On Mon, Oct 22, 2018 at 09:41:51AM -0700, Palmer Dabbelt wrote: > On Fri, 19 Oct 2018 13:45:57 PDT (-0700), robh+dt@xxxxxxxxxx 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" > > > > 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. Rights to the ISA and cores may be different, but how chips are built is not really all that different (or doesn't have to be). > 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. So all this discussion is purely SiFive specific and really has nothing to do with RISC-V ecosystem. Putting the DT into the ROM isn't something I'd do. It's simply not going to work timeline wise IMO. > 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 Relying on people to catch whether changes are important or not is bound to fail. It's really got to be built into the design flow. Even just updating a version register I've experienced the h/w designers forgetting to update it. > 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). What about config options? Say the UART has a configurable FIFO size. What about major vs. minor version changes? Respins of chips would need to make minor changes if picking up major changes are deemed too risky. > 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. This is all good information and is essentially what I'm looking for. I just don't want it lost in a reply to an email, but something you can reference. Look at bindings/arm/primecell.txt for example. That describes a family of IP blocks and not any specific device. Whether the versioning is sufficient or not, I don't really care as long as you docuemnt what it is so it is consistent. Since you have a common schema across IP blocks, that means you should have a common document. Rob
