Re: [PATCH v2 27/33] drm/xe: Add IOVA-based xe_res_cursor variant

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Low] Unreachable dead code block added for IOVA cursor segment advancement in xe_res_next().
--

--- Patch [27]: [PATCH v2 27/33] drm/xe: Add IOVA-based xe_res_cursor variant ---
commit 8700355546957aacaa7e49fc841d6e860b3ed9e7
Author: Matthew Brost <matthew.brost@intel.com>

drm/xe: Add IOVA-based xe_res_cursor variant

Introduce xe_res_first_iova() to walk a buffer that was DMA mapped using
the IOVA-based DMA API. Such a mapping is described by a struct
dma_iova_state and is a single contiguous IOVA range, so the cursor walks
it as one segment.

> diff --git a/drivers/gpu/drm/xe/xe_res_cursor.h b/drivers/gpu/drm/xe/xe_res_cursor.h
> index 0522caafd89d7..f0f1d699e16e5 100644
> --- a/drivers/gpu/drm/xe/xe_res_cursor.h
> +++ b/drivers/gpu/drm/xe/xe_res_cursor.h

[ ... ]

> @@ -255,6 +264,41 @@ static inline void xe_res_first_dma(const struct drm_pagemap_addr *dma_addr,
> +static inline void xe_res_first_iova(struct dma_iova_state *state,
> +				     u64 start, u64 size,
> +				     struct xe_res_cursor *cur)
> +{
> +	XE_WARN_ON(!state);
> +	XE_WARN_ON(start + size > dma_iova_size(state));
> +
> +	cur->node = NULL;
> +	cur->start = start;
> +	cur->remaining = size;
> +	cur->size = size;
> +	cur->dma_addr = NULL;
> +	cur->sgl = NULL;
> +	cur->dma_start = state->addr;
> +	cur->dma_seg_size = dma_iova_size(state);
> +	cur->mem_type = XE_PL_TT;
> +	cur->is_iova = true;
>  }

[ ... ]

> @@ -283,6 +327,13 @@ static inline void xe_res_next(struct xe_res_cursor *cur, u64 size)
>  	if (cur->size > size) {
>  		cur->size -= size;
>  		cur->start += size;
>  		return;
>  	}
>  
> +	if (cur->is_iova) {
> +		/* Single contiguous IOVA segment. */
> +		cur->start += size;
> +		cur->size = cur->remaining;
> +		return;
> +	}
> +

[Severity: Low]
Is this block unreachable dead code?

In xe_res_first_iova(), the cursor is initialized as a single segment where
cur->size is equal to cur->remaining:

    cur->remaining = size;
    cur->size = size;

When xe_res_next(cur, size) is called, it first subtracts size from
cur->remaining. If the entire remaining segment is consumed, it returns
early via if (!cur->remaining).

If only a portion of the segment is consumed, size is strictly less than
cur->size. This causes the function to always take the early return path
immediately above this newly added block:

    if (cur->size > size) {
        cur->size -= size;
        cur->start += size;
        return;
    }

Because an IOVA cursor is a single contiguous segment where cur->size and
cur->remaining start perfectly synchronized, does it ever need to bypass
these early returns and evaluate the if (cur->is_iova) block?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260710215442.2444235-1-matthew.brost@intel.com?part=27