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[2a00:1028:83b8:1e7a:3010:3bd6:8521:caf1]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4538a406ab6sm50817745e9.30.2025.06.27.06.02.02 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 27 Jun 2025 06:02:03 -0700 (PDT) Date: Fri, 27 Jun 2025 15:02:00 +0200 From: Petr Tesarik To: Christoph Hellwig Cc: Robin Murphy , Marek Szyprowski , Bagas Sanjaya , Jonathan Corbet , Andrew Morton , Leon Romanovsky , Keith Busch , Caleb Sander Mateos , Sagi Grimberg , Jens Axboe , John Garry , "open list:DOCUMENTATION" , open list , "open list:MEMORY MANAGEMENT" , iommu@lists.linux.dev Subject: Re: [PATCH 7/8] docs: dma-api: update streaming DMA API physical address constraints Message-ID: <20250627150200.2192c6b1@mordecai.tesarici.cz> In-Reply-To: References: <20250624133923.1140421-1-ptesarik@suse.com> <20250624133923.1140421-8-ptesarik@suse.com> <20250626070602.3d42b607@mordecai.tesarici.cz> <20250626154818.46b0cfb1@mordecai.tesarici.cz> <0f95be6d-2e13-4281-98b5-6d4311a3b98f@arm.com> X-Mailer: Claws Mail 4.3.1 (GTK 3.24.50; x86_64-suse-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 8CB7F100022 X-Stat-Signature: 5yzfewh874x1iomh8j81cow9gjtkbtsm X-Rspam-User: X-Rspamd-Server: rspam01 X-HE-Tag: 1751029326-9538 X-HE-Meta: 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 KnXcCjeb p8XNwoezRtFyqYqFq8/S7zdUWYSUJlZscczDjmCeUSUYcIa3hUvXVgFJaMl9+Hjq7BJZ80BgZr6BwoQ4xBBuKxKQpv9TOcpOZI+qe77SfjYHiT7b6q5IUZYfA7XgVEAvKM3tPRmnYefZSUer5X1JEwErz6gO3fOxqLizlxQ+0pYpRYbZnyMwSu9KOJ5WZapvB4PlkqZY2rc0YpsZjkVNiKsoxzvxvdBHwswQdbsU5RPV6Lt71lbuNjoSVqLdPGvPMPrf+zML22V+zoaKDEzv1AUhuNJTXJtNvY62UgJh2nq6Y06QVXiWTwT0+PxHpEUULwbkUXTiQCZhIDca4UeyYeQCEcB1IIbsO5+ps2T/0Tx/oJaIbBTQV6uNRoQ== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, 27 Jun 2025 05:55:09 -0700 Christoph Hellwig wrote: > 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. Wow, thank you for this insight! There's one item on my TODO list: convert SLAB_CACHE_DMA caches to dmapool. But now I see it would introduce a regression (accessing DMA-coherent pages may be much slower). I could implement a variant of dmapool which allocates normal pages from a given physical address range, and it seems it would be actually useful. Petr T