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

        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

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