Re: [PATCH 1/3] drm/i915: Stop putting GGTT VMA at the head of the list

From: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
To: Chris Wilson <chris@chris-wilson.co.uk>, Intel-gfx@lists.freedesktop.org
Subject: Re: [PATCH 1/3] drm/i915: Stop putting GGTT VMA at the head of the list
Date: Thu, 04 Dec 2014 10:30:30 +0000	[thread overview]
Message-ID: <548037C6.2030004@linux.intel.com> (raw)
In-Reply-To: <20141204101740.GI25773@nuc-i3427.alporthouse.com>

On 12/04/2014 10:17 AM, Chris Wilson wrote:
> On Thu, Dec 04, 2014 at 10:02:19AM +0000, Tvrtko Ursulin wrote:
>>
>> On 12/04/2014 09:48 AM, Chris Wilson wrote:
>>> On Wed, Dec 03, 2014 at 02:59:24PM +0000, Tvrtko Ursulin wrote:
>>>> From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>>>
>>>> Multiple GGTT VMAs per object will be introduced in the near future which will
>>>> make it impossible to guarantee normal GGTT view is at the head of the list.
>>>>
>>>> Purpose of this patch is to break this assumption straight away so any
>>>> potential hidden assumptions in the code base can be bisected to this
>>>> simple patch.
>>>
>>> No, please don't. The rationale for putting ggtt first is because it
>>> puts the one vma that every object is likely to use at the front, and
>>> makes it also the first in the list for debugging.
>>
>> This came from talking with Daniel, but I don't understand why every
>> object is likely to have a GGTT mapping?
>
> Because the userspace usage pattern is such that it is typical that
> every object is accessed through the GTT at some point in its life.
>
> It's actually becoming less so as we use alternative mmappings, but it
> will remain so for quite some time yet.
>
>> With multiple GGTT views it may happen that only single GGTT VMA
>> exists in the list but it is not the one current code would expect.
>> So the idea was to break that to flesh out any potential hidden
>> assumptions in the code. (I did not find any by just looking
>> though.)
>>
>> What kind of debugging you have in mind, error capture? Or actually
>> inspecting memory of a live kernel?
>
> Error capture inspection of memory of a live kernel.

So personally you don't think it should be of any concern if a GGTT VMA 
is at the head of the list, but it is not the same GGTT VMA which you 
would find there in majority of cases?

Regards,

Tvrtko
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