Author Filter not working

Hi there. I have some problems to be able to filter by author (id).

I made a shortcode in functions.php

/* add shortcode home posts */
add_shortcode('post_home', 'post_home1');
function post_home1( $atts ) { 
    ob_start();
    ?>
    <div class="wpsp-related-posts grid-container">
    <?php
        // titulo
        if(isset($atts['title'])){
            echo "<h2>".$atts['title']."</h2>";
        }

        // autor
        if(isset($atts['author']) && !empty($atts['author'])){
            // comprobamos texto
            $args['author']=$atts['author'];
        }

        $list = get_page_by_title( 'related', 'OBJECT', 'wp_show_posts' );

        wpsp_display( $list->ID, $args );
    ?>

    </div>

<?php
    return ob_get_clean();
}

And I try to use that shortcode in a hook element

<?php
global $post;
$author = $post->post_author;
?>
[post_home author='<?php echo $author; ?>']

For some reason, it’s not working and the shortcode gives me all the posts without filtering the author.

How can I fix it?

Thanks a lot!!