linux-kernel - Re: [RFC PATCH v3 2/6] swiotlb: Add restricted DMA pool

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Date:   Thu, 7 Jan 2021 10:09:14 -0800
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Claire Chang <tientzu@...omium.org>
Cc:     Rob Herring <robh+dt@...nel.org>, mpe@...erman.id.au,
        benh@...nel.crashing.org, paulus@...ba.org,
        "list@....net:IOMMU DRIVERS" <iommu@...ts.linux-foundation.org>,
        Joerg Roedel <joro@...tes.org>, joro@...tes.org,
        will@...nel.org, Frank Rowand <frowand.list@...il.com>,
        boris.ostrovsky@...cle.com, jgross@...e.com,
        sstabellini@...nel.org, Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>, grant.likely@....com,
        xypron.glpk@....de, Thierry Reding <treding@...dia.com>,
        mingo@...nel.org, bauerman@...ux.ibm.com, peterz@...radead.org,
        Greg KH <gregkh@...uxfoundation.org>,
        Saravana Kannan <saravanak@...gle.com>,
        rafael.j.wysocki@...el.com, heikki.krogerus@...ux.intel.com,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        rdunlap@...radead.org, dan.j.williams@...el.com,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        linux-devicetree <devicetree@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        linuxppc-dev@...ts.ozlabs.org,
        "list@....net:IOMMU DRIVERS" <iommu@...ts.linux-foundation.org>,
        Joerg Roedel <joro@...tes.org>,
        iommu@...ts.linux-foundation.org, xen-devel@...ts.xenproject.org,
        Tomasz Figa <tfiga@...omium.org>,
        Nicolas Boichat <drinkcat@...omium.org>
Subject: Re: [RFC PATCH v3 2/6] swiotlb: Add restricted DMA pool

On 1/7/21 9:57 AM, Konrad Rzeszutek Wilk wrote:
> On Fri, Jan 08, 2021 at 01:39:18AM +0800, Claire Chang wrote:
>> Hi Greg and Konrad,
>>
>> This change is intended to be non-arch specific. Any arch that lacks DMA access
>> control and has devices not behind an IOMMU can make use of it. Could you share
>> why you think this should be arch specific?
> 
> The idea behind non-arch specific code is it to be generic. The devicetree
> is specific to PowerPC, Sparc, and ARM, and not to x86 - hence it should
> be in arch specific code.

In premise the same code could be used with an ACPI enabled system with
an appropriate service to identify the restricted DMA regions and unlock
them.

More than 1 architecture requiring this function (ARM and ARM64 are the
two I can think of needing this immediately) sort of calls for making
the code architecture agnostic since past 2, you need something that scales.

There is already code today under kernel/dma/contiguous.c that is only
activated on a CONFIG_OF=y && CONFIG_OF_RESERVED_MEM=y system, this is
no different.
-- 
Florian