With GnuPG master (2.1.21-beta7) on debian jessie I sometimes (happened now for the second time) can't sign / authenticate after waking my system from sleep (suspend to ram). Because the operation fails with the unhelpful error message "Invalid value". Only killing scdaemon resolves the issue.
The debug output is
The Smartcard I was trying to access was a GNUK Token.
It looks like the device got suspended and resumed.
But the application (scdaemon) didn't get noticed by libusb.
So, scdaemon kept communicating as usual, but got unexpected msg type = 0x81,
which is error report status (RDR_to_PC_DataBlock).
This unexpected error report should be handled correctly, at least.
By the commit: rGf053f99ed0b0: scd: Handle unexpected suspend/resume by CCID driver., I put some code to handle such an expected return from the device.
Please try.
I couldn't reproduce this on my machine. (Suspend-to-RAM keeps my USB device running.)
I learned suspend-to-ram functionality. Currently, for Linux, if we have USB driver in kernel, there are methods to handle suspend-to-ram and resume events. For user space driver by libusb, there is nothing and it should all work well by reseting after resume.
There is nothing we can do by scdaemon.
So, closing.
If more fine-grained control is needed with suspend-to-ram, we need to write kernel driver for USB access.