when does btrfs create sparse extents?

[Marek Behun <marek.behun@xxxxxx>]

Hello,

there was a bug fixed recently in U-Boot's btrfs driver - the driver
failed to read files with sparse extents. This causes that sometimes
device failes to boot Linux, since the kernel fails to load from
storage.

So when does kernel's btrfs driver write sparse extents? Is it always
when it finds a PAGE_SIZEd and aligned all-zeros block? Or is it more
complicated?

Thanks.

Marek