linux-kernel - Re: [PATCH v4 09/16] dma-mapping: handle MMIO flow in dma_map|unmap

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Message-ID: <20250828151730.GH9469@nvidia.com>
Date: Thu, 28 Aug 2025 12:17:30 -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 v4 09/16] dma-mapping: handle MMIO flow in
 dma_map|unmap_page

On Tue, Aug 19, 2025 at 08:36:53PM +0300, Leon Romanovsky wrote:
> From: Leon Romanovsky <leonro@...dia.com>
> 
> Extend base DMA page API to handle MMIO flow and follow
> existing dma_map_resource() implementation to rely on dma_map_direct()
> only to take DMA direct path.

I would reword this a little bit too

dma-mapping: implement DMA_ATTR_MMIO for dma_(un)map_page_attrs()

Make dma_map_page_attrs() and dma_map_page_attrs() respect
DMA_ATTR_MMIO.

DMA_ATR_MMIO makes the functions behave the same as dma_(un)map_resource():
 - No swiotlb is possible
 - Legacy dma_ops arches use ops->map_resource()
 - No kmsan
 - No arch_dma_map_phys_direct()

The prior patches have made the internl funtions called here support
DMA_ATTR_MMIO.

This is also preparation for turning dma_map_resource() into an inline
calling dma_map_phys(DMA_ATTR_MMIO) to consolidate the flows.

> @@ -166,14 +167,25 @@ 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);

PPC is the only user of arch_dma_map_phys_direct() and it looks like
it should be called on MMIO memory. Seems like another inconsistency
with map_resource. I'd leave it like the above though for this series.

>  	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;

Probably written like:

		if (ops->map_resource)
			addr = ops->map_resource(dev, phys, size, dir, attrs);
		else
			addr = DMA_MAPPING_ERROR;

As I think some of the design here is to run the trace even on the
failure path?

Otherwise looks OK

Reviewed-by: Jason Gunthorpe <jgg@...dia.com>

Jason