get_permalink argument must be passed in quotes

[sillybean]

It seems that the following should work:

<?php echo get_permalink($post->post_parent); ?>

It doesn't, however, unless you enclose the argument in quotes:

<?php echo get_permalink("$post->post_parent"); ?>

[DD32]

I see no logical reason why either of your codes should not work.
It works fine for me:

$subpageid = 315;
$subpage = get_post($subpageid);

echo get_permalink($subpage->post_parent) , '<br />';
echo get_permalink($subpage->ID);

http://dd32/wordpress/mainpage/
http://dd32/wordpress/mainpage/subpage/