This is just a short blog post to say that I landed multi-head support on plymouth master yesterday.  More details here:

http://lists.freedesktop.org/archives/plymouth/2009-September/000188.html

What this means is the boot splash redraws itself once per monitor,  in a way that looks good for the native resolution each monitor.  Previously, we’d draw right on the fb console, which is cloned across all monitors.  That made the splash have large black margins on some monitors, or look stretched.

One implication of this change is we now use libdrm directly when possible instead of going through the /dev/fb interface.

We still don’t do any accelerated rendering.  We don’t really need it, since plymouth splashes are normally designed to be very simple (Think “cell phone splash” not “3D video grame”).


bool write_it_harder (int fd, void *buf, size_t size_of_buf)
{
while (there are still bytes to write) {
the_number_of_bytes_written = write (fd, buf, the_number_of_bytes_left_to_write);
/* check for errors then figure out the_number_of_bytes_left_to_write */
}
return true;
}


THIS IS WRONG. It’s not a good idea to do in a gui program. It means that your app will

1) block until all the bytes are written (which could be a while, if the thing getting written to is slow)
2) will completely fall over on some devices

It’s really better (although harder to code) to treat read()/write() as one-shot operations. You do them once, get a result, and then should go back to poll() and wait for it to report that the fd you are working with is readable or writable again.

Another common one is:


again:
bytes_read = read (fd, buf, sizeof (buf));

if (bytes_read < 0 && errno == EAGAIN)
goto again;

Again, THIS IS WRONG.

1) you may get into a situation where read() will return EAGAIN over and over again in a tight, battery killing, cpu heating loop
2) there is no guarantee that you’ll get EAGAIN on successive calls anyway. pseudo-terminals for instance, will give you EAGAIN once, and then EIO for successive calls.

So, only every call read() or write() when poll() says its okay to call them, and realize that if poll() says its okay to call them, it’s only saying it’s okay to call them ONCE, and you need to ask poll() permission to call them again.

update
I’ve put a sample program up here to show what I mean.