linux-kernel - Re: [PATCH 05/11] of: Ratify of_dma

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Date:   Thu, 3 Oct 2019 20:53:09 -0500
From:   Rob Herring <robh@...nel.org>
To:     Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
Cc:     devicetree@...r.kernel.org,
        Florian Fainelli <f.fainelli@...il.com>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Arnd Bergmann <arnd@...db.de>, PCI <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Christoph Hellwig <hch@...radead.org>,
        Marek Vasut <marek.vasut@...il.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Oza Pawandeep <oza.oza@...adcom.com>,
        Stefan Wahren <wahrenst@....net>,
        Simon Horman <horms+renesas@...ge.net.au>,
        Frank Rowand <frowand.list@...il.com>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH 05/11] of: Ratify of_dma_configure() interface

On Tue, Oct 1, 2019 at 10:43 AM Nicolas Saenz Julienne
<nsaenzjulienne@...e.de> wrote:
>
> On Mon, 2019-09-30 at 16:24 -0500, Rob Herring wrote:
> > On Mon, Sep 30, 2019 at 8:32 AM Nicolas Saenz Julienne
> > <nsaenzjulienne@...e.de> wrote:
> > > On Mon, 2019-09-30 at 05:57 -0700, Christoph Hellwig wrote:
> > > > On Thu, Sep 26, 2019 at 07:24:49PM -0500, Rob Herring wrote:
> > > > > -int of_dma_configure(struct device *dev, struct device_node *np, bool
> > > > > force_dma)
> > > > > +int of_dma_configure(struct device *dev, struct device_node *parent,
> > > > > bool
> > > > > force_dma)
> > > >
> > > > This creates a > 80 char line.
> > > >
> > > > >  {
> > > > >     u64 dma_addr, paddr, size = 0;
> > > > >     int ret;
> > > > >     bool coherent;
> > > > >     unsigned long offset;
> > > > >     const struct iommu_ops *iommu;
> > > > > +   struct device_node *np;
> > > > >     u64 mask;
> > > > >
> > > > > +   np = dev->of_node;
> > > > > +   if (!np)
> > > > > +           np = parent;
> > > > > +   if (!np)
> > > > > +           return -ENODEV;
> > > >
> > > > I have to say I find the older calling convention simpler to understand.
> > > > If we want to enforce the invariant I'd rather do that explicitly:
> > > >
> > > >       if (dev->of_node && np != dev->of_node)
> > > >               return -EINVAL;
> > >
> > > As is, this would break Freescale Layerscape fsl-mc bus' dma_configure():
> >
> > This may break PCI too for devices that have a DT node.
> >
> > > static int fsl_mc_dma_configure(struct device *dev)
> > > {
> > >         struct device *dma_dev = dev;
> > >
> > >         while (dev_is_fsl_mc(dma_dev))
> > >                 dma_dev = dma_dev->parent;
> > >
> > >         return of_dma_configure(dev, dma_dev->of_node, 0);
> > > }
> > >
> > > But I think that with this series, given the fact that we now treat the lack
> > > of
> > > dma-ranges as a 1:1 mapping instead of an error, we could rewrite the
> > > function
> > > like this:
> >
> > Now, I'm reconsidering allowing this abuse... It's better if the code
> > which understands the bus structure in DT for a specific bus passes in
> > the right thing. Maybe I should go back to Robin's version (below).
> > OTOH, the existing assumption that 'dma-ranges' was in the immediate
> > parent was an assumption on the bus structure which maybe doesn't
> > always apply.
> >
> > diff --git a/drivers/of/device.c b/drivers/of/device.c
> > index a45261e21144..6951450bb8f3 100644
> > --- a/drivers/of/device.c
> > +++ b/drivers/of/device.c
> > @@ -98,12 +98,15 @@ int of_dma_configure(struct device *dev, struct
> > device_node *parent, bool force_
> >         u64 mask;
> >
> >         np = dev->of_node;
> > -       if (!np)
> > -               np = parent;
> > +       if (np)
> > +               parent = of_get_dma_parent(np);
> > +       else
> > +               np = of_node_get(parent);
> >         if (!np)
> >                 return -ENODEV;
> >
> > -       ret = of_dma_get_range(np, &dma_addr, &paddr, &size);
> > +       ret = of_dma_get_range(parent, &dma_addr, &paddr, &size);
> > +       of_node_put(parent);
> >         if (ret < 0) {
> >                 /*
> >                  * For legacy reasons, we have to assume some devices need
>
> I spent some time thinking about your comments and researching. I came to the
> realization that both these solutions break the usage in
> drivers/gpu/drm/sun4i/sun4i_backend.c:805. In that specific case both
> 'dev->of_node' and 'parent' exist yet the device receiving the configuration
> and 'parent' aren't related in any way.

I knew there was some reason I didn't like those virtual DT nodes...

That does seem to be the oddest case. Several of the others are just
non-DT child platform devices. Perhaps we need a "copy the DMA config
from another struct device (or parent struct device)" function to
avoid using a DT function on a non-DT device.

> IOW we can't just use 'dev->of_node' as a starting point to walk upwards the
> tree. We always have to respect whatever DT node the bus provided, and start
> there. This clashes with the current solutions, as they are based on the fact
> that we can use dev->of_node when present.

Yes, you are right.

> My guess at this point, if we're forced to honor that behaviour, is that we
> have to create a new API for the PCI use case. Something the likes of
> of_dma_configure_parent().

I think of_dma_configure just has to work with the device_node of
either the device or the device parent and dev->of_node is never used
unless the caller sets it.

Rob