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Date: Thu, 24 Jul 2025 00:30:34 -0700
From: Christoph Hellwig <hch@infradead.org>
To: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Christoph Hellwig <hch@infradead.org>,
	Yonatan Maman <ymaman@nvidia.com>,
	=?iso-8859-1?B?Suly9G1l?= Glisse <jglisse@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Leon Romanovsky <leon@kernel.org>, Lyude Paul <lyude@redhat.com>,
	Danilo Krummrich <dakr@kernel.org>,
	David Airlie <airlied@gmail.com>, Simona Vetter <simona@ffwll.ch>,
	Alistair Popple <apopple@nvidia.com>,
	Ben Skeggs <bskeggs@nvidia.com>,
	Michael Guralnik <michaelgur@nvidia.com>,
	Or Har-Toov <ohartoov@nvidia.com>,
	Daisuke Matsuda <dskmtsd@gmail.com>, Shay Drory <shayd@nvidia.com>,
	linux-mm@kvack.org, linux-rdma@vger.kernel.org,
	dri-devel@lists.freedesktop.org, nouveau@lists.freedesktop.org,
	linux-kernel@vger.kernel.org, Gal Shalom <GalShalom@nvidia.com>
Subject: Re: [PATCH v2 4/5] RDMA/mlx5: Enable P2P DMA with fallback mechanism
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References: <20250718115112.3881129-1-ymaman@nvidia.com>
 <20250718115112.3881129-5-ymaman@nvidia.com>
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On Wed, Jul 23, 2025 at 12:55:22AM -0300, Jason Gunthorpe wrote:
> On Mon, Jul 21, 2025 at 12:03:41AM -0700, Christoph Hellwig wrote:
> > On Fri, Jul 18, 2025 at 02:51:11PM +0300, Yonatan Maman wrote:
> > > From: Yonatan Maman <Ymaman@Nvidia.com>
> > > 
> > > Add support for P2P for MLX5 NIC devices with automatic fallback to
> > > standard DMA when P2P mapping fails.
> > 
> > That's now how the P2P API works.  You need to check the P2P availability
> > higher up.
> 
> How do you mean?
> 
> This looks OKish to me, for ODP and HMM it has to check the P2P
> availability on a page by page basis because every single page can be
> a different origin device.
> 
> There isn't really a higher up here...

The DMA API expects the caller to already check for connectability,
why can't HMM do that like everyone else?