linux-kernel - Re: [PATCH 3/5] PCI: rcar: Add R-Car PCIe endpoint device tree bindings

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Date:   Thu, 7 Nov 2019 21:08:35 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     "Lad, Prabhakar" <prabhakar.csengg@...il.com>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Magnus Damm <magnus.damm@...il.com>,
        Kishon Vijay Abraham I <kishon@...com>,
        Marek Vasut <marek.vasut+renesas@...il.com>,
        Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>,
        linux-pci <linux-pci@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Murray <andrew.murray@....com>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        Chris Paterson <Chris.Paterson2@...esas.com>,
        "Lad, Prabhakar" <prabhakar.mahadev-lad.rj@...renesas.com>
Subject: Re: [PATCH 3/5] PCI: rcar: Add R-Car PCIe endpoint device tree bindings

Hi Prabhakar,

On Thu, Nov 7, 2019 at 10:26 AM Lad, Prabhakar
<prabhakar.csengg@...il.com> wrote:
> On Thu, Nov 7, 2019 at 8:44 AM Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
> > On Wed, Nov 6, 2019 at 8:36 PM Lad Prabhakar <prabhakar.csengg@...il.com> wrote:
> > > From: "Lad, Prabhakar" <prabhakar.mahadev-lad.rj@...renesas.com>
> > >
> > > This patch adds the bindings for the R-Car PCIe endpoint driver.
> > >
> > > Signed-off-by: Lad, Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>
> >
> > Thanks for your patch!
> >
> > > --- /dev/null
> > > +++ b/Documentation/devicetree/bindings/pci/rcar-pci-ep.txt
> > > @@ -0,0 +1,43 @@
> > > +* Renesas R-Car PCIe Endpoint Controller DT description
> > > +
> > > +Required properties:
> > > +           "renesas,pcie-ep-r8a774c0" for the R8A774C0 SoC;
> > > +           "renesas,pcie-ep-rcar-gen3" for a generic R-Car Gen3 or
> > > +                                    RZ/G2 compatible device.
> >
> > Unless I'm missing something, this is for the exact same hardware block as
> > Documentation/devicetree/bindings/pci/rcar-pci.txt?
> > So shouldn't you amend those bindings, instead of adding new compatible
> > values?
> > Please remember that DT describes hardware, not software policy.
> > So IMHO choosing between host and endpoint is purely a configuration
> > issue, and could be indicated by the presence or lack of some DT properties.
> > E.g. host mode requires both "bus-range" and "device_type" properties,
> > so their absence could indicate endpoint mode.
> >
> yes its the same hardware block as described in the rcar-pci.txt, I
> did think about amending it
> but  it might turn out to be bit messy,
>
> required properties host ======required properties Endpoint
> ====================||==================
> 1: reg                                || reg
> 2:bus-range                      || reg names
> 3: device_type                  || resets
> 4: ranges                          || clocks
> 5: dma-ranges                  || clock-names
> 6: interrupts                      ||
> 7: interrupt-cells               ||
> 8: interrupt-map-mask     ||
> 9: clocks                          ||
> 10: clock-names             ||

We have a similar situation with SPI, where a controller can operate in
master or slave mode, based on the absence or presence of the
"spi-slave" DT property.

> and if I go ahead with the same compatible string that would mean to
> add support for endpoint
> mode in the host driver itself. I did follow the examples of

You can still have two separate drivers, binding against the same
compatible value.  Just let the .probe() function return -ENODEV if it
discovers (by looking at DT properties) if the node is configured for
the other mode.
Which brings us to my next questions: is there any code that could be
shared between the drivers for the two modes?

> rockchip/cadence/designware where
> its the same hardware block but has two different binding files one
> for host mode and other for
> endpoint mode.

Having two separate DT binding documents sounds fine to me, if unifying
them makes things too complex.
However, I think they should use the same compatible value, because the
hardware block is the same, but just used in a different mode.

Rob/Mark: Any input from the DT maintainers?

> > > +- reg: Five register ranges as listed in the reg-names property
> > > +- reg-names: Must include the following names
> > > +       - "apb-base"
> > > +       - "memory0"
> > > +       - "memory1"
> > > +       - "memory2"
> > > +       - "memory3"
> >
> > What is the purpose of the last 4 regions?
> > Can they be chosen by the driver, at runtime?
> >
> no the driver cannot choose them at runtime, as these are the only
> PCIE memory(0/1/2/3) ranges
> in the AXI address space where host memory can be mapped.

Are they fixed by the PCIe hardware, i.e. could they be looked up by the
driver based on the compatible value?

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds