linux-kernel - Re: [PATCH v1 11/16] dma-mapping: export new dma_*map

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Message-ID: <20250807133848.GL184255@nvidia.com>
Date: Thu, 7 Aug 2025 10:38:48 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Leon Romanovsky <leon@...nel.org>
Cc: Marek Szyprowski <m.szyprowski@...sung.com>,
	Leon Romanovsky <leonro@...dia.com>,
	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 v1 11/16] dma-mapping: export new dma_*map_phys()
 interface

On Mon, Aug 04, 2025 at 03:42:45PM +0300, Leon Romanovsky wrote:
> From: Leon Romanovsky <leonro@...dia.com>
> 
> Introduce new DMA mapping functions dma_map_phys() and dma_unmap_phys()
> that operate directly on physical addresses instead of page+offset
> parameters. This provides a more efficient interface for drivers that
> already have physical addresses available.
> 
> The new functions are implemented as the primary mapping layer, with
> the existing dma_map_page_attrs() and dma_unmap_page_attrs() functions
> converted to simple wrappers around the phys-based implementations.

Briefly explain how the existing functions are remapped into wrappers
calling the phys functions.

> +dma_addr_t dma_map_page_attrs(struct device *dev, struct page *page,
> +		size_t offset, size_t size, enum dma_data_direction dir,
> +		unsigned long attrs)
> +{
> +	phys_addr_t phys = page_to_phys(page) + offset;
> +
> +	if (unlikely(attrs & DMA_ATTR_MMIO))
> +		return DMA_MAPPING_ERROR;
> +
> +	if (IS_ENABLED(CONFIG_DMA_API_DEBUG))
> +		WARN_ON_ONCE(!pfn_valid(PHYS_PFN(phys)));

This is not useful, if we have a struct page and did page_to_phys then
pfn_valid is always true.

Instead this should check for any ZONE_DEVICE page and reject that.
And handle the error:

  if (WARN_ON_ONCE()) return DMA_MAPPING_ERROR;

I'd add another debug check inside dma_map_phys that if !ATTR_MMIO
then pfn_valid, and not zone_device

> @@ -337,41 +364,18 @@ EXPORT_SYMBOL(dma_unmap_sg_attrs);
>  dma_addr_t dma_map_resource(struct device *dev, phys_addr_t phys_addr,
>  		size_t size, enum dma_data_direction dir, unsigned long attrs)
>  {

> -	const struct dma_map_ops *ops = get_dma_ops(dev);
> -	dma_addr_t addr = DMA_MAPPING_ERROR;
> -
> -	BUG_ON(!valid_dma_direction(dir));
> -
> -	if (WARN_ON_ONCE(!dev->dma_mask))
> +	if (IS_ENABLED(CONFIG_DMA_API_DEBUG) &&
> +	    WARN_ON_ONCE(pfn_valid(PHYS_PFN(phys_addr))))
>  		return DMA_MAPPING_ERROR;
>  
> -	if (dma_map_direct(dev, ops))
> -		addr = dma_direct_map_resource(dev, phys_addr, size, dir, attrs);
> -	else if (use_dma_iommu(dev))
> -		addr = iommu_dma_map_resource(dev, phys_addr, size, dir, attrs);
> -	else if (ops->map_resource)
> -		addr = ops->map_resource(dev, phys_addr, size, dir, attrs);
> -
> -	trace_dma_map_resource(dev, phys_addr, addr, size, dir, attrs);
> -	debug_dma_map_resource(dev, phys_addr, size, dir, addr, attrs);
> -	return addr;
> +	return dma_map_phys(dev, phys_addr, size, dir, attrs | DMA_ATTR_MMIO);
>  }
>  EXPORT_SYMBOL(dma_map_resource);

I think this makes alot of sense at least.

Jason