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Date: Fri, 24 Apr 2026 15:21:00 +0200
From: Christoph Hellwig <hch@lst.de>
To: "Ionut Nechita (Wind River)" <ionut.nechita@windriver.com>
Cc: "James E . J . Bottomley" <James.Bottomley@HansenPartnership.com>,
	"Martin K . Petersen" <martin.petersen@oracle.com>,
	linux-scsi@vger.kernel.org, linux-kernel@vger.kernel.org,
	stable@vger.kernel.org, hch@lst.de, dlemoal@kernel.org,
	robin.murphy@arm.com, john.g.garry@oracle.com, axboe@kernel.dk,
	m.szyprowski@samsung.com, ahuang12@lenovo.com,
	ionut_n2001@yahoo.com, sunlightlinux@gmail.com
Subject: Re: [PATCH v7 1/1] scsi: sas: skip opt_sectors when DMA reports no
 real optimization hint
Message-ID: <20260424132100.GA15553@lst.de>
References: <20260415071849.25693-1-ionut.nechita@windriver.com> <20260415071849.25693-2-ionut.nechita@windriver.com>
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In-Reply-To: <20260415071849.25693-2-ionut.nechita@windriver.com>
User-Agent: Mutt/1.5.17 (2007-11-01)

On Wed, Apr 15, 2026 at 10:18:49AM +0300, Ionut Nechita (Wind River) wrote:
> +/*
> + * Set shost->opt_sectors from the DMA optimal mapping size, but only
> + * when dma_opt_mapping_size() is strictly less than dma_max_mapping_size(),
> + * indicating a genuine optimization hint from an IOMMU or DMA backend.
> + * When the two are equal (e.g. IOMMU disabled / passthrough), no real
> + * hint exists, so leave opt_sectors at 0 to avoid bogus optimal_io_size
> + * values that break filesystem geometry (e.g. mkfs.xfs stripe alignment).
> + */
> +static void sas_dma_setup_opt_sectors(struct Scsi_Host *shost)
> +{
> +	struct device *dma_dev = shost->dma_dev;
> +	size_t opt, max;
> +	unsigned int opt_sectors;
> +
> +	if (!dma_dev->dma_mask)
> +		return;

Upper layers have no real busines looking at dma_dev->dma_mask. What
is this check intended to do?

> +
> +	opt = dma_opt_mapping_size(dma_dev);
> +	max = dma_max_mapping_size(dma_dev);
> +
> +	if (opt >= max)
> +		return;
> +
> +	opt_sectors = min_t(unsigned int, opt >> SECTOR_SHIFT,
> +			    shost->max_sectors);
> +	if (!opt_sectors)
> +		return;
> +
> +	shost->opt_sectors = rounddown_pow_of_two(opt_sectors);

Please add comments explaining the logic.