prev contains parm
cur contains =
compgen -W "a b c" -- = outputs nothing

You can use _get_comp_words_by_ref from bash-completion:

source bash-completion # /usr/share/bash-completion/bash_completion on Ubuntu with bash-complete installed

function _completion_command() {
  local cur prev
  _get_comp_words_by_ref -n = cur prev
  case "$prev" in
    -u)
      case "$cur" in
        parm=*)
          COMPREPLY=($(compgen -W "a b c" -- ${cur#*=}))
          return 0;
          ;;
        *)
          compopt -o nospace
          COMPREPLY=($(compgen -S\= -W "parm" -- $cur))
          return 0;
          ;;
      esac
      ;;
  esac

  COMPREPLY=($(compgen  -W "-u" -- $cur))
}

complete -F _completion_command command

prev contains parm
cur contains =
compgen -W "a b c" -- = outputs nothing

You can use _get_comp_words_by_ref from bash-completion:

source bash-completion # /usr/share/bash-completion/bash_completion on Ubuntu with bash-complete installed

function _completion_command() {
  local cur prev
  _get_comp_words_by_ref -n = cur prev
  case "$prev" in
    -u)
      case "$cur" in
        parm=*)
          COMPREPLY=($(compgen -W "a b c" -- ${cur#*=}))
          return 0;
          ;;
        *)
          compopt -o nospace
          COMPREPLY=($(compgen -S\= -W "parm" -- $cur))
          return 0;
          ;;
      esac
      ;;
  esac

  COMPREPLY=($(compgen  -W "-u" -- $cur))
}

complete -F _completion_command command

prev contains parm
cur contains =
compgen -W "a b c" -- = outputs nothing

You can use _get_comp_words_by_ref from bash-completion:

source bash-completion # /usr/share/bash-completion/bash_completion on Ubuntu with bash-complete installed

function _completion_command() {
  _get_comp_words_by_ref -n = cur prev
  case "$prev" in
    -u)
      case "$cur" in
        parm=*)
          COMPREPLY=($(compgen -W "a b c" -- ${cur#*=}))
          return 0;
          ;;
        *)
          compopt -o nospace
          COMPREPLY=($(compgen -S\= -W "parm" -- $cur))
          return 0;
          ;;
      esac
      ;;
  esac

  COMPREPLY=($(compgen  -W "-u" -- $cur))
}

complete -F _completion_command command

prev contains parm
cur contains =
compgen -W "a b c" -- = outputs nothing

You can use _get_comp_words_by_ref from bash-completion:

source bash-completion # /usr/share/bash-completion/bash_completion on Ubuntu with bash-complete installed

function _completion_command() {
  local cur prev
  _get_comp_words_by_ref -n = cur prev
  case "$prev" in
    -u)
      case "$cur" in
        parm=*)
          COMPREPLY=($(compgen -W "a b c" -- ${cur#*=}))
          return 0;
          ;;
        *)
          compopt -o nospace
          COMPREPLY=($(compgen -S\= -W "parm" -- $cur))
          return 0;
          ;;
      esac
      ;;
  esac

  COMPREPLY=($(compgen  -W "-u" -- $cur))
}

complete -F _completion_command command