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Date: Mon, 23 Jun 2025 09:27:30 -0600
From: Keith Busch <kbusch@kernel.org>
To: Christoph Hellwig <hch@lst.de>
Cc: Jens Axboe <axboe@kernel.dk>, Sagi Grimberg <sagi@grimberg.me>,
	Chaitanya Kulkarni <kch@nvidia.com>,
	Kanchan Joshi <joshi.k@samsung.com>,
	Leon Romanovsky <leon@kernel.org>,
	Nitesh Shetty <nj.shetty@samsung.com>,
	Logan Gunthorpe <logang@deltatee.com>, linux-block@vger.kernel.org,
	linux-nvme@lists.infradead.org
Subject: Re: [PATCH 3/8] nvme-pci: refactor nvme_pci_use_sgls
Message-ID: <aFlyYjALviyhQ-IE@kbusch-mbp>
References: <20250623141259.76767-1-hch@lst.de>
 <20250623141259.76767-4-hch@lst.de>
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In-Reply-To: <20250623141259.76767-4-hch@lst.de>

On Mon, Jun 23, 2025 at 04:12:25PM +0200, Christoph Hellwig wrote:
> @@ -888,7 +899,9 @@ static blk_status_t nvme_map_data(struct nvme_dev *dev, struct request *req,
>  		goto out_free_sg;
>  	}
>  
> -	if (nvme_pci_use_sgls(dev, req, iod->sgt.nents))
> +	if (use_sgl == SGL_FORCED ||
> +	    (use_sgl == SGL_SUPPORTED &&
> +	     (!sgl_threshold || nvme_pci_avg_seg_size(req) >= sgl_threshold)))
>  		ret = nvme_pci_setup_sgls(nvmeq, req, &cmnd->rw);

We historically interpreted sgl_threshold set to 0 to mean disable SGL
usage, maybe because the controller is broken or something. It might be
okay to have 0 mean to not consider segment sizes, but I just wanted to
point out this is a different interpretation of the user parameter.