[PATCH v2 1/2] dt-bindings: serial: add documentation for the SiFive UART driver
paul.walmsley at sifive.com
Tue Oct 23 10:05:40 PDT 2018
On 10/20/18 7:21 AM, Rob Herring wrote:
> On Fri, Oct 19, 2018 at 5:06 PM Paul Walmsley <paul.walmsley at sifive.com> wrote:
>> On 10/19/18 1:45 PM, Rob Herring wrote:
>>> On Fri, Oct 19, 2018 at 1:48 PM Paul Walmsley <paul.walmsley at sifive.com> wrote:
>>>> Add DT binding documentation for the Linux driver for the SiFive
>>>> asynchronous serial IP block. Nothing too exotic.
>>>> Cc: linux-serial at vger.kernel.org
>>>> Cc: devicetree at vger.kernel.org
>>>> Cc: linux-riscv at lists.infradead.org
>>>> Cc: linux-kernel at vger.kernel.org
>>>> Cc: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
>>>> Cc: Rob Herring <robh+dt at kernel.org>
>>>> Cc: Mark Rutland <mark.rutland at arm.com>
>>>> Cc: Palmer Dabbelt <palmer at sifive.com>
>>>> Reviewed-by: Palmer Dabbelt <palmer at sifive.com>
>>>> Signed-off-by: Paul Walmsley <paul.walmsley at sifive.com>
>>>> Signed-off-by: Paul Walmsley <paul at pwsan.com>
>>>> .../bindings/serial/sifive-serial.txt | 21 +++++++++++++++++++
>>>> 1 file changed, 21 insertions(+)
>>>> create mode 100644 Documentation/devicetree/bindings/serial/sifive-serial.txt
>>>> diff --git a/Documentation/devicetree/bindings/serial/sifive-serial.txt b/Documentation/devicetree/bindings/serial/sifive-serial.txt
>>>> new file mode 100644
>>>> index 000000000000..8982338512f5
>>>> --- /dev/null
>>>> +++ b/Documentation/devicetree/bindings/serial/sifive-serial.txt
>>>> @@ -0,0 +1,21 @@
>>>> +SiFive asynchronous serial interface (UART)
>>>> +Required properties:
>>>> +- compatible: should be "sifive,fu540-c000-uart0" or "sifive,uart0"
>>> As I mentioned for the
>>> intc and now the pwm block bindings, if you are going to do version
>>> numbers please document the versioning scheme.
>>> Will add that to the binding document.
> I don't seem to be making my point clear. I don't want any of this
> added to a binding doc for particular IP blocks. Write a common doc
> that explains the scheme and addresses the questions I asked. Then
> just reference that doc here.
> Maybe this is documented somewhere already? Otherwise, if one is
> creating a new IP block, how do they know what the versioning scheme
> is or what goes in the DT ROM?
Seems like there might be some confusion between IP blocks as integrated
on an SoC vs. IP blocks in isolation. It's not necessarily the SoC
integrator that sets an IP block version number; this can come from the
IP block vendor itself. So each IP block may have its own version
numbering practices for the IP block alone.
For SiFive IP blocks, we at SiFive could probably align on a common
version numbering structure for what's in the sifive-blocks repository.
But other IP blocks from other vendors may not align to that, or may not
have version numbers exposed at all. In those cases there's no way for
software folks to find out what they are, as you pointed out earlier.
This is the case with most DT compatible strings in the kernel tree.
For example, we've integrated the NVDLA IP block, from NVIDIA, on some
designs. Any NVIDIA version numbers in that IP block will probably not
follow the SiFive version numbering scheme. I'd propose the right thing
to do for an IP block compatible string is to follow the vendor's
practice, and then use the SoC integrator's version numbering practice
for the SoC-integrated compatible string.
In effect, an SoC integration DT compatible string like
"sifive,fu540-c000-uart" implicitly states an IP block version number:
"whatever came out of the fab on the chip"[**]. I'd propose that even
in these cases, there's an advantage to keeping the "0" on the end,
since it uniquely identifies an SoC-independent IP block, rather than
just the type of the IP block. But if the "0" on the end of the SoC
integration DT compatible string is problematic for you, we can
certainly drop that last 0 from the SoC integration DT compatible
string, and only suffer a slight lack of clarity as to what version was
integrated on that chip.
But for IP block-specific version strings like "sifive,uart0", I think
we can address your concern, at least for these public IP blocks. Since
the SiFive UART and some other peripheral IP blocks are open-source, the
public can have a pretty good idea of what DT version number corresponds
to the source RTL, since the RTL is public. The version number
identifies a specific programming model, without tying that programming
model to any SoC-specific workarounds, etc. So for these cases, I think
there's a pretty good case for having IP block-specific version numbers
in DT compatible strings, and I hope you'll agree.
The advantage for all of us is that there's then no need to embed
chip-specific DT match strings in these drivers, for the most part. We
just match on "sifive,uart0" and that's it, assuming no chip-specific
workarounds are needed.
>>> Where does the
>>> number come from?
>> It comes from the RTL, which is public:
> I'm not going to go read your RTL, sorry.
There's no need, but you did ask where it came from. Sorry you didn't
like the answer.
Please let us know what you want us to do.
Thanks for your review
** The caveat is that even with SoC identifiers in the Linux DT
compatible strings, there's not enough information in many of the
existing kernel DT compatibility strings to uniquely identify chip
versions. Taking OMAP and Tegra as examples, there are several
different chip versions for a given SoC generation that came out of the
fab. OMAP chip version strings usually began with "ES"; Tegra version
numbers, as I recall, were a letter and two numbers. For the most part,
those versions were never specifically identified in the upstream kernel
DT strings or in DT file names. (There are some exceptions with OMAP
where we did identify specific chip version numbers, because sizable
numbers of folks had boards with early silicon, and we were committed to
supporting them at the time.) Sadly even adding these additional chip
version identifiers to the DT strings wouldn't be perfect: I've seen at
least one large vendor implementing metal-only ECOs without incrementing
public chip version numbers. The point here is that we're already not
uniquely identifying IP blocks with our current Linux DT compatibility
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