Re: [PATCH v4] scsi: sas: skip opt_sectors when DMA reports no real optimization hint

[Ionut Nechita (Wind River)]

On Wed, 19 Mar 2026 11:07:00 +0000, Damien Le Moal wrote:
> Why return 0 ? This is a valid case, so this should get through the
> alignment below.

Hi Damien,

Thanks for the review.

The opt == max case is specifically the bug this patch fixes.

When the IOMMU is disabled or in passthrough mode and no DMA ops
provide an opt_mapping_size callback, dma_opt_mapping_size() falls
back to min(SIZE_MAX, dma_max_mapping_size()), which equals
dma_max_mapping_size().  So opt == max.

If we let that value through, rounddown_pow_of_two() produces a
huge power-of-two, and min_t() caps it at max_sectors (32767).
That gives opt_sectors = 32767, which is exactly the bogus value
that breaks mkfs.xfs:

  swidth = 16773120 / 4096 = 4095
  sunit  = 8192 / 4096     = 2
  4095 % 2 != 0  ->  "SB stripe unit sanity check failed"

The key insight (from Robin Murphy's v1 review) is that when no
backend provides a real optimization constraint, the DMA core
returns the largest efficient size == the largest size.  That is
correct DMA semantics, but it means opt == max signals "no
preference", not "the optimal size happens to equal the maximum".

Returning 0 in that case means "no preference", which leaves
opt_sectors at 0 and lets the disk's own geometry (or lack
thereof) determine the I/O size.

Regarding the Cc list: noted, I will trim it for v5 if needed.

Thanks,
Ionut