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

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Date:   Thu, 7 Jan 2021 16:19:37 -0500
From:   Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     Claire Chang <tientzu@...omium.org>,
        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>, 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, 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 Thu, Jan 07, 2021 at 10:09:14AM -0800, Florian Fainelli wrote:
> 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.

Which this patchset is not.

> 
> 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.

I believe the use-case is for ARM64 at this moment.

> 
> 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