linux-kernel - Re: [PATCH 1/2] dma-mapping: add a dma_ops

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Date:   Tue, 24 Mar 2020 08:55:14 +0100
From:   Christoph Hellwig <hch@....de>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
Cc:     Alexey Kardashevskiy <aik@...abs.ru>,
        Christoph Hellwig <hch@....de>,
        iommu@...ts.linux-foundation.org, linuxppc-dev@...ts.ozlabs.org,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Joerg Roedel <joro@...tes.org>,
        Robin Murphy <robin.murphy@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] dma-mapping: add a dma_ops_bypass flag to struct
 device

On Tue, Mar 24, 2020 at 12:00:09PM +0530, Aneesh Kumar K.V wrote:
> dma_addr_t dma_direct_map_page(struct device *dev, struct page *page,
> 		unsigned long offset, size_t size, enum dma_data_direction dir,
> 		unsigned long attrs)
> {
> 	phys_addr_t phys = page_to_phys(page) + offset;
> 	dma_addr_t dma_addr = phys_to_dma(dev, phys);
> 
> 	if (unlikely(!dma_capable(dev, dma_addr, size, true))) {
> 			return iommu_map(dev, phys, size, dir, attrs);
> 
> 		return DMA_MAPPING_ERROR;

If powerpc hardware / firmware people really come up with crap that
stupid you'll have to handle it yourself and will always pay the
indirect call penality.