linux-kernel - Re: [PATCH v12 2/3] rust: add dma coherent allocator abstraction.

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Message-ID: <20250306161818.GG354511@nvidia.com>
Date: Thu, 6 Mar 2025 12:18:18 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: Danilo Krummrich <dakr@...nel.org>
Cc: Simona Vetter <simona.vetter@...ll.ch>,
	Abdiel Janulgue <abdiel.janulgue@...il.com>, aliceryhl@...gle.com,
	robin.murphy@....com, daniel.almeida@...labora.com,
	rust-for-linux@...r.kernel.org, Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Trevor Gross <tmgross@...ch.edu>,
	Valentin Obst <kernel@...entinobst.de>,
	open list <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@....de>,
	Marek Szyprowski <m.szyprowski@...sung.com>, airlied@...hat.com,
	"open list:DMA MAPPING HELPERS" <iommu@...ts.linux.dev>
Subject: Re: [PATCH v12 2/3] rust: add dma coherent allocator abstraction.

On Thu, Mar 06, 2025 at 04:54:14PM +0100, Danilo Krummrich wrote:
> (For some reason, when replying to this mail, mutt removed Sima from To: and
> instead switched Cc: to To:, hence resending.)

It is normal, Simona's mail client is setup to do that.

> > I think for basic driver allocations that you just need to run the device
> > stuffing it all into devres is ok.
> 
> What exactly do you mean with that? DMA memory allocations or "normal" memory
> allocations?

Simona means things like a coherent allocation backing something
allocated once like a global queue for talking to the device.

Ie DMA API usage that is not on the performance path.

> > But for dma mappings at runtime this will be too slow.
> 
> What exactly do you mean with "DMA mappings at runtime"? What to you think is
> is slow in this aspect?

Things like dma_map_sg(), dma_map_page(), etc, etc.

You cannot propose to add any runtime overhead to those paths and get
any support from the kernel community. They are performance paths
optimized to be fast.

For example: proposing to wrap their allocation in a devm container -
allocate a tracking structure, acquire the per-device spinlock and
thread the tracking into the devm linked list. For every single IO on
the performance path.

That idea would get a very hard NAK.

Jason