Not sure I understand why yet. HP driver doesn't support this operationg correctly? Is it giving incorrect values or even non-success when calling it? The result of the initial pending count is used in the TransferReadyEventArgs so it's not an unnecessary call.

It is giving incorrect value, even after setting capabilities correctly. Initial value is set to 1 and on next EndXfer value is decremented to 0, even though the count is passed in TransferReadyEventArgs, since the initial value is set to 'DoNotCare' - will it be an issue? Also on testing on TWACKER, it doesn't seem to send the pending count initially.

If problem is happening after EndXfer then the PR shouldn't fix it (unless it does?). It may be a bug where TWPendingXfers count needs to be reset to DoNotCare before making calls with it.

Doesn't seem to be the problem with EndXfer, it returns the correct pending count in each iteration. But only when pending count is initially 1, it just decrements the value. With pending set DoNotCare at first, it's working fine.