On Wed, Jul 10, 2024 at 02:33:29PM +0100, André Draszik wrote: > As pointed out in [1] before, the hand-over between earlycon and serial > console is fragile due to clocking issues: > > ... causing earlycon to stop to work sometime into the boot for two > reasons: > * peric0_top1_ipclk_0 requires its parent gout_cmu_peric0_ip to be > running, but because earlycon doesn't deal with clocks that > parent will be disabled when none of the other drivers that > actually deal with clocks correctly require it to be running and > the real serial driver (which does deal with clocks) hasn't taken > over yet > > The console UART, and I2C bus 8 are on the same cmu_peric0 controller, > and that cmu_peric0 has two clocks coming from cmu_top, ip and bus. For > I2C8 & UART to work, both of these clocks from cmu_top need to to be on > as they are the parent of the i2c8-(ip|pclk) and uart-(ip|pclk) each. > > The bootloader leaves those clocks running, yes. So earlycon works (for > a while). > > At some point into the boot, one of two things happens: > 1) Linux will load the i2c driver. That driver does clock handling > (correctly), it will initialise and then it has nothing to do, therefore > it disables cmu_peric0's i2c8 ip and pclk clocks. Because at that stage > nothing appears to be using the cmu_peric0's ip clock (the real serial > driver hasn't initialised yet), Linux decides to also disable the parent > ip clock coming from cmu_top. > > At this stage, the earlycon driver stops working, as the parent ip clock > of the uart ip clock is not running any more. No serial output can be > observed from this stage onwards. I think what is probably happening is > that the console uart FIFO doesn't get emptied anymore, and earlycon > will simply wait forever for space to become available in the FIFO (but > I didn't debug this). > > Anyway, the boot doesn't progress, the system appears to hang. In any > case it's not usable as we have no other means of using it at this stage > (network / usb / display etc.). > > 2) Alternatively, the UART driver will load at this stage. Again, it > will tweak the clocks and after probe it will leave its clocks disabled. > The serial console driver hasn't taken over at this stage and earlycon > is still active. Again, the system will hang, because IP and PCLK have > been disabled by the UART driver. Once the serial console is enabled, > clocks are being enabled again, but because earlycon is still waiting > for progress, the boot doesn't progress past disabling ip and pclk. It > never gets to enabling the serial console (re-enabling the clocks). > > So in both cases we get some output from earlycon, but the system hangs > once the first consumer driver of an IP attached to cmu_peric0 has > completed probing. > > ... > > If earlycon is not enabled in kernel command line, everything works > fine, the kernel buffers its messages and once the real serial console > driver starts, all messages since boot are being printed. > > As requested, add a comment to the code for posterity, so the > information is not lost. The patch referenced in the comment can be > found at [2]. That should also be in the comment in the .c file, right? Along with the git id that you feel should be reverted? thanks, greg k-h
