Re: [PATCH] drm/i915: change node clearing from memset to initialization

[Tom Rix <trix@redhat.com>]

On 4/16/22 2:04 PM, Joe Perches wrote:
> On Sat, 2022-04-16 at 13:48 -0700, Tom Rix wrote:
>> On 4/16/22 11:33 AM, Joe Perches wrote:
>>> On Sat, 2022-04-16 at 13:23 -0400, Tom Rix wrote:
>>>> In insert_mappable_node(), the parameter node is
>>>> cleared late in node's use with memset.
>>>> insert_mappable_node() is a singleton, called only
>>>> from i915_gem_gtt_prepare() which itself is only
>>>> called by i915_gem_gtt_pread() and
>>>> i915_gem_gtt_pwrite_fast() where the definition of
>>>> node originates.
>>>>
>>>> Instead of using memset, initialize node to 0 at it's
>>>> definitions.
>>> trivia: /it's/its/
>>>
>>> Only reason _not_ to do this is memset is guaranteed to
>>> zero any padding that might go to userspace.
>>>
>>> But it doesn't seem there is any padding anyway nor is
>>> the struct available to userspace.
>>>
>>> So this seems fine though it might increase overall code
>>> size a tiny bit.
>>>
>>> I do have a caveat: see below:
>>>
>>>> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
>>> []
>>>> @@ -328,7 +327,6 @@ static struct i915_vma *i915_gem_gtt_prepare(struct drm_i915_gem_object *obj,
>>>>    		goto err_ww;
>>>>    	} else if (!IS_ERR(vma)) {
>>>>    		node->start = i915_ggtt_offset(vma);
>>>> -		node->flags = 0;
>>> Why is this unneeded?
>> node = {} initializes all of node's elements to 0, including this one.
> true, but could the call to insert_mappable_node combined with the
> retry goto in i915_gem_gtt_prepare set this to non-zero?

Yikes!

I'll add that back.

Thanks for pointing it out.

Tom

>
>