linux-kernel - Re: [PATCH v2 4/5] RDMA/mlx5: Enable P2P DMA with fallback mechanism

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Message-ID: <aIBdKhzft4umCGZO@ziepe.ca>
Date: Wed, 23 Jul 2025 00:55:22 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Christoph Hellwig <hch@...radead.org>
Cc: Yonatan Maman <ymaman@...dia.com>,
	Jérôme Glisse <jglisse@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Leon Romanovsky <leon@...nel.org>, Lyude Paul <lyude@...hat.com>,
	Danilo Krummrich <dakr@...nel.org>,
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	Alistair Popple <apopple@...dia.com>,
	Ben Skeggs <bskeggs@...dia.com>,
	Michael Guralnik <michaelgur@...dia.com>,
	Or Har-Toov <ohartoov@...dia.com>,
	Daisuke Matsuda <dskmtsd@...il.com>, Shay Drory <shayd@...dia.com>,
	linux-mm@...ck.org, linux-rdma@...r.kernel.org,
	dri-devel@...ts.freedesktop.org, nouveau@...ts.freedesktop.org,
	linux-kernel@...r.kernel.org, Gal Shalom <GalShalom@...dia.com>
Subject: Re: [PATCH v2 4/5] RDMA/mlx5: Enable P2P DMA with fallback mechanism

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@...dia.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...

Jason