Re: [PATCH] drm/i915/bdw: Don't allow the FBC base to be 0

From: "Ville Syrjälä" <ville.syrjala@linux.intel.com>
To: Ben Widawsky <ben@bwidawsk.net>
Cc: Art Runyan <arthur.j.runyan@intel.com>,
	Intel GFX <intel-gfx@lists.freedesktop.org>,
	Ben Widawsky <benjamin.widawsky@intel.com>
Subject: Re: [PATCH] drm/i915/bdw: Don't allow the FBC base to be 0
Date: Thu, 15 May 2014 17:05:05 +0300	[thread overview]
Message-ID: <20140515140505.GC27580@intel.com> (raw)
In-Reply-To: <20140503014851.GA22130@bwidawsk.net>

On Fri, May 02, 2014 at 06:48:52PM -0700, Ben Widawsky wrote:
> On Fri, May 02, 2014 at 09:35:20PM +0100, Chris Wilson wrote:
> > On Fri, May 02, 2014 at 10:00:01AM -0700, Ben Widawsky wrote:
> > > On Fri, May 02, 2014 at 04:00:25PM +0300, Ville Syrjälä wrote:
> > > > On Fri, May 02, 2014 at 09:38:11AM +0100, Chris Wilson wrote:
> > > > > On Fri, May 02, 2014 at 11:19:27AM +0300, Ville Syrjälä wrote:
> > > > > > On Thu, May 01, 2014 at 06:47:54PM -0700, Ben Widawsky wrote:
> > > > > > > "Restriction :
> > > > > > > The offset must be greater than 4K bytes, avoiding the first 4KB of
> > > > > > > stolen memory."
> > > > > > 
> > > > > > Isn't this a more generic issue that we must avoid the first 4k? If so
> > > > > > I think we should just reserve the first 4k permanently at driver init
> > > > > > time.
> > > 
> > > Is anyone opposed to this plan? Realistically it won't make a
> > > difference.
> > > 
> > > > > 
> > > > > What? On many machines the vga framebuffer is allocated from offset 0. I
> > > > > think some explanation is in order.
> > > 
> > > The chopped off part of the commit message explained it (if I understood
> > > your point).
> > 
> > How do we handle the inherited fb if it starts at offset 0?
> > -Chris
> > 
> 
> I must be missing something important. The FBC buffer is the only one
> requiring a non-zero offset from the base of stolen memory.

Nothing important can be placed there since the CS apparently writes
there without anyone explicitly telling it to do so. That's what I
gathered from the hsd anyway.

I don't have a real answer for the inherited config thing. Should we
just not inherit in that case? One would hope that the BIOS doesn't
put the FB at offset 0, but this being the BIOS I'm pretty sure it
does. I guess we could always make a copy and flip, but it would be
nice if we didn't have to. Or maybe just copy the first page somewhere
else and set up the PTE to compensate? Though that might violate some
assumption we have about stolen being always contiguous...

On a related note our FB takeover code is anywya rather optimistic since
it blindly assumes that the ggtt offset matches the stolen offet. We
never actually look at the PTEs to confirm that.

-- 
Ville Syrjälä
Intel OTC