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Date:   Mon, 23 Mar 2020 09:50:59 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Alexey Kardashevskiy <aik@ozlabs.ru>
Cc:     Christoph Hellwig <hch@lst.de>, iommu@lists.linux-foundation.org,
        linuxppc-dev@lists.ozlabs.org, Lu Baolu <baolu.lu@linux.intel.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Joerg Roedel <joro@8bytes.org>,
        Robin Murphy <robin.murphy@arm.com>,
        linux-kernel@vger.kernel.org,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
Subject: Re: [PATCH 1/2] dma-mapping: add a dma_ops_bypass flag to struct
 device
Message-ID: <20200323085059.GA32528@lst.de>
References: <20200320141640.366360-1-hch@lst.de> <20200320141640.366360-2-hch@lst.de> <2f31d0dd-aa7e-8b76-c8a1-5759fda5afc9@ozlabs.ru> <20200323083705.GA31245@lst.de>
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On Mon, Mar 23, 2020 at 09:37:05AM +0100, Christoph Hellwig wrote:
> > > +	/*
> > > +	 * Allows IOMMU drivers to bypass dynamic translations if the DMA mask
> > > +	 * is large enough.
> > > +	 */
> > > +	if (dev->dma_ops_bypass) {
> > > +		if (min_not_zero(dev->coherent_dma_mask, dev->bus_dma_limit) >=
> > > +				dma_direct_get_required_mask(dev))
> > > +			return true;
> > > +	}
> > 
> > 
> > Why not do this in dma_map_direct() as well?
> 
> Mostly beacuse it is a relatively expensive operation, including a
> fls64.

Which I guess isn't too bad compared to a dynamic IOMMU mapping.  Can
you just send a draft patch for what you'd like to see for ppc?