linux-kernel - Re: [PATCH 5/6] dma-mapping: support fsl-mc bus

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Message-ID: <1729ae21-d08c-b413-51a3-f22c394b388d@arm.com>
Date:   Mon, 5 Mar 2018 18:51:56 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Christoph Hellwig <hch@....de>
Cc:     Nipun Gupta <nipun.gupta@....com>, will.deacon@....com,
        mark.rutland@....com, catalin.marinas@....com,
        iommu@...ts.linux-foundation.org, robh+dt@...nel.org,
        m.szyprowski@...sung.com, gregkh@...uxfoundation.org,
        joro@...tes.org, leoyang.li@....com, shawnguo@...nel.org,
        linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, bharat.bhushan@....com,
        stuyoder@...il.com, laurentiu.tudor@....com
Subject: Re: [PATCH 5/6] dma-mapping: support fsl-mc bus

On 05/03/18 18:39, Christoph Hellwig wrote:
> On Mon, Mar 05, 2018 at 03:48:32PM +0000, Robin Murphy wrote:
>> Unfortunately for us, fsl-mc is conceptually rather like PCI in that it's
>> software-discoverable and the only thing described in DT is the bus "host",
>> thus we need the same sort of thing as for PCI to map from the child
>> devices back to the bus root in order to find the appropriate firmware
>> node. Worse than PCI, though, we wouldn't even have the option of
>> describing child devices statically in firmware at all, since it's actually
>> one of these runtime-configurable "build your own network accelerator"
>> hardware pools where userspace gets to create and destroy "devices" as it
>> likes.
> 
> I really hate the PCI special case just as much.  Maybe we just
> need a dma_configure method on the bus, and move PCI as well as fsl-mc
> to it.

Hmm, on reflection, 100% ack to that idea. It would neatly supersede 
bus->force_dma *and* mean that we don't have to effectively pull pci.h 
into everything, which I've never liked. In hindsight dma_configure() 
does feel like it's grown into this odd choke point where we munge 
everything in just for it to awkwardly unpick things again.

Robin.