Re: [PATCH v2 1/2] dt-bindings: serial: add documentation for the SiFive UART driver

From: Paul Walmsley
Date: Tue Oct 23 2018 - 13:05:48 EST


On 10/20/18 7:21 AM, Rob Herring wrote:
On Fri, Oct 19, 2018 at 5:06 PM Paul Walmsley <paul.walmsley@xxxxxxxxxx> wrote:
On 10/19/18 1:45 PM, Rob Herring wrote:
On Fri, Oct 19, 2018 at 1:48 PM Paul Walmsley <paul.walmsley@xxxxxxxxxx> wrote:
Add DT binding documentation for the Linux driver for the SiFive
asynchronous serial IP block. Nothing too exotic.

Cc: linux-serial@xxxxxxxxxxxxxxx
Cc: devicetree@xxxxxxxxxxxxxxx
Cc: linux-riscv@xxxxxxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Rob Herring <robh+dt@xxxxxxxxxx>
Cc: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Palmer Dabbelt <palmer@xxxxxxxxxx>
Reviewed-by: Palmer Dabbelt <palmer@xxxxxxxxxx>
Signed-off-by: Paul Walmsley <paul.walmsley@xxxxxxxxxx>
Signed-off-by: Paul Walmsley <paul@xxxxxxxxx>
---
.../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"[**].ÂÂ 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. Â 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:

https://github.com/sifive/sifive-blocks/blob/master/src/main/scala/devices/uart/UART.scala#L43
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


- Paul


** 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 string scheme.