linux-kernel - Re: [PATCH v7 14/15] dt-bindings: of: Add restricted DMA pool

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Date:   Thu, 27 May 2021 13:53:28 +0100
From:   Will Deacon <will@...nel.org>
To:     Claire Chang <tientzu@...omium.org>
Cc:     heikki.krogerus@...ux.intel.com, thomas.hellstrom@...ux.intel.com,
        peterz@...radead.org, benh@...nel.crashing.org,
        joonas.lahtinen@...ux.intel.com, dri-devel@...ts.freedesktop.org,
        chris@...is-wilson.co.uk, grant.likely@....com, paulus@...ba.org,
        Frank Rowand <frowand.list@...il.com>, mingo@...nel.org,
        sstabellini@...nel.org, Saravana Kannan <saravanak@...gle.com>,
        mpe@...erman.id.au,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Christoph Hellwig <hch@....de>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        bskeggs@...hat.com, linux-pci@...r.kernel.org,
        xen-devel@...ts.xenproject.org,
        Thierry Reding <treding@...dia.com>,
        intel-gfx@...ts.freedesktop.org, matthew.auld@...el.com,
        linux-devicetree <devicetree@...r.kernel.org>,
        Jianxiong Gao <jxgao@...gle.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        maarten.lankhorst@...ux.intel.com, airlied@...ux.ie,
        Dan Williams <dan.j.williams@...el.com>,
        linuxppc-dev@...ts.ozlabs.org, jani.nikula@...ux.intel.com,
        Rob Herring <robh+dt@...nel.org>, rodrigo.vivi@...el.com,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        boris.ostrovsky@...cle.com,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        jgross@...e.com, Nicolas Boichat <drinkcat@...omium.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        lkml <linux-kernel@...r.kernel.org>,
        "list@....net:IOMMU DRIVERS" <iommu@...ts.linux-foundation.org>,
        Jim Quinlan <james.quinlan@...adcom.com>, xypron.glpk@....de,
        Robin Murphy <robin.murphy@....com>, bauerman@...ux.ibm.com
Subject: Re: [PATCH v7 14/15] dt-bindings: of: Add restricted DMA pool

On Thu, May 27, 2021 at 08:48:59PM +0800, Claire Chang wrote:
> On Thu, May 27, 2021 at 7:35 PM Will Deacon <will@...nel.org> wrote:
> >
> > On Thu, May 27, 2021 at 07:29:20PM +0800, Claire Chang wrote:
> > > On Wed, May 26, 2021 at 11:53 PM Will Deacon <will@...nel.org> wrote:
> > > >
> > > > On Wed, May 26, 2021 at 01:13:22PM +0100, Will Deacon wrote:
> > > > > On Tue, May 18, 2021 at 02:42:14PM +0800, Claire Chang wrote:
> > > > > > @@ -138,4 +160,9 @@ one for multimedia processing (named multimedia-memory@...00000, 64MiB).
> > > > > >             memory-region = <&multimedia_reserved>;
> > > > > >             /* ... */
> > > > > >     };
> > > > > > +
> > > > > > +   pcie_device: pcie_device@0,0 {
> > > > > > +           memory-region = <&restricted_dma_mem_reserved>;
> > > > > > +           /* ... */
> > > > > > +   };
> > > > >
> > > > > I still don't understand how this works for individual PCIe devices -- how
> > > > > is dev->of_node set to point at the node you have above?
> > > > >
> > > > > I tried adding the memory-region to the host controller instead, and then
> > > > > I see it crop up in dmesg:
> > > > >
> > > > >   | pci-host-generic 40000000.pci: assigned reserved memory node restricted_dma_mem_reserved
> > > > >
> > > > > but none of the actual PCI devices end up with 'dma_io_tlb_mem' set, and
> > > > > so the restricted DMA area is not used. In fact, swiotlb isn't used at all.
> > > > >
> > > > > What am I missing to make this work with PCIe devices?
> > > >
> > > > Aha, looks like we're just missing the logic to inherit the DMA
> > > > configuration. The diff below gets things working for me.
> > >
> > > I guess what was missing is the reg property in the pcie_device node.
> > > Will update the example dts.
> >
> > Thanks. I still think something like my diff makes sense, if you wouldn't mind including
> > it, as it allows restricted DMA to be used for situations where the PCIe
> > topology is not static.
> >
> > Perhaps we should prefer dev->of_node if it exists, but then use the node
> > of the host bridge's parent node otherwise?
> 
> Sure. Let me add in the next version.

Brill, thanks! I'll take it for a spin once it lands on the list.

Will