Re: [PATCH v2 06/23] dma-buf: Let the exporter select mapping type at pin/map

[Karl Mehltretter <kmehltretter@gmail.com>]

On Thu, Nov 05, 2026 at 05:40:41PM +0100, Alistair Popple wrote:
> + * A pin or map that fails after dma_buf_mapping_select() may reopen the
> + * choice only if it was the one that made it. A selection that predates the
> + * call belongs to a live pin or mapping and must survive, otherwise the next
> + * pin or map would reselect under it without an invalidate.
> + */
> +void dma_buf_mapping_unselect(struct dma_buf_attachment *attach,
> +			      bool was_selected)
> +{
> +	dma_resv_assert_held(attach->dmabuf->resv);
> +
> +	if (was_selected)
> +		return;
> +
> +	attach->selected = false;
> +	attach->selected_idx = 0;
> +}
> +

After dma_buf_invalidate_mappings(), attach->selected is false while
attach->selected_idx still holds that mapping's entry for a pending
unmap. 

If a pin/map fails before that unmap runs, was_selected is false and
dma_buf_mapping_unselect() resets attach->selected_idx to 0.

A subsequent dma_buf_unmap_sgt() then uses matches[0].

>  int dma_buf_pin(struct dma_buf_attachment *attach)
>  {
>  	struct dma_buf *dmabuf = attach->dmabuf;
> +	bool selected = attach->selected;
>  	int ret = 0;
>  
>  	WARN_ON(!attach->importer_ops);
>  
>  	dma_resv_assert_held(dmabuf->resv);
>  
> -	if (dmabuf->ops->pin)
> +	ret = dma_buf_mapping_select(attach);
> +	if (ret)
> +		return ret;
> +
> +	if (dmabuf->ops->pin) {
>  		ret = dmabuf->ops->pin(attach);
> +		if (ret)
> +			dma_buf_mapping_unselect(attach, selected);
> +	}

dma_buf_mapping_select() sets attach->selected = true before ops->pin()
and the map callbacks run.

In patches 14/15, xe/amdgpu migrate a VRAM buffer to system memory
during pin and map when the attachment does not accept P2P.

That migration calls dma_buf_invalidate_mappings() and clears
attach->selected on every attachment, also the one being pinned/mapped. 

When the callback succeeds, the attachment stays pinned or mapped with
attach->selected == false.

A later map/pin on the same attachment can then reselect without an
invalidation.

Thanks,
Karl