[Intel-gfx] [RFC] drm/i915: Smarten up and use to_i915() everywhere

Jani Nikula jani.nikula at linux.intel.com
Thu Mar 17 18:46:05 UTC 2016

On Thu, 17 Mar 2016, Tvrtko Ursulin <tvrtko.ursulin at linux.intel.com> wrote:
> From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> There is a lot of ways to get to our dev_priv depending on which
> object is at hand and often what was chosen by the developer.
> We can make to_i915() accept different pointers by using compile
> time magic. Like:
>   dev_priv = to_i915(request);
>   dev_priv = to_i915(engine);
>   dev_priv = to_i915(ctx);
>   dev_priv = to_i915(dev);
>   dev_priv = to_i915(guc);
>   dev_priv = to_i915(device);
> If an unknown pointer is passed to the function it will cause
> a compile time failure.
> Main advantage is that with this in place we could add and
> remove shourtcuts to dev_priv from supported structures easily
> and without touching the code which uses it. If we wanted to
> fiddle with the balance of structure sizes and number of pointer
> dereferencing for example. And it makes the code a bit tidier
> and uniform.
> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> Cc: Chris Wilson <chris at chris-wilson.co.uk>
> --
> However the churn is huge so I don't really think this is a
> must have.

The "magic" __I915__() macro was added to support a transition from
using dev pointer to using dev_priv pointer. I like the transition, and
we slowly keep doing it.

IMO there have been two problems with that. First, the transition is
slow, because there's nothing forcing us to switch. This was expected,
as we explicitly didn't want a huge patch (like this). Second, it
appears to *still* confuse people after over a year that you can pass
either type of pointer to the macros in C.

I object to this patch both because it's huge (and I'll get my fair
share of the conflicts) and, more importantly, because it promotes an
appearance of a sort of dynamic typing in a statically typed
language. The latter contains an element of surprise to the programmer,
and surprising is not a quality I want to associate with code.

I think I'd rather promote a sensible set of T_to_i915() macros for
cleanly doing the conversion from various types we often use, and slowly
keep transitioning.


Jani Nikula, Intel Open Source Technology Center

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