Am Mittwoch, 8. Juli 2015, 21:00:06 schrieb Heiko St?bner: > Most newer Rockchip SoCs provide the possibility to use a usb-phy > as passthrough for the debug uart (uart2), making it possible to > get console output without needing to open the device. > > This patch adds an early_initcall to enable this functionality > conditionally via the commandline and also disables the corresponding > usb controller in the devicetree. > > Currently only data for the rk3288 is provided, but at least the > rk3188 and arm64 rk3368 also provide this functionality and will be > enabled later. > > On a spliced usb cable the signals are tx on white wire(D+) and > rx on green wire(D-). > > Signed-off-by: Heiko Stuebner <heiko at sntech.de> dumping some thoughts that stem from comments in my g+ post about this: Arnd Bergmann, yesterday 19:40: Would it help to move this code into the console init section of the UART driver to avoid losing the initial output?? Heiko St?bner, yesterday 20:08 I don't think so. The functionality has nothing to do with the uart itself - in fact for the system everything stays the same - only the routing becomes different. It should even work with earlyprintk/earlycon. The only output you loose is up to the point when the early_initcall is executed - which is around the smp bringup time. And I guess if somebody is doing kernel changes for this early stage, they will probably use a board with a real uart or modify their uboot to already also set this option.? Arnd Bergmann, yesterday 20:52 Isn't early_initcall /much/ later than console_init, or even earlycon? The console output gets buffered until the first console gets found and is then flushed to that. If you switch the MUX just before registering the console, you should not lose anything, and also put this code in a more natural location.? Heiko St?bner, today 23:03 Yep as I said, twi switch currently runs around the time of the smp bringup . I'm probably just daft, but also still not sure how this should fit into the uart driver/console stuff. Especially as it is not even a property of the (designware) uart ips, but instead it might even be more a property of the usb phy. In terms of natural location, the usbphy driver might then be a more suitable position? In terms of when to run, it also looks like early_initcall is one of the earliest times to run, as the code needs syscons, devicetree matching and also devicetree updates. If just the regular console is included, nothing is lost and I guess when somebody needs to touch (and debug) such low-low-level parts like before the smp bringup they won't do it on a board without a real serial port I guess. I guess we should also move this to a mailing list ... I guess I'll just copy our thread here into a reply to my patch ;-) .?
