Thanks Alan for the reply.
Regards,
Mohan
Hello All,
According to the USB 2.0 spec, "bInterval" value for the High Speed Bulk
OUT endpoint must specify the maximum NAK rate of the endpoint. (pg.
271). But on page 273 of the specs, its written
"For high-speed bulk and control OUT endpoints, the bInterval field is
only used for compliance purposes; the host controller is not required
to change its behavior based on the value in this field."
I wanted to know, for High speed Bulk OUT endpoints, what should be the
value of bInterval.
It _should_ be the value defined in the spec, i.e., the maximum NAK
rate.
If it is set to 0, it indicates that the endpoint never NAKs. So, does
this mean that device can never NAK?
It means that _endpoint_ will never NAK (assuming the host behaves
correctly). The device can still NAK on other endpoints.
Consider the following case:
The host is sending data, device sends an ACK for first chunk;
The host will send the next chunk; if for some reason, the device is not
able to receive data, will the host retry if bInterval = 0?
Yes, it will. All the host controllers I know about ignore the
bInterval value for bulk endpoints.
However by specifying bInterval = 0 and returning ACK for the first
chunk, the device has promised that it _will_ be able to receive the
second chunk. So this case should never occur.
I am asking this by considering the USB android gadget composite driver,
gadget/f_mass_storage.c where the bInterval is set to 1 for high speed
bulk out endpoint for mass storage interface.
I believe that value was chosen simply as a default to be safe. It
doesn't really mean anything, since it would have to depend on the
device controller hardware.
Alan Stern
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