WordPress: Inject custom post type posts, that are attached to a given term of a custom taxonomy, into the main loop and the feed.

functions.php

/**
 * Inject posts of the custom post type 'usp_post', that are attached to the 'featured' term of the 
 * 'firstpage' taxonomy, into the main loop and the feed.
 * @see http://www.wpquestions.com/question/showChronoLoggedIn/id/10370  
 */
 
add_filter( 'pre_get_posts', 'wpq_pre_get_posts', 99 );

function wpq_pre_get_posts( $query )
{
    if ( ( is_front_page() && $query->is_main_query() ) || is_feed() )
    {
        $query->set( 'post_type', array( 'post' )  ); 
        add_filter( 'posts_where', 'wpq_posts_where', PHP_INT_MAX );
    }
}      

function wpq_posts_where( $where )
{
    global $wpdb;
    remove_filter( current_filter(), __FUNCTION__, PHP_INT_MAX );

    // Modify this to your needs for the injected posts:  
    $taxonomy  = 'firstpage';    // <-- This should be the taxonomy.
    $term_slug = 'featured';     // <-- This should be the term slug. 
    $cpt       = 'usp_post';     // <-- This should be the custom post type.

    // Get the term info for the term_taxonomy_id:
    $term = get_term_by( 'slug', $term_slug, $taxonomy );

    // Modify the SQL query:
    if( ! is_wp_error( $term ) )
    {
       $where .= " OR {$wpdb->posts}.ID IN (
           SELECT object_id FROM {$wpdb->term_relationships} WHERE term_taxonomy_id IN ( {$term->term_taxonomy_id} ) )
            AND {$wpdb->posts}.post_status = 'publish'
            AND {$wpdb->posts}.post_type = '{$cpt}' ";
    }
    return $where;
}


/**
 * Some other modifications that you might have:
 */
 
function limit_posts_per_home_page( $query ) 
{
    $paged            = ( get_query_var('paged') ) ? get_query_var('paged') : 1;
    $first_page_limit = 11;
    $limit            = get_option('posts_per_page');

    if ( is_front_page() )
    {
        if ( $paged == 1 )
        {
            $limit = $first_page_limit;
        }
        else
        {
            $offset = $first_page_limit + ( ( $paged - 2 ) * $limit );
            set_query_var( 'offset', $offset );   
        }
    }
    set_query_var('posts_per_archive_page', $limit);
    set_query_var('posts_per_page', $limit);
}

add_filter('pre_get_posts', 'limit_posts_per_home_page');

you should do add_action() if the function is not returning any value