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Date: Tue, 17 Jun 2025 19:43:26 +0200
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Reply-To: Daniel Gomez <da.gomez@kernel.org>
Subject: Re: [PATCH 7/9] nvme-pci: convert the data mapping blk_rq_dma_map
To: Christoph Hellwig <hch@lst.de>
Cc: Jens Axboe <axboe@kernel.dk>, Keith Busch <kbusch@kernel.org>,
 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
References: <20250610050713.2046316-1-hch@lst.de>
 <20250610050713.2046316-8-hch@lst.de>
 <5c4f1a7f-b56f-4a97-a32e-fa2ded52922a@kernel.org>
 <20250612050256.GH12863@lst.de>
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From: Daniel Gomez <da.gomez@kernel.org>
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On 12/06/2025 07.02, Christoph Hellwig wrote:
> On Wed, Jun 11, 2025 at 02:15:10PM +0200, Daniel Gomez wrote:
>>>  #define NVME_MAX_SEGS \
>>> -	min(NVME_CTRL_PAGE_SIZE / sizeof(struct nvme_sgl_desc), \
>>> -	    (PAGE_SIZE / sizeof(struct scatterlist)))
>>> +	(NVME_CTRL_PAGE_SIZE / sizeof(struct nvme_sgl_desc))
>>
>> The 8 MiB max transfer size is only reachable if host segments are at least 32k.
>> But I think this limitation is only on the SGL side, right?
> 
> Yes, PRPs don't really have the concept of segments to start with.

SGLs don't have the same MPS limitation we have in PRPs. So I think the correct
calculation for NVME_MAX_SEGS is PAGE_SIZE / sizeof(struct nvme_sgl_desc).