On Thu, 5 Apr 2018, Guido Kiener wrote: > > Well, I'm quite happy with the performance of the new ioctl functions > > for generic read/write and I don't want to start a lengthy discussion > > here. > > However I need some expert knowledge and hints about the following questions: > > 1. We can detect short USB packages. A ZLP (Zero Length Package) can > > happen when the previous package has the maximum packet length. > > The driver could be simplified if it would be possible to detect ZLPs. > > How can it be simplified? > > [gk] A generic_read operation (from Bulk IN) could return immediately upon detecting a short package or a ZLP. [The correct term is "packet", not "package".] > When a driver reads a package with maximum package length then it cannot decide whether another package will follow or not. Not true; it _can_ tell. If the total amount of data received so far was less than the amount that the URB requested, then another packet _will_ follow. If the total amount of data is equal to the amount that the URB requested then another packet will _not_ follow. (And if the total amount of data is larger than the amount that the URB requested, an overrun error has occurred.) > That means when I submit an URB with 4kB and the driver reads exactly 4kB + ZLP then the URB is completed but it does not signal that a ZLP was received as well. Submitting an extra URB will just wait for the next data package and does not return, since a ZLP is like an empty string. This is incorrect, and it indicates a misunderstanding of how the USB bulk protocol works. Suppose you submit a 4-KB URB, and the driver reads exactly 4 KB. The URB will be complete at that time; no ZLP will be sent because the host will not ask the device to send any more packets. In short, the device will send a bulk ZLP _only_ under the following conditions: The amount of data it wants to send is less than what the host expects to receive, and the amount is a multiple of the maximum packet length; or The host wants to receive a zero-length message (rather unlikely, but I suppose this could happen). If your communication protocol does not allow the device to know how much data the host expects to receive, but a ZLP can mess up the protocol, then the protocol is buggy and needs to be fixed. > Of course we can work around this problem when we read the header with a single URB and calculate the expected package size and then submit more URBs. There should not be any problem to work around. > > Is there any way (without changing the USB subsystem) to detect ZLPs? > > On an urb you send, just set URB_ZERO_PACKET, right? What else do you need? > > [gk] For Bulk OUT transfers the USBTMC protocol does not use ZLP. The generic_write operation works pretty well. Bulk-OUT transfers never use ZLP, unless for some reason the host wants to send a message containing no data. Alan Stern -- 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
