is not the norm (i.e. unless firmware has to be programmed, etc.). Thanks, Blake On Fri, May 16, 2014 at 2:31 PM, Preston Fick <pffick@xxxxxxxxx> wrote: > This comment and functionality in the cp210x driver has been around since > the cp2101 was reverse engineered and committed to the kernel originally > (probably about 8 or 9 years ago). I don't know exactly what the behavior is > or if the comment is referring to buffers on the host or the device. > > I can say that for all other CP210x based drivers (on Windows, OSX and other > embedded platforms) a USB port reset is *not* required on open for any > reason. In my opinion removing this line all together would be the best > patch, although we have not gone through any regression testing yet to prove > that is the case and I don't know the implications within the Linux kernel > itself - but from a pure device standpoint it's not necessary. > > Kind Regards - > Preston > > > On Thu, May 15, 2014 at 5:53 PM, Miner, Blake <bminer@xxxxxxxxxxxxxxxx> > wrote: >> >> After debugging further, I figured out what was causing the problem. >> >> In the driver for the cp210x, there is a line of code in the `.attach` >> function that calls `usb_reset_device(serial->dev);`. See the source >> code here: >> https://github.com/torvalds/linux/blob/master/drivers/usb/serial/cp210x.c#L859 >> >> This call to `usb_reset_device` causes the device to be reset just >> before the second interface is detected. For whatever reason, the >> reset prevents the second interface from being created in `/dev/`. >> >> By simply removing this line, both interfaces are properly detected >> the first time. Unfortunately, there is a comment above this line >> that reads "/* cp210x buffers behave strangely unless device is reset >> */" Therefore, this reset might actually be necessary in order to >> clear the buffers of the CP210x. In this case, when is the >> appropriate time to perform a device reset so that both interfaces are >> properly detected? >> >> Thanks, >> >> Blake >> -- >> To unsubscribe from this list: send the line "unsubscribe linux-usb" in >> the body of a message to majordomo@xxxxxxxxxxxxxxx >> More majordomo info at http://vger.kernel.org/majordomo-info.html > > -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
