linux-kernel - Re: GEM-related desktop sluggishness due to linear-time arch_get_unmapped_area

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Date:	Tue, 29 Mar 2011 20:01:04 +0200
From:	Lucas Stach <dev@...xeye.de>
To:	Jerome Glisse <j.glisse@...il.com>
Cc:	r6144 <rainy6144@...il.com>, linux-kernel@...r.kernel.org,
	dri-devel@...ts.freedesktop.org
Subject: Re: GEM-related desktop sluggishness due to linear-time
 arch_get_unmapped_area_topdown()

Am Dienstag, den 29.03.2011, 11:23 -0400 schrieb Jerome Glisse:
> 2011/3/29 r6144 <rainy6144@...il.com>:
> > 在 2011-03-29二的 10:22 -0400，Jerome Glisse写道：
> >
> >> Killer solution would be to have no mapping and a decent
> >> upload/download ioctl that can take userpage.
> >
> > Doesn't this sound like GEM's read/write interface implemented by e.g.
> > the i915 driver?  But if I understand correctly, a mmap-like interface
> > should still be necessary if we want to implement e.g. glMapBuffer()
> > without extra copying.
> >
> > r6144
> >
> >
> glMapBuffer should not be use, it's really not a good way to do stuff.
> Anyway the extra copy might be unavoidable given that sometime the
> front/back might either be in unmappable vram or either have memory
> layout that is not the one specify at buffer creation (this is very
> common when using tiling for instance). So even considering MapBuffer
> or a like function i believe it's a lot better to not allow buffer
> mapping in userspace but provide upload/download hooks that can use
> userpage to avoid as much as possible extra copy.
> 
> Cheers,
> Jerome
> 

Wouldn't this give us a performance penalty for short lived resources
like vbo's which are located in GART memory? Mmap allows us to write
directly to this drm controlled portion of sysram. With a copy based
implementation we would have to allocate the buffer in sysram just to
copy it over to another portion of sysram which seems a little insane to
me, but I'm not an expert here.

-- Lucas


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