Re: [PATCH 1/1] scsi: sas: skip opt_sectors when DMA reports no real optimization hint

[Christoph Hellwig]

>  	if (dma_dev->dma_mask) {
> -		shost->opt_sectors = min_t(unsigned int, shost->max_sectors,
> -				dma_opt_mapping_size(dma_dev) >> SECTOR_SHIFT);
> +		size_t opt = dma_opt_mapping_size(dma_dev);
> +
> +		/*
> +		 * Only set opt_sectors when the DMA layer reports a
> +		 * genuine optimization constraint.  When opt equals
> +		 * dma_max_mapping_size() no backend provided a real
> +		 * hint — the value is just the DMA maximum, which is
> +		 * not useful as an optimal I/O size and can cause
> +		 * mkfs.xfs to compute invalid stripe geometry.
> +		 */
> +		if (opt < dma_max_mapping_size(dma_dev))
> +			shost->opt_sectors = min_t(unsigned int,
> +					shost->max_sectors,
> +					opt >> SECTOR_SHIFT);

This looks reasonable, but please also round down the opt value
to a power of two when you touch this anyway.

And especially with that this logic is complicated enough that it
warrants a little helper that is clearly split out.