On 20/07/15 19:25, Johan Hovold wrote: > What kernel version are you using? I'm using linux 4.1.2. > The idea of adding another layer of buffering in the cdc-acm driver has > been suggested in the past but was rejected (or at least questioned). > See for example this thread: > > https://lkml.kernel.org/r/20110608164626.22bc893c@xxxxxxxxxxxxxxxxxxx Yes, that is indeed pretty much the same problem and the same solution. Answering to the questions brought up in that thread: > a) Why is your setup filling 64K in the time it takes the throttle > response to occur As far as I understand, the throttle happens only when there's less than 128 bytes of free space in the tty buffer. Data can already be lost before the tty even decides it should start throttling, simply because the throttle threshold is smaller than the amount of data potentially in each urb. Also (excuse my cluelessness) it seems that when exactly the throttling happens depends on some scheduling "jitter" as well. Additionally, the response of the cdc_acm driver to a throttle request is not very prompt; it might have a queue of up to 16kB (16 urbs) pending. > b) Do we care (is the right thing to do to lose bits anyway at > that point) This I cannot answer, I don't know enough about the architecture or standards. I can just say that for my case, there's a lot of losses; this it not an issue which happens after hours when the system is under heavy load, it happens after just a few seconds reproducably. > The tty buffers are quite large these days, but could possibly be bumped > further if needed to give the ldisc some more time to throttle the > device at very high speeds. I do not like this solution. It will again be based on luck, and you will still be unable to rely on the delivery guarantee made by the USB stack (at least when using bulk). My suggestion instead stops the host system from accepting any more data from the device when its buffers are full, forcing the device to wait before sending out more data (which many kinds of devices might very well be able to do). Also note that this patch does not introduce an extra layer of buffering. The buffers are already there; this change just alters the process which decides when to submit the buffers to the tty, and when to free them for more input data from the device. Sven
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