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Date: Tue, 25 Nov 2025 07:35:40 +0100
From: Christoph Hellwig <hch@lst.de>
To: Keith Busch <kbusch@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>, Keith Busch <kbusch@meta.com>,
	dm-devel@lists.linux.dev, linux-block@vger.kernel.org,
	snitzer@kernel.org, axboe@kernel.dk, ebiggers@kernel.org
Subject: Re: [PATCHv2 1/3] block: remove stacking default dma_alignment
Message-ID: <20251125063540.GA14873@lst.de>
References: <20251124170903.3931792-1-kbusch@meta.com> <20251124170903.3931792-2-kbusch@meta.com> <20251124171230.GA29490@lst.de> <aSUZP6yP8mvi-q7v@kbusch-mbp>
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User-Agent: Mutt/1.5.17 (2007-11-01)

On Mon, Nov 24, 2025 at 07:49:35PM -0700, Keith Busch wrote:
> On Mon, Nov 24, 2025 at 06:12:30PM +0100, Christoph Hellwig wrote:
> > On Mon, Nov 24, 2025 at 09:09:01AM -0800, Keith Busch wrote:
> > > From: Keith Busch <kbusch@kernel.org>
> > > 
> > > The dma_alignment becomes 511 anyway if the caller doesn't explicitly
> > > set it. But setting this default prevents the stacked device from
> > > requesting a lower value even if it can handle lower alignments.
> > 
> > Given how much trouble we had with drivers doing software processing
> > and unaligned buffers I'd feel more comfortable keeping this default
> > limitation.  Drivers that want to relax it can still trivially do
> > that right after calling blk_set_stacking_limits.
> 
> That should have the same result, but it's totally fine with me if you
> prefer the caller overwrite the default instead.

I think it's much safer if we require the opt-in.