linux-kernel - Re: [PATCH v1 09/16] dma-mapping: handle MMIO flow in dma_map|unmap

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Message-ID: <20250807130811.GI184255@nvidia.com>
Date: Thu, 7 Aug 2025 10:08:11 -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 09/16] dma-mapping: handle MMIO flow in
 dma_map|unmap_page

On Mon, Aug 04, 2025 at 03:42:43PM +0300, Leon Romanovsky wrote:
> From: Leon Romanovsky <leonro@...dia.com>
> 
> Extend base DMA page API to handle MMIO flow.

I would mention here this follows the long ago agreement that we don't
need to enable P2P in the legacy dma_ops area. Simply failing when
getting an ATTR_MMIO is OK.

> --- a/kernel/dma/mapping.c
> +++ b/kernel/dma/mapping.c
> @@ -158,6 +158,7 @@ dma_addr_t dma_map_page_attrs(struct device *dev, struct page *page,
>  {
>  	const struct dma_map_ops *ops = get_dma_ops(dev);
>  	phys_addr_t phys = page_to_phys(page) + offset;
> +	bool is_mmio = attrs & DMA_ATTR_MMIO;
>  	dma_addr_t addr;
>  
>  	BUG_ON(!valid_dma_direction(dir));
> @@ -166,12 +167,23 @@ dma_addr_t dma_map_page_attrs(struct device *dev, struct page *page,
>  		return DMA_MAPPING_ERROR;
>  
>  	if (dma_map_direct(dev, ops) ||
> -	    arch_dma_map_phys_direct(dev, phys + size))
> +	    (!is_mmio && arch_dma_map_phys_direct(dev, phys + size)))
>  		addr = dma_direct_map_phys(dev, phys, size, dir, attrs);

I don't know this area, maybe explain a bit in the commit message how
you see ATTR_MMIO interacts with arch_dma_map_phys_direct ?

>  	else if (use_dma_iommu(dev))
>  		addr = iommu_dma_map_phys(dev, phys, size, dir, attrs);
> -	else
> +	else if (is_mmio) {
> +		if (!ops->map_resource)
> +			return DMA_MAPPING_ERROR;
> +
> +		addr = ops->map_resource(dev, phys, size, dir, attrs);
> +	} else {
> +		/*
> +		 * All platforms which implement .map_page() don't support
> +		 * non-struct page backed addresses.
> +		 */
>  		addr = ops->map_page(dev, page, offset, size, dir, attrs);

Comment could be clearer maybe just:

 The dma_ops API contract for ops->map_page() requires kmappable memory, while
 ops->map_resource() does not.

But this approach looks good to me, it prevents non-kmappable phys
from going down to the legacy dma_ops map_page where it cannot work.

>From here you could do what Marek and Christoph asked to flush the
struct page out of the ops->map_page() and replace it with
kmap_local_phys().

Jason