Re: [PATCH 4/4] drm/i915: Reserve space for FBC (fbcon)

[Ben Widawsky <ben@bwidawsk.net>]

On Thu, Jun 19, 2014 at 08:28:11PM +0100, Chris Wilson wrote:
> On Thu, Jun 19, 2014 at 12:06:13PM -0700, Ben Widawsky wrote:
> > This is one part in a few fixes needed to make FBC work with limited
> > stolen memory and large resolution displays. It is not the full
> > solution, but one (easy) step.
> > 
> > The patch is straight-forward, it attempts to check there will be room
> > for FBC before trying to "reclaim"
> 
> But it special cases one particular allocation. Why don't you just
> reserve stolen upfront for FBC? Compute the maximum buffer size the
> hardware could support and try to claim it during stolen init.
> -Chris
> 

I agree this would be the best approach (and what I had planned to do).
For one, I didn't find the interfaces I wanted in the drm_mm to do what
I needed (though I didn't look very hard). I ended up getting stuck with
having to decide whether to reclaim the scanout (and fastboot), or FBC.
I believe this should be a decision left to the user, where user is the
distro packaging.

I'd like to just point out some math at this point too.
Common stolen size is 32M
3840 x 2160  x 4 = 31.64M

So we have a real problem if we want to reuse any of stolen memory,
which the first 3 patches address to some extent.

Anyway, I think I was pretty clear that the patch is incomplete, and
primarily meant to motivate the relevant parties to figure out how they
want to handle the stolen reclaim.

-- 
Ben Widawsky, Intel Open Source Technology Center