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Date: Fri, 20 Feb 2026 17:20:07 +0100
From: Christoph Hellwig <hch@lst.de>
To: Caleb Sander Mateos <csander@purestorage.com>
Cc: Christoph Hellwig <hch@lst.de>, Keith Busch <kbusch@kernel.org>,
	Jens Axboe <axboe@kernel.dk>, Sagi Grimberg <sagi@grimberg.me>,
	Chaitanya Kulkarni <kch@nvidia.com>, linux-nvme@lists.infradead.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/6] nvme: update nvme_id_ns OPTPERF constants
Message-ID: <20260220162007.GA15462@lst.de>
References: <20260220032809.758089-1-csander@purestorage.com> <20260220032809.758089-3-csander@purestorage.com> <20260220160741.GC15071@lst.de> <CADUfDZrx0pvf4HLk73vMmJNLQ8cE8S0FLLTj-hUE7a4f=uE-RQ@mail.gmail.com>
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On Fri, Feb 20, 2026 at 08:17:29AM -0800, Caleb Sander Mateos wrote:
> On Fri, Feb 20, 2026 at 8:07 AM Christoph Hellwig <hch@lst.de> wrote:
> >
> > On Thu, Feb 19, 2026 at 08:28:05PM -0700, Caleb Sander Mateos wrote:
> > > Since NVM Command Spec 1.1, OPTPERF comprises both bits 4 and 5 of
> > > NSFEAT in the Identify Namespace structure. Replace NVME_NS_FEAT_IO_OPT,
> > > which represented only bit 4, with NVME_NS_FEAT_OPTPERF_SHIFT and
> > > NVME_NS_FEAT_OPTPERF_MASK.
> >
> > As you're deeper in this than me:  do the older specs require the
> > other bit to be cleared to zero and this is all compatible?  Or do
> > we need to guard it somehow?
> 
> In NVM Command Set spec 1.0 (NVMe version 2.0), OPTPERF is only bit 4
> of NSFEAT and the higher bits are reserved. I think it seems
> relatively safe to assume bit 5 will be reported as 0 on controllers
> advertising older NVMe versions, but you're correct that the host
> should technically check for version >= 2.1 before using bit 5. Let me
> know which approach you'd prefer.

Not entirely sure, but either way the code should have a big fat
comment about this.