On Sun, Jun 24, 2018 at 05:40:37PM +0200, Matthias Dieter Wallnöfer wrote: > Hi people, > > as I am working with a PL2303-based USB-to-serial converter I discovered > that the SW-based flow control was unsupported for all kernels till the > upcoming 4.18 release. Thanks to the patch by Florian Zumbiehl this > deficit was finally resolved and also my tests showed that now the flow > characters get finally sent as one would expect it. > > According to my tests the HW (RTS/CTD) and SW flow control is now > working correctly in the direction *inbound* to USB. Unfortunately I am > less convinced that it does so also the other way around. > > Let me explain why: as the datasheets available on the web state, the > chip is equipped with both internal receive and send buffers. Consider > enabling the HW or SW flow control and filling up both the PL2303's > internal and the device's target recv buffer, for example with a string > of 12'000 characters (in my case it was a PC connected with a nullmodem > link). > > First, the target device's buffer fills up which is fine. When the > buffer's high watermark is reached, the Prolific chip gets informed that > it should not send anymore. This means that now fresh chars arriving > from the USB host end up in the internal send buffer which grows and > grows. Filled up also this one I am not aware of any mechanism which > informs the host to stop doing so, that in the end data gets lost. I think that is because this device does not have any way to do so. When USB to serial devices first came out, a number of devices did have this type of capability, and it was complex. The devices that did this also were "rock solid" but unfortunatly did not sell all that well, and over the decades we ended up with the dirt-cheap devices we have today without the full capabilities of old-style UARTS because it was "good enough". If you really worry about this type of problem, I suggest you get one of those other types of devices (io-edgeport is one example, I think there are others). Last time I looked at the data sheet of the pl2303 device, I do not think it had this type of funcationality at all. Also note that there are different types of this device, as well as "cheap clones" which _almost_ work the same, but not always for some of the "odd" features, like flow control :) Sorry, and good luck! greg k-h -- 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
