Re: [PATCH] drm/i915: Add flex arrays to struct i915_syncmap

[Tvrtko Ursulin]

On 08/02/2024 18:13, Erick Archer wrote:
> The "struct i915_syncmap" uses a dynamically sized set of trailing
> elements. It can use an "u32" array or a "struct i915_syncmap *"
> array.
>
> So, use the preferred way in the kernel declaring flexible arrays [1].
> Because there are two possibilities for the trailing arrays, it is
> necessary to declare a union and use the DECLARE_FLEX_ARRAY macro.
>
> The comment can be removed as the union is now clear enough.
>
> Also, avoid the open-coded arithmetic in the memory allocator functions
> [2] using the "struct_size" macro.
>
> Moreover, refactor the "__sync_seqno" and "__sync_child" functions due
> to now it is possible to use the union members added to the structure.
> This way, it is also possible to avoid the open-coded arithmetic in
> pointers.
>
> Link: https://www.kernel.org/doc/html/next/process/deprecated.html#zero-length-and-one-element-arrays [1]
> Link: https://www.kernel.org/doc/html/next/process/deprecated.html#open-coded-arithmetic-in-allocator-arguments [2]
> Signed-off-by: Erick Archer <erick.archer@xxxxxxx>

Looks good to me too so I've pushed it to drm-intel-gt-next, thanks!

Regards,

Tvrtko

> ---
>   drivers/gpu/drm/i915/i915_syncmap.c | 19 ++++++++-----------
>   1 file changed, 8 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_syncmap.c b/drivers/gpu/drm/i915/i915_syncmap.c
> index 60404dbb2e9f..df6437c37373 100644
> --- a/drivers/gpu/drm/i915/i915_syncmap.c
> +++ b/drivers/gpu/drm/i915/i915_syncmap.c
> @@ -75,13 +75,10 @@ struct i915_syncmap {
>   	unsigned int height;
>   	unsigned int bitmap;
>   	struct i915_syncmap *parent;
> -	/*
> -	 * Following this header is an array of either seqno or child pointers:
> -	 * union {
> -	 *	u32 seqno[KSYNCMAP];
> -	 *	struct i915_syncmap *child[KSYNCMAP];
> -	 * };
> -	 */
> +	union {
> +		DECLARE_FLEX_ARRAY(u32, seqno);
> +		DECLARE_FLEX_ARRAY(struct i915_syncmap *, child);
> +	};
>   };
>
>   /**
> @@ -99,13 +96,13 @@ void i915_syncmap_init(struct i915_syncmap **root)
>   static inline u32 *__sync_seqno(struct i915_syncmap *p)
>   {
>   	GEM_BUG_ON(p->height);
> -	return (u32 *)(p + 1);
> +	return p->seqno;
>   }
>
>   static inline struct i915_syncmap **__sync_child(struct i915_syncmap *p)
>   {
>   	GEM_BUG_ON(!p->height);
> -	return (struct i915_syncmap **)(p + 1);
> +	return p->child;
>   }
>
>   static inline unsigned int
> @@ -200,7 +197,7 @@ __sync_alloc_leaf(struct i915_syncmap *parent, u64 id)
>   {
>   	struct i915_syncmap *p;
>
> -	p = kmalloc(sizeof(*p) + KSYNCMAP * sizeof(u32), GFP_KERNEL);
> +	p = kmalloc(struct_size(p, seqno, KSYNCMAP), GFP_KERNEL);
>   	if (unlikely(!p))
>   		return NULL;
>
> @@ -282,7 +279,7 @@ static noinline int __sync_set(struct i915_syncmap **root, u64 id, u32 seqno)
>   			unsigned int above;
>
>   			/* Insert a join above the current layer */
> -			next = kzalloc(sizeof(*next) + KSYNCMAP * sizeof(next),
> +			next = kzalloc(struct_size(next, child, KSYNCMAP),
>   				       GFP_KERNEL);
>   			if (unlikely(!next))
>   				return -ENOMEM;
> --
> 2.25.1