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

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Date:   Mon, 23 Mar 2020 09:50:59 +0100
From:   Christoph Hellwig <hch@....de>
To:     Alexey Kardashevskiy <aik@...abs.ru>
Cc:     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,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
Subject: Re: [PATCH 1/2] dma-mapping: add a dma_ops_bypass flag to struct
 device

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?