On 27/07/2022 16:07, Doug Anderson wrote: > Hi, > > On Wed, Jul 27, 2022 at 1:47 AM Krzysztof Kozlowski > <krzysztof.kozlowski@xxxxxxxxxx> wrote: >> >> On 21/07/2022 01:42, Julius Werner wrote: >>> Sorry, got distracted from this for a bit. Sounds like we were pretty >>> much on the same page about how the updated binding should look like >>> here, the remaining question was just about the compatible string. >>> >>>>>> Yes, we can. You still would need to generate the compatible according >>>>>> to the current bindings. Whether we can change it I am not sure. I think >>>>>> it depends how much customization is possible per vendor, according to >>>>>> JEDEC spec. If we never ever have to identify specific part, because >>>>>> JEDEC spec and registers tell us everything, then we could skip it, >>>>>> similarly to lpddr2 and jedec,spi-nor. >>>>> >>>>> Shouldn't that be decided per use case? In general LPDDR is a pretty >>>>> rigid set of standards and memory controllers are generally compatible >>>>> with any vendor without hardcoding vendor-specific behavior, so I >>>>> don't anticipate that this would be likely (particularly since there >>>>> is no "real" kernel device driver that needs to initialize the full >>>>> memory controller, after all, these bindings are mostly >>>>> informational). >>>> >>>> If decided per use case understood as "decided depending how to use the >>>> bindings" then answer is rather not. For example Linux implementation is >>>> usually not the best argument to shape the bindings and usually to such >>>> arguments answer is: "implementation does not matter". >>>> >>>> If by "use case" you mean actual hardware or specification >>>> characteristics, then yes, of course. This is why I wrote "it depends". >>> >>> By "use case" I mean our particular platform and firmware requirements >>> -- or rather, the realities of building devices with widely >>> multi-sourced LPDDR parts. One cannot efficiently build firmware that >>> can pass an exact vendor-and-part-specific compatible string to Linux >>> for this binding for every single LPDDR part used on such a platform. >> >> Why cannot? You want to pass them as numerical values which directly map >> to vendor ID and some part, don't they? > > If you really want this to be in the "compatible" string, maybe the > right answer is to follow the lead of USB which encodes the VID/PID in > the compatible string > (Documentation/devicetree/bindings/usb/usb-device.yaml). It's solving > this exact same problem of avoiding needing a table translating from > an ID provided by a probable device to an human-readable string. This makes sense. I would still argue that number of vendors is small thus strings could be translated (there is like 20 of them in JEP166D - JC-42.6), but for device ID this would work. > > >>> And I don't see why that should be needed, either... that's kinda the >>> point of having an interoperability standard, after all, that you can >>> just assume the devices all work according to the same spec and don't >>> need to hardcode details about each specific instance. >> >> If we talk about standard, then DT purpose is not for autodetectable >> pieces. These values are autodetectable, so such properties should not >> be encoded in DT. > > In the case of DDR, I think that the firmware can auto-detect them but > not the kernel. So from the kernel's point of view the DDR info should > be in DT, right? True, I thought memory controllers could provide such information, but now I checked Exynos5422 DMC and it does not expose such register. Best regards, Krzysztof
