[PATCH 3/5] DRM: Armada: Add support for ARGB 32x64 or 64x32 hardware cursors

[Russell King - ARM Linux]

On Mon, Oct 07, 2013 at 12:09:22PM +0200, Jean-Francois Moine wrote:
> On Mon, 7 Oct 2013 10:40:08 +0100
> Russell King - ARM Linux <linux at arm.linux.org.uk> wrote:
> 
> > > > This patch adds ARGB hardware cursor support to the DRM driver for the
> > > > Marvell Armada SoCs.  ARGB cursors are supported at either 32x64 or
> > > > 64x32 resolutions.  
> > > 	[snip]
> > > 
> > > I don't see the interest of such cursors. Actually, most often, the
> > > cursors are 64x64 and 'A' is either 0 or 0xff. As the Armada 510
> > > supports 64x64 cursors with transparency, it would be more useful to
> > > implement these ones...  
> > 
> > Sorry, you're completely wrong there.  Xorg uses an alphablended cursor.
> > This patch is a result of trialling each of the Armada's cursor options
> > and this is the only one which results in a reasonable looking cursor.
> 
> Strange. I am using the 64x64 cursor with transparency of the 510 for
> many months and I never saw any problem.

When I tried it, all cursors had variable alpha, and converting the
alpha to a single transparency bit made the cursor look rubbish against
certain backgrounds.

> If you absolutely want to have all transparency shades, you should
> accept 64x64 cursors and test if they may be rendered as 64x32 or
> 32x64, i.e. test that there is only pure transparency in the non-
> rendered rectangles...

That is policy, and that is something which can be done by the X server
rather than the kernel.  The kernel exposes the hardware capabilities,
which are to allow a 64x32 or a 32x64 cursor.  Userspace can make the
decision dynamically which it wishes to use.

However, what you're suggesting is dangerous.  What do you do when you're
presented with a cursor which is 64x64 ?  Do you:

(a) not display it
(b) crash the X server

There isn't a fallback to using software cursors available in the X server
because there's no error reporting for a failed hardware cursor update.