linux-kernel - Re: [PATCH v4 00/16] dma-mapping: migrate to physical address-based API

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Message-ID: <20250901222302.GA186519@nvidia.com>
Date: Mon, 1 Sep 2025 19:23:02 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Marek Szyprowski <m.szyprowski@...sung.com>
Cc: Leon Romanovsky <leon@...nel.org>,
	Abdiel Janulgue <abdiel.janulgue@...il.com>,
	Alexander Potapenko <glider@...gle.com>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Hellwig <hch@....de>, Danilo Krummrich <dakr@...nel.org>,
	iommu@...ts.linux.dev, Jason Wang <jasowang@...hat.com>,
	Jens Axboe <axboe@...nel.dk>, Joerg Roedel <joro@...tes.org>,
	Jonathan Corbet <corbet@....net>, Juergen Gross <jgross@...e.com>,
	kasan-dev@...glegroups.com, Keith Busch <kbusch@...nel.org>,
	linux-block@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-nvme@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org,
	linux-trace-kernel@...r.kernel.org,
	Madhavan Srinivasan <maddy@...ux.ibm.com>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Miguel Ojeda <ojeda@...nel.org>,
	Robin Murphy <robin.murphy@....com>, rust-for-linux@...r.kernel.org,
	Sagi Grimberg <sagi@...mberg.me>,
	Stefano Stabellini <sstabellini@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	virtualization@...ts.linux.dev, Will Deacon <will@...nel.org>,
	xen-devel@...ts.xenproject.org
Subject: Re: [PATCH v4 00/16] dma-mapping: migrate to physical address-based
 API

On Mon, Sep 01, 2025 at 11:47:59PM +0200, Marek Szyprowski wrote:
> I would like to give those patches a try in linux-next, but in meantime 
> I tested it on my test farm and found a regression in dma_map_resource() 
> handling. Namely the dma_map_resource() is no longer possible with size 
> not aligned to kmalloc()'ed buffer, as dma_direct_map_phys() calls 
> dma_kmalloc_needs_bounce(),

Hmm, it's this bit:

	capable = dma_capable(dev, dma_addr, size, !(attrs & DMA_ATTR_MMIO));
	if (unlikely(!capable) || dma_kmalloc_needs_bounce(dev, size, dir)) {
		if (is_swiotlb_active(dev) && !(attrs & DMA_ATTR_MMIO))
			return swiotlb_map(dev, phys, size, dir, attrs);

		goto err_overflow;
	}

We shouldn't be checking dma_kmalloc_needs_bounce() on mmio as there
is no cache flushing so the "dma safe alignment" for non-coherent DMA
does not apply.

Like you say looks good to me, and more of the surrouding code can be
pulled in too, no sense in repeating the boolean logic:

	if (attrs & DMA_ATTR_MMIO) {
		dma_addr = phys;
		if (unlikely(!dma_capable(dev, dma_addr, size, false)))
			goto err_overflow;
	} else {
		dma_addr = phys_to_dma(dev, phys);
		if (unlikely(!dma_capable(dev, dma_addr, size, true)) ||
		    dma_kmalloc_needs_bounce(dev, size, dir)) {
			if (is_swiotlb_active(dev))
				return swiotlb_map(dev, phys, size, dir, attrs);

			goto err_overflow;
		}
		if (!dev_is_dma_coherent(dev) &&
		    !(attrs & DMA_ATTR_SKIP_CPU_SYNC))
			arch_sync_dma_for_device(phys, size, dir);
	}

Jason