On Thu, Dec 17, 2020 at 06:39:55PM -0800, Dan Williams wrote:
There is room for documentation improvement here. I realize reading it back now that much of the justification for "why not platform bus?" happened on the list, but only a small mention made it into the
It wasn't clear from the list discussions either TBH, or at least the bits I happened to see (I did miss several versions of this).
document. It turns out that platform-bus has some special integrations and hacks with platform-firmware implementations. For example, the ACPI companion magic
Could you be more specific about the problems that these cause for users of the bus?
and specific platform firmware integrations inplatform_match(). It's also an awkward bus name to use because these
Going through a bunch of possible firmware interfaces is standard for buses that can't be enumerated, SPI has almost exactly the same code for example. Again, I'm not clear what problem this causes?
devices do not belong to the platform. The platform bus is for devices that do not have an enumeration mechanism besides board files or firmware descriptions.
This is the one thing I was getting from what I did see, it was an abstraction thing. I'm still unclear what the abstraction is supposed to be - I had thought that it was supposed to be purely for MMIO devices but in a parallel reply Greg is suggesting that it applies to at least "firmware direct" devices which I guess is things enumerated by board files or firmware but that makes things even less clear for me as it's kind of random if people try to describe the internals of devices in DT or not, and ACPI goes the other way and doesn't really describe some things that physically exist.
We already have a bunch of drivers in tree that have to share a state and register other drivers from other subsystems for the same device. How is the auxiliary bus different?
There's also custom subsystem buses that do this. Why not other alternatives? They didn't capture the simultaneous mindshare of RDMA, SOF, and NETDEV developers. Personally my plans for using
At least in the case of SOF they were getting active pushback from somewhere telling them not to use MFD.
auxiliary-bus do not map cleanly to anything else in the tree. I want to use it for attaching an NPEM driver (Native PCIE Enclosure Management) to any PCI device driver that opts-in, but it would be overkill to go create an "npem" bus for this.
This is why everyone is using platform devices here - people were making custom buses but people (including Greg!) pointed out that these were just carbon copies of the platform bus.