Re: [PATCH 4/5] dt-binding: mtd: onenand/samsung: Add device tree support

[Tomasz Figa]

2019å5æ2æ(æ) 16:21 Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx>:
>
> On Thu, 2 May 2019 15:58:24 +0900
> Tomasz Figa <tomasz.figa@xxxxxxxxx> wrote:
>
> > 2019å5æ2æ(æ) 15:55 Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx>:
> > >
> > > On Thu, 2 May 2019 15:42:59 +0900
> > > Tomasz Figa <tomasz.figa@xxxxxxxxx> wrote:
> > >
> > > > 2019å5æ2æ(æ) 15:36 Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx>:
> > > > >
> > > > > Hi Tomasz,
> > > > >
> > > > > On Thu, 2 May 2019 15:23:33 +0900
> > > > > Tomasz Figa <tomasz.figa@xxxxxxxxx> wrote:
> > > > >
> > > > > > 2019å5æ2æ(æ) 10:54 Rob Herring <robh@xxxxxxxxxx>:
> > > > > > >
> > > > > > > On Fri, Apr 26, 2019 at 06:42:23PM +0200, PaweÅ Chmiel wrote:
> > > > > > > > From: Tomasz Figa <tomasz.figa@xxxxxxxxx>
> > > > > > > >
> > > > > > > > This patch adds dt-bindings for Samsung OneNAND driver.
> > > > > > > >
> > > > > > > > Signed-off-by: Tomasz Figa <tomasz.figa@xxxxxxxxx>
> > > > > > > > Signed-off-by: PaweÅ Chmiel <pawel.mikolaj.chmiel@xxxxxxxxx>
> > > > > > > > ---
> > > > > > > >  .../bindings/mtd/samsung-onenand.txt          | 46 +++++++++++++++++++
> > > > > > > >  1 file changed, 46 insertions(+)
> > > > > > > >  create mode 100644 Documentation/devicetree/bindings/mtd/samsung-onenand.txt
> > > > > > > >
> > > > > > > > diff --git a/Documentation/devicetree/bindings/mtd/samsung-onenand.txt b/Documentation/devicetree/bindings/mtd/samsung-onenand.txt
> > > > > > > > new file mode 100644
> > > > > > > > index 000000000000..341d97cc1513
> > > > > > > > --- /dev/null
> > > > > > > > +++ b/Documentation/devicetree/bindings/mtd/samsung-onenand.txt
> > > > > > > > @@ -0,0 +1,46 @@
> > > > > > > > +Device tree bindings for Samsung SoC OneNAND controller
> > > > > > > > +
> > > > > > > > +Required properties:
> > > > > > > > + - compatible : value should be either of the following.
> > > > > > > > +   (a) "samsung,s3c6400-onenand" - for onenand controller compatible with
> > > > > > > > +       S3C6400 SoC,
> > > > > > > > +   (b) "samsung,s3c6410-onenand" - for onenand controller compatible with
> > > > > > > > +       S3C6410 SoC,
> > > > > > > > +   (c) "samsung,s5pc100-onenand" - for onenand controller compatible with
> > > > > > > > +       S5PC100 SoC,
> > > > > > > > +   (d) "samsung,s5pv210-onenand" - for onenand controller compatible with
> > > > > > > > +       S5PC110/S5PV210 SoCs.
> > > > > > > > +
> > > > > > > > + - reg : two memory mapped register regions:
> > > > > > > > +   - first entry: control registers.
> > > > > > > > +   - second and next entries: memory windows of particular OneNAND chips;
> > > > > > > > +     for variants a), b) and c) only one is allowed, in case of d) up to
> > > > > > > > +     two chips can be supported.
> > > > > > > > +
> > > > > > > > + - interrupt-parent : phandle of interrupt controller to which the OneNAND
> > > > > > > > +   controller is wired,
> > > > > > >
> > > > > > > This is implied and can be removed.
> > > > > > >
> > > > > > > > + - interrupts : specifier of interrupt signal to which the OneNAND controller
> > > > > > > > +   is wired; should contain just one entry.
> > > > > > > > + - clock-names : should contain two entries:
> > > > > > > > +   - "bus" - bus clock of the controller,
> > > > > > > > +   - "onenand" - clock supplied to OneNAND memory.
> > > > > > >
> > > > > > > If the clock just goes to the OneNAND device, then it should be in the
> > > > > > > nand device node rather than the controller node.
> > > > > > >
> > > > > >
> > > > > > (Trying hard to recall the details about this hardware.)
> > > > > > AFAIR the clock goes to the controller and the controller then feeds
> > > > > > it to the memory chips.
> > > > > >
> > > > > > Also I don't think we should have any nand device nodes here, since
> > > > > > the memory itself is only exposed via the controller, which offers
> > > > > > various queries to probe the memory at runtime, so there is no need to
> > > > > > describe it in DT.
> > > > >
> > > > > It's probably true, though not providing this controller/device
> > > > > separation for NAND controller/devices has proven to be a mistake for
> > > > > raw NAND controllers (some props apply to the controllers and others to
> > > > > the NAND device, not to mention that some controllers support
> > > > > interacting with several chips), so, if that's a new binding, I'd
> > > > > recommend having this separation even if it's not strictly required.
> > > > >
> > > >
> > > > Note that OneNAND is a totally different thing than the typical NAND
> > > > memory with NAND interface. OneNAND chips have a NOR-like interface,
> > > > with internal controller and buffers inside, so technically they can
> > > > be even used without any special controller on the SoC, via a generic
> > > > parallel host interface and possibly some regular DMA engine for CPU
> > > > offload.
> > >
> > > Yes, I know that.
> > >
> > > >
> > > > The controller design of the SoCs in question further abstracts the
> > > > OneNAND's programming interface into a number of high level operations
> > > > and attempts to hide the details of the underlying memory, so I don't
> > > > see the point of describing the memory in DT here, it would actually
> > > > defeat the purpose of this controller.
> > >
> > > I don't see how having a subnode for the NAND chip would change
> > > anything on how the controller interacts with the NAND device. My point
> > > is, if we ever need to add props that apply to the device rather than
> > > the controller itself, having a single node to represent both elements
> > > is not that great.
> >
> > You mean, just having a very generic onenand@0 node that doesn't
> > really include any information, except maybe the partition table?
>
> Yes.
>
> > I
> > guess that wouldn't have any negative side effects indeed.
> >
> > My point was that we don't want to put things like chip vendor, size,
> > etc. in DT, since that's enumerable.
>
> Oh, definitely not, and that's exactly how we do it for NAND devices.
> Everything that's discoverable is not described in the DT, but some
> things can't be discovered this way (like when you want to override the
> ECC strength and use SW-based implem instead of the HW-based one). I
> know none of this applies to OneNAND yet, I'm just over-cautious about
> that since DT bindings changes are hard to make once the bindings are
> in use.

Makes sense. Thanks for clarifying!