Re: [PATCH 17/49] drm/i915/bdw: A bit more advanced context init/fini

From: Ben Widawsky <ben@bwidawsk.net>
To: Damien Lespiau <damien.lespiau@intel.com>
Cc: "intel-gfx@lists.freedesktop.org"
	<intel-gfx@lists.freedesktop.org>,
	"Widawsky, Benjamin" <benjamin.widawsky@intel.com>
Subject: Re: [PATCH 17/49] drm/i915/bdw: A bit more advanced context init/fini
Date: Tue, 1 Apr 2014 21:07:57 -0700	[thread overview]
Message-ID: <20140402040756.GA2507@bwidawsk.net> (raw)
In-Reply-To: <20140401210512.GH16291@strange.amr.corp.intel.com>

On Tue, Apr 01, 2014 at 10:05:12PM +0100, Damien Lespiau wrote:
> On Tue, Apr 01, 2014 at 12:18:24PM -0700, Ben Widawsky wrote:
> > On Tue, Apr 01, 2014 at 02:51:27PM +0100, Damien Lespiau wrote:
> > > On Tue, Apr 01, 2014 at 02:47:19PM +0100, Mateo Lozano, Oscar wrote:
> > > > > > --- a/drivers/gpu/drm/i915/i915_lrc.c
> > > > > > +++ b/drivers/gpu/drm/i915/i915_lrc.c
> > > > > > @@ -41,7 +41,45 @@
> > > > > >  #include <drm/i915_drm.h>
> > > > > >  #include "i915_drv.h"
> > > > > >
> > > > > > +#define GEN8_LR_CONTEXT_SIZE (21 * PAGE_SIZE)
> > > > > 
> > > > > I'm a bit puzzled by that number:
> > > > >   - I found a sentence saying: "the Context Image for the rendering
> > > > >     engine consists of 20 4K pages", which seems that it includes the
> > > > >     HWS page (on the same page it says context layout = HWS Page +
> > > > >     register state context).
> > > > >   - When looking at the register state context for the render engine:
> > > > >     18096 dwords -> 18 pages, so in total it'd be 19 pages (need to add
> > > > >     the HWS Page)
> > > > >   - Clearly I must be missing something :)
> > > > >   - That's only for the render engine, other engines have a much smaller
> > > > >     context, smaller enough that it's worth looking at their exact size.
> > > > >   - It'd be nice to work out the real size from the *CXT_*SIZE
> > > > >     registers.
> > > > 
> > > > Hmmmm... I´ll try to get the real context sizes from the registers and
> > > > compare. At least for RCS, VCS and BCS since there doesn´t seem to be
> > > > a register for VECS?
> > > 
> > > Couldn't find it either. I guess we'll need to ask the help of a friend.
> > > Or the 50/50 joker maybe.
> > > 
> > > -- 
> > > Damien
> > 
> > CXT_SIZE is total garbage on anything past Ivybridge. That's why we
> > don't use it for HSW either... I know, right? We should request the spec
> > get updated. I have no excuse for not requesting that sooner.
> 
> (talking about BDW only)
> 
> For the render ring:
> 
> HWSP: 4KB
> Ring context: CTX_SIZE[26:24] 5 cache lines -> offsets (in DW) 0x0 to 0x4f (= 5 * 64 / 4)
> Render context: CTX_SIZE[23:16] -> 0x65 caches lines -> offets (in DW) 0x50 to 0x69f (= 0x50 + 0x65 * 64 / 4 - 1)
> VF/VFE context CTX_SIZE[7:0] -> 0x82 cache lines -> offsets (in DW) 0x6A0 to 0xebf (= 0x6a0 + 0x82*64/4 - 1)
> Atomic storage is the max that you can allocate, 32KB ie 8192 DWords
> 
> So we're almost there. What's missing here is the RS context size, couldn't find
> it in the spec :/ Maybe because that is a "well known" value.
> 
> Note that I don't actually know what we read back from hw.
> 
> Considering that the BCS context size seems to be 2 pages, I think it's worth
> digging a bit more to save ~66KB per BCS context (for instance). Even if we
> have to hardcode the different context sizes.
> 
> -- 
> Damien

I guess I should have checked first. Looks like there are actually quite
a few changes since I wrote the code originally.

Carry on.

-- 
Ben Widawsky, Intel Open Source Technology Center
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