Advanced Printer Sharing with hotplug

Hotplug is a set of scripts running upon certain predefined conditions. On today's Linux system, hotplug can do many things from auto mounting USB drive, pairing bluetooth devices to network configuration.

Seldom mentioned, but DD-WRT does offer (very preliminary) hotplug support. With almost no info on the web, the job is done by digging though DD-WRT's source code and various Linux documents.

1. Replace DD-WRT's event handler with our own
   By default, DD-WRT registers /sbin/hotplug to handle all hotplug requests but the program, /sbin/hotplug doesn't follow standard hotplug rules, and, per my understanding, is a dummy and does nothing at all. So let's replace it with our own
   
   Create a startup script /opt/etc/init.d/hotplugd with only one line:
   
   echo /opt/sbin/hotplug > /proc/sys/kernel/hotplug
   
   Then set it to run upon start up
   chmod a+x /opt/etc/init.d/hotplugd
   ln -s /opt/etc/init.d/hotplugd /opt/etc/init.d/S95hotplugd
   
   run
   /opt/etc/init.d/hotplugd
   and check the result:
   cat /proc/sys/kernel/hotplug
   
   The output should be /opt/sbin/hotplug, which is our new event handler.
   
   
2. Test drive hotplug
   So how does the hotplug work? below script will give you some idea to start with:
   Create /opt/sbin/hotplug with following:
   
   #!/bin/sh

   #For Debugging
   echo "Customized hotplug" >> /tmp/hotplug.log
   echo -------------$(date)----------- >> /tmp/hotplug.log
   set >> /tmp/hotplug.log
   
   set it as executable
   chmod a+x /opt/sbin/hotplug
   
   Unplug the printer, wait few seconds and plug the printer back.  Check the log by typing:
   cat /tmp/hotplug.log
   
   You can see there are bunch of environmental variables logged, two of them are worth noting:
   ACTION The value for "ACTION" variable can be "add" or "remove", corresponding to device plugin and removal.
   PRODUCT This is the unique device ID for the USB device, my printer is HP 1020 and value is 3f0/2b17/100.
   
   In a full Linux system, hotplug is much more complex. My script is a quick dirty hack but should be sufficient to handle USB printers.
   

3. Configure hotplug to upload firmware to the printer
   Some GDI printer(aka Windows printer) will require extra work on Linux. Below is an example of my HP Laserjet 1020 but also applies to other HP laser printers like 1000, 1005, 1018, P1005, P1008, P1505 etc..
   You'll need two things to start with:
   The printer's firmware -- this must be in the printer before printing. Some of popular HP printers' can be downloaded from http://oleg.wl500g.info/hplj/
   usb_printerid -- this is used to detect if the firmware is already in place to prevent duplicated firmware uploading. Above link has one but that's for Broadcom devices. So I compile one for Atheros routers. Download here. Extract and put it to the router(samba, ftp or WinSCP).
   
   Assume we have the firmware in /opt/usr/local/lib/ and usb_printerid in /opt/usr/local/bin/, run
   chmod a+x /opt/usr/local/bin/usb_printerid
   
   And finally, here is the full script /opt/sbin/hotplug

   #!/bin/sh

   #For Debugging
   #echo "Customized hotplug" >> /tmp/hotplug.log
   #echo -------------$(date)----------- >> /tmp/hotplug.log
   #set >> /tmp/hotplug.log

   P910ND='/opt/usr/sbin/p910nd'
   PRINTERID='/opt/usr/local/bin/usb_printerid'
   FIRMWARE='/opt/usr/local/lib/sihp1020.dl'
   #replace it with your printer's firmware.

   if [ $DEVTYPE == 'usb_device' ] && [ $PRODUCT == '3f0/2b17/100' ]; then
   #replace with your printer's device id.
     if [  $ACTION == 'add' ]; then
       #check and upload firmware
       ${PRINTERID} /dev/usb/lp0 | grep -q FWVER || cat ${FIRMWARE} > /dev/usb/lp0
       #kill and restart p910nd
       kill -9 $(pidof p9100d)
       ${P910ND}  -b -f /dev/usb/lp0 0
     fi

     if [ $ACTION == 'remove' ]; then
       #printer removed, lets stop p910nd
       kill -9 $(pidof p9100d)
     fi
   fi
   
   The above script will upload firmware to printer then start p910nd daemon upon printer plug-in and stop p910nd when it's removed.
   

4. Modified p910nd startup script
   With above script, DD-WRT will automatically configure the printer after the router is fully booted. However, similar approach has to be done during startup as the hotplug does not take effect at that moment.
   For unknown reasons, DD-WRT will NOT create /dev/usb/lp0 for printer even we have printer support enabled. As a result, we can't use this device node to detect the printer's presence.
   
   Here is a modified version of /opt/etc/init.d/p910nd startup script.
   
   source /mnt/root/.profile

   mkdir -m 755 -p /dev/usb
   mknod -m 660 /dev/usb/lp0 c 180 0
   sleep 1

   P910ND='/opt/usr/sbin/p910nd'
   PRINTER_ID='/opt/usr/local/bin/usb_printerid'
   FIRMWARE='/opt/usr/local/lib/sihp1020.dl'
   #replace with yours
   PNAME='LaserJet'
   #printer name id string, see below

   i=$(find /sys/devices/platform/ -name product -exec cat {} \; | grep -c ${PNAME})

   if [ $i != '0' ]; then
        ${PRINTER_ID} /dev/usb/lp0 | grep -q FWVER || cat ${FIRMWARE} > /dev/usb/lp0
        kill -9 `pidof p9100d`
        ${P910ND}  -b -f /dev/usb/lp0 0
   fi
   
   Notice the PNAME='LaserJet' line. The 'LaserJet" is part of the printer's name and can be used to check if the printer is plugged. To determine yours, run
   
   find /sys/devices/platform/ -name product -exec cat {} \;
   
   The result looks like that:
   HP LaserJet 1020 #printer
   DataTravelerMini #USB flash drive
   Atheros AR91xx built-in EHCI controller #USB controller
   

After that, follow the previous tutorial to install printer in Windows if you haven't done so. Now enjoy the convenience of hotplug.