linux-kernel - Re: [PATCH 7/8] docs: dma-api: update streaming DMA API physical address constraints

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Message-ID: <aF6Urf667AKtVGXr@infradead.org>
Date: Fri, 27 Jun 2025 05:55:09 -0700
From: Christoph Hellwig <hch@...radead.org>
To: Robin Murphy <robin.murphy@....com>
Cc: Petr Tesarik <ptesarik@...e.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Bagas Sanjaya <bagasdotme@...il.com>,
	Jonathan Corbet <corbet@....net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Leon Romanovsky <leon@...nel.org>, Keith Busch <kbusch@...nel.org>,
	Caleb Sander Mateos <csander@...estorage.com>,
	Sagi Grimberg <sagi@...mberg.me>, Jens Axboe <axboe@...nel.dk>,
	John Garry <john.g.garry@...cle.com>,
	"open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
	open list <linux-kernel@...r.kernel.org>,
	"open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
	iommu@...ts.linux.dev
Subject: Re: [PATCH 7/8] docs: dma-api: update streaming DMA API physical
 address constraints

On Thu, Jun 26, 2025 at 05:45:18PM +0100, Robin Murphy wrote:
> Indeed that might actually end up pushing things in the opposite direction,
> at least in some cases. Right now, a driver with, say, a 40-bit DMA mask is
> usually better off not special-casing DMA buffers, and just making plain
> GFP_KERNEL allocations for everything (on the assumption that 64-bit systems
> with masses of memory *should* have SWIOTLB to cover things in the worst
> case), vs. artificially constraining its DMA buffers to GFP_DMA32 and having
> to deal with allocation failure more often. However with a more precise and
> flexible allocator, there's then a much stronger incentive for such drivers
> to explicitly mark *every* allocation that may be used for DMA, in order to
> get the optimal behaviour.

It really should be using dma_alloc_pages to ensure it gets addressable
memory for these cases.  For sub-page allocations it could use dmapool,
but that's a little annoying because it does coherent allocations which
95% of the users don't actually need.