Hi, On Fri, Apr 29, 2016 at 3:44 PM, Russell King - ARM Linux <linux@xxxxxxxxxxxxxxxx> wrote: > My reply would be... why should MMC have special handling that no > other subsystem has? No other subsystem? * i2c allows numbering devices by alias * rtc allows numbering devices by alias. * serial allows numbering devices by alias. * spi allows numbering devices by alias. * watchdog allows numbering devices by alias. ...at least that's my impression doing a grep for of_alias_get_id(), which I suggested earlier in this thread but apparently wasn't done. > Here's another example. Plug in several USB serial adapters. Which > USB serial /dev/ttyUSB* device corresponds to which adapter? The > answer is... it depends on the order you plug them in, which could > well be different from the order in which they are found if the > machine reboots. That's a very real problem. USB is, by definition, hotplug and probable and there is no ordering. For peripherals built-in to a SoC there is a sane ordering. Thus hotplug peripherals and builtin peripherals shouldn't have the same requirements. Quite honestly, it _would_ be quite convenient that if you are on a SoC and you know it has builtin USB controllers to have the root hubs numbered in a sane and consistent manner. An area for a future patch, maybe. > However, the answer to this problem is not to find some way to bind > a particular /dev/ttyUSB* node to a particular USB device, but to > use the solution(s) already provided - iow, /dev/serial/by-id or > /dev/serial/by-path trees which give a stable view of these devices. > > Now, while that allows the appropriate /dev/ttyUSB* device to be > found, it doesn't solve the "which ttyUSB* entry in the kernel > message log corresponds with which adapter" (which is the basis > of your point #1.) That's not solved, and isn't purposely isn't > solved for any kernel subsystem. > > Another example - I have boards here with multiple RTCs... guess > what happens with those? Yep, same problem. They get dynamic > /dev/rtc* assignment. Are you quite certain about that? See above. > Another example - I have a board with three ethernet devices... > yep, same problem again, the order depends on the driver probe > order, and they get dynamically assigned eth* names. Ethernet is often provided by USB and thus hotplug and probable. Quite honestly if there was a builtin Ethernet adapter provided on a SoC (not connected over USB), it would be super handy if it was forced to be "eth0" (and if there were more than one if they could be ordered in a way that made sense for that SoC). Dynamic ordering could come after. > The list of subsystems that have this property is large, because > it's all dependent on the device/driver probing order. Sure. For hotplug, there is no sane device ordering. For things built in to an SoC, there is sane device ordering. Dynamic ordering should come after static. > So, please answer this question: why should _only_ MMC be treated > as a special case? Please give a _technical_ reason rather than a > _personal_ reason. Because technically it makes it easier for people to understand their system to have a sane ordering for builtin peripherals. -Doug -- To unsubscribe from this list: send the line "unsubscribe devicetree-spec" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
