Re: [PATCH v3 1/3] drm/i915: simplify allocation of driver-internal requests

From: Dave Gordon <david.s.gordon@intel.com>
To: Chris Wilson <chris@chris-wilson.co.uk>,
	intel-gfx@lists.freedesktop.org,
	Jesse Barnes <jbarnes@virtuousgeek.org>
Subject: Re: [PATCH v3 1/3] drm/i915: simplify allocation of driver-internal requests
Date: Mon, 11 Jan 2016 12:45:20 +0000	[thread overview]
Message-ID: <5693A3E0.9000309@intel.com> (raw)
In-Reply-To: <20160107165639.GS652@nuc-i3427.alporthouse.com>

On 07/01/16 16:56, Chris Wilson wrote:
> On Thu, Jan 07, 2016 at 12:34:39PM +0000, Dave Gordon wrote:
>> On 07/01/16 11:58, Chris Wilson wrote:
>>> On Thu, Jan 07, 2016 at 10:20:50AM +0000, Dave Gordon wrote:
>>>> There are a number of places where the driver needs a request, but isn't
>>>> working on behalf of any specific user or in a specific context. At
>>>> present, we associate them with the per-engine default context. A future
>>>> patch will abolish those per-engine context pointers; but we can already
>>>> eliminate a lot of the references to them, just by making the allocator
>>>> allow NULL as a shorthand for "an appropriate context for this ring",
>>>> which will mean that the callers don't need to know anything about how
>>>> the "appropriate context" is found (e.g. per-ring vs per-device, etc).
>>>>
>>>> So this patch renames the existing i915_gem_request_alloc(), and makes
>>>> it local (static inline), and replaces it with a wrapper that provides
>>>> a default if the context is NULL, and also has a nicer calling
>>>> convention (doesn't require a pointer to an output parameter). Then we
>>>> change all callers to use the new convention:
>>>> OLD:
>>>> 	err = i915_gem_request_alloc(ring, user_ctx, &req);
>>>> 	if (err) ...
>>>> NEW:
>>>> 	req = i915_gem_request_alloc(ring, user_ctx);
>>>> 	if (IS_ERR(req)) ...
>>>> OLD:
>>>> 	err = i915_gem_request_alloc(ring, ring->default_context, &req);
>>>> 	if (err) ...
>>>> NEW:
>>>> 	req = i915_gem_request_alloc(ring, NULL);
>>>> 	if (IS_ERR(req)) ...
>>>
>>> Nak. You haven't fixed i915_gem_request_alloc() at all.
>>>
>>> http://cgit.freedesktop.org/~ickle/linux-2.6/commit/?h=breadcrumbs&id=82c72e1a2b4385f0ab07dccee45acef38303e96f
>>> is the patch I have been carrying ever since.
>>> -Chris
>>
>> I think you'll find that the version of i915_gem_request_alloc()
>> I've implemented is equivalent to yours, with the *additional* (and
>> better) semantic of not requiring the caller to specify
>> (ring->default_param) as the context parameter (which is the main
>> point, as far as I'm concerned; making the calling convention nicer
>> was just incidental).
>
> No. Specifying the context is crucial to request allocation. The issue
> in the current function call chains are the requests appear out of
> nowhere rather than being created with explicit context.
>
> Hiding the context is bad.
> -Chris

Why would it be better to name dev_priv->kernel_context at every 
callsite rather than once in the called function (especially in my 
version where it's very obvious that the new wrapper provides that 
default before calling the original underlying allocator).

The context is only important to callers who have one in hand. To the 
others, it's an irrelevance; they're just saying, "hey, I want a request 
but it's not connected to any context I know of, so gimme a default". 
The proof of that is that we can switch them from a per-engine default 
to a per-driver default without changing anything else about them. They 
really don't need to know how to find the default context, so let the 
allocator find it for them.

<aside>
Suppose we some day decide that perhaps there should be *multiple* 
/kernel contexts/ (maybe for greater parallelism); my approach will let 
us spread non-user requests across all suitable contexts just by 
implementing that choice in one place (the new wrapper). By contrast, 
naming the default kernel context at every callsite would make this a 
far more invasive change.
</aside>

It is a commonplace of modern software practice that encapsulating (and 
thus hiding) irrelevant detail is GOOD practice; it is the exposing of 
internal implementation detail that is BAD.

.Dave.
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