netdev - Re: [RFC net-next 1/4] net: Allow non parent devices to be used for ZC DMA

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Message-ID: <c006f353-8b35-43c5-b010-d058954ff993@gmail.com>
Date: Tue, 8 Jul 2025 12:08:48 +0100
From: Pavel Begunkov <asml.silence@...il.com>
To: Dragos Tatulea <dtatulea@...dia.com>, Parav Pandit <parav@...dia.com>,
 Jakub Kicinski <kuba@...nel.org>
Cc: "almasrymina@...gle.com" <almasrymina@...gle.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>,
 Saeed Mahameed <saeedm@...dia.com>, Tariq Toukan <tariqt@...dia.com>,
 Cosmin Ratiu <cratiu@...dia.com>,
 "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC net-next 1/4] net: Allow non parent devices to be used for
 ZC DMA

On 7/4/25 14:11, Dragos Tatulea wrote:
> On Thu, Jul 03, 2025 at 01:58:50PM +0200, Parav Pandit wrote:
>>
>>> From: Jakub Kicinski <kuba@...nel.org>
>>> Sent: 03 July 2025 02:23 AM
...>> In an offline discussion, Dragos mentioned that io_uring already
>> operates at the queue level, may be some ideas can be picked up
>> from io_uring?
> The problem for devmem is that the device based API is already set in
> stone so not sure how we can change this. Maybe Mina can chime in.
> 
> To sum the conversation up, there are 2 imperfect and overlapping
> solutions:
> 
> 1) For the common case of having a single PCI device per netdev, going one
>     parent up if the parent device is not DMA capable would be a good
>     starting point.
> 
> 2) For multi-PF netdev [0], a per-queue get_dma_dev() op would be ideal
>     as it provides the right PF device for the given queue. io_uring
>     could use this but devmem can't. Devmem could use 1. but the
>     driver has to detect and block the multi PF case.
> 
> I think we need both. Either that or a netdev op with an optional queue
> parameter. Any thoughts?

No objection from zcrx for either approach, but it sounds like a good
idea to have something simple for 1) sooner than later, and perhaps
marked as a fix.

-- 
Pavel Begunkov