[Mesa-dev] [PATCH] r600g: order atom emission

[Alex Deucher]

On Thu, Sep 6, 2012 at 10:54 AM, Jerome Glisse <j.glisse at gmail.com> wrote:
> On Thu, Sep 6, 2012 at 6:20 AM, Dave Airlie <airlied at gmail.com> wrote:
>> On Thu, Sep 6, 2012 at 5:21 PM, Philipp Klaus Krause <pkk at spth.de> wrote:
>>> On 06.09.2012 07:35, j.glisse at gmail.com wrote:
>>>> From: Jerome Glisse <jglisse at redhat.com>
>>>>
>>>> To avoid GPU lockup registers must be emited in a specific order
>>>> (no kidding ...). This patch rework atom emission so order in which
>>>> atom are emited in respect to each other is always the same. We
>>>> don't have any informations on what is the correct order so order
>>>> will need to be infered from fglrx command stream.
>>>
>>> Shouldn't this be stated in comments, so the next person who comes along
>>> and makes a change in this code doesn't inadvertently change the order?
>>
>> Also a comment on what ordering matters most, like I suspect this is
>> just hiding a real issue.
>>
>> Dave.
>
> No it's not hiding an issue, afaict it's how the hw works. The hw do
> what some amd document call states validations. So here is how i
> understand how things happen and i can be completely wrong. Hw process
> register write in order it receive them and to avoid postponing state
> validation the hw do state validation while processing register. That
> means if writing register A trigger state validation that use some
> field of register B the hw might not redo state validation when
> register B is latter written. ie only some register trigger the state
> validation no matter on what they depends on. I believe state
> validation is only use as pipeline optimization by the hw, so the hw
> knows it can take some short cut. But in some rare case if short cut
> are taken for wrong reasons we end up in GPU lockup.
>
> No matter if my guess is right or wrong, i know for a fact that
> register order is important in some situation, that's the hard bottom
> line, no matter what is the reasons inside the hw.
>
> This patch is far from having all the order right, it's just a first
> step, i am atomizing everything and it's what needed to go forward
> without regression.

I've talked to the internal hw and sw guys and they said there isn't
any specific ordering required and the closed driver doesn't impose
any specific order.  The pipeline doesn't get kicked off until a draw
command is issued, so I don't see why the state update order would
matter.  It's possible there are subtle ordering requirements and the
closed driver just happened to get it right.  There are dependencies
and hw bug workarounds however.  E.g., some blocks snoop registers
from other blocks so you need to make sure those dependant registers
have been initialized before drawing.  I don't know if it's the
ordering so much as making sure we emit all the necessary state when
needed.  The closed driver tends to update a lot more state the is
minimally required for a lot of things.  That said, it probably
wouldn't hurt to mirror the closed driver more closely.

Alex

>
> Note that i have been told that in the r100/r200 days same issue came
> up and registers needed to be written in some specific order (well
> only some register matter but i doubt we have a good knowledge on
> that).
>
> Cheers,
> Jerome
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