linux-kernel - Changing radeon KMS cs+gem ioctl to merge read & write domain

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Date:	Thu, 22 Oct 2009 00:49:09 +0200
From:	Jerome Glisse <glisse@...edesktop.org>
To:	DRI Development Mailing List <dri-devel@...ts.sourceforge.net>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Changing radeon KMS cs+gem ioctl to merge read & write domain

Hi,

I think we should merge the read & write domain of radeon KMS
into a single domains information. I don't think there is a
good reason for separate read & write domain, we did copy intel
model for that and intel use this mostly for cache coherency &
cache flushing as far as i understand. We make no good use of
this inside the kernel. In order to make this change less disruptive
and easier to introduce i propose we keep libdrm-radeon api
intact thus userspace xf86video-ati & mesa 3d driver doesn't
need a single line patch to adapt. Attached is a proof of concept,
a patch against libdrm which merge read & write domain and only
use the read domain to communicate with the kernel. I am still
in process of stress testing this patch but so far neither X
or 3D had any glitches.

I want to take advantage of this change to the cs reloc to the
following:
struct drm_radeon_cs_reloc {
»·······uint32_t»·······»·······handle;
»·······uint32_t»·······»·······domains;
»·······uint32_t»·······»·······unused;
»·······uint32_t»·······»·······flags;
};

With the following rules: a domain is a 4bit value (more than
enough i believe). Userspace can then provide domain preference
for each relocation. For instance :
0 Invalid|CPU
1 VRAM
2 GTT

domains = (VRAM << 0) | (GTT << 4)
would mean try to place object in VRAM first, if not enough
VRAM place it in GTT.

domains = (GTT << 0)
object can only be in GTT
...

I believe this would be a lot more useful information that
read|write domain. We would also now assume that userspace
knows what it's doing inside a single submited cs and that
userspace issue necessary flush if a bo is used in different
way. Which is what the ddx does.

I believe the only argument in favor of read & write split
is broken AGP chipset where GPU can't write to GART. So far
we don't use this information to work around the issue,
we don't even always test AGP writeback. Thus i believe this
change won't impact current user. Note that i am working on
code to work around bad AGP chipset (fallback to PCI GART
for GPU write + detection of broken writeback).

I really think we should take advantage of being in staging
driver to get the ioctl right before we have to freeze them.

Cheers,
Jerome Glisse

View attachment "0001-libdrm-libradeon-Unify-read-write-domain-into-a-sing.patch" of type "text/x-patch" (12272 bytes)