linux-kernel - Re: [PATCH net] net: Allow non parent devices to be used for ZC DMA

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Message-ID: <20250710164729.275f62fd@kernel.org>
Date: Thu, 10 Jul 2025 16:47:29 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Mina Almasry <almasrymina@...gle.com>
Cc: Dragos Tatulea <dtatulea@...dia.com>, asml.silence@...il.com, Andrew
 Lunn <andrew+netdev@...n.ch>, "David S. Miller" <davem@...emloft.net>, Eric
 Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>, Simon
 Horman <horms@...nel.org>, Jens Axboe <axboe@...nel.dk>, Simona Vetter
 <simona.vetter@...ll.ch>, Willem de Bruijn <willemb@...gle.com>, Kaiyuan
 Zhang <kaiyuanz@...gle.com>, cratiu@...dia.com, parav@...dia.com, Tariq
 Toukan <tariqt@...dia.com>, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, io-uring@...r.kernel.org
Subject: Re: [PATCH net] net: Allow non parent devices to be used for ZC DMA

On Wed, 9 Jul 2025 15:56:16 -0700 Mina Almasry wrote:
> > > nit: This doesn't seem like a fix? The current code supports all
> > > devices that are not SF well enough, right? And in the case of SF
> > > devices, I expect net_devmem_bind_dmabuf() to fail gracefully as the
> > > dma mapping of a device that doesn't support it, I think, would fail
> > > gracefully. So to me this seems like an improvement rather than a bug
> > > fix.
> > >  
> > dma_buf_map_attachment_unlocked() will return a sg_table with 0 nents.
> > That is graceful. However this will result in page_pools that will
> > always be returning errors further down the line which is very confusing
> > regarding the motives that caused it.
> >
> > I am also fine to not make it a fix btw. Especially since the mlx5
> > devmem code was just accepted.
> 
> If you submit another version I'd rather it be a non-fix, especially
> since applying the io_uring hunk will be challenging when backporting
> this patch, but I assume hunk can be dropped while backporting, so I'm
> fine either way.

+1 for non-fix. The core is working as expected, if we want a Fixes tag
I'd aim it at mlx5 and make the patch reject queue binding to SFs
_within mlx5_ until the core bits are figured out.