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

[Palmer Dabbelt <palmer@sifive.com>]

On Fri, 19 Oct 2018 13:45:57 PDT (-0700), robh+dt@kernel.org wrote:
> On Fri, Oct 19, 2018 at 1:48 PM Paul Walmsley <paul.walmsley@sifive.com> wrote:
>>
>> Add DT binding documentation for the Linux driver for the SiFive
>> asynchronous serial IP block.  Nothing too exotic.
>>
>> Cc: linux-serial@vger.kernel.org
>> Cc: devicetree@vger.kernel.org
>> Cc: linux-riscv@lists.infradead.org
>> Cc: linux-kernel@vger.kernel.org
>> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
>> Cc: Rob Herring <robh+dt@kernel.org>
>> Cc: Mark Rutland <mark.rutland@arm.com>
>> Cc: Palmer Dabbelt <palmer@sifive.com>
>> Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
>> Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>
>> Signed-off-by: Paul Walmsley <paul@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"
>
> I assume once again, the last '0' is a version? 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. Palmer mentioned the
> compatible string is part of the IP block repository? Where does the
> number come from? What's the next version? Major vs. minor versions?
> ECO fixes? Is the version s/w readable? How do you ensure it gets
> updated? All that should be addressed.

The RISC-V ecosystem is a bit different than that of ARM, MIPS, or Intel in 
that the ISA is an royalty-free open standard that anyone can implement (ie, 
without even signing a license agreement), with only the "RISC-V" trademark 
being held behind a pay+conformance wall.  As a result, we don't actually have 
any control over who builds a RISC-V chip so all we at SiFive can really to is 
try to demonstrate good practices in software land and go from there.

As far as SiFive's codebase is concerned, the version number is embedded in the 
RTL generator, and a device tree is generated along with the RTL.  This device 
tree is then embedded into a mask ROM on the chip, which allows the earliest 
stage of boot to proceed.  As I'm sure you know, boot is a very complicated 
process and as a result the device tree passed to Linux doesn't necessarily 
look like what's in the ROM, but the intent is to keep iterating until we can 
get these as similar as possible -- that's why we're submitting every 
devicetree binding to the standard.

Specifically as far as the UART is concerned, the compat string that's not 
chip-specific lives here (the "sifive,fu540-c000-uart" string lives in an 
internal chip repo that I can't point to):

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

The version numbering scheme right now is pretty simple: I try to pay as much 
attention as possible to how the hardware changes (both by looking and with 
some automation), and I go yell at anyone who does something stupid.  I know 
it's not the most scalable of schemes, but it's the best we have.  The UART is 
actually an interesting case right now because we have an outstanding pull 
request that adds a bit to the UART and then adds "sifive,uart1" to the compat 
string

    https://github.com/sifive/sifive-blocks/pull/90

My intent is to ensure that the device tree's compat string uniquely identifies 
the software interface to a block.  Thus, whenever a device's implementation 
changes in a software-visible way (bug fix or feature addition) we change the 
compat string -- either adding one (as is the case of the UART, where the 
compat string will be both "sifive,uart1" and "sifive,uart0" since the new 
feature is backwards compatible with the old software) or changing one (if the 
interface change is not compatible with old software).

Like I said above, this is all a manual process right now and this only applies 
to SiFive's implementations.  I'm confident that I can at least ensure that, 
for any given SiFive implementation, a block's compat string will uniquely 
identify the software interface to it.  For the rest of the RISC-V world all we 
can do is set a good example and review the software.

> Otherwise, don't do version numbers because we have no visibility to
> what they mean.
>
>> +- reg: address and length of the register space
>> +- interrupt-parent: should contain a phandle pointing to the SoC interrupt
>> +    controller device node that the UART interrupts are connected to
>
> Don't need to document interrupt-parent here.
>
>> +- interrupts: Should contain the UART interrupt identifier
>> +- clocks: Should contain a clock identifier for the UART's parent clock
>> +
>> +
>> +Example:
>> +
>> +uart0: serial@10010000 {
>> +       compatible = "sifive,uart0";
>> +       interrupt-parent = <&plic0>;
>> +       interrupts = <80>;
>> +       reg = <0x0 0x10010000 0x0 0x1000>;
>> +       clocks = <&prci PRCI_CLK_TLCLK>;
>> +};
>> --
>> 2.19.1
>>

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