Hi Matthias, On 03/23/2016 10:40 AM, Matthias Schiffer wrote: > On 03/22/2016 04:38 PM, Peter Hurley wrote: >> On 03/22/2016 06:07 AM, Matthias Schiffer wrote: >>> I've tried your patch and I can't reproduce the issue anymore with it; I >>> have no idea if this actually has to do something with the issue, or the >>> change of the code path just hid the bug again. >>> >>> Regarding your other mail: with "small change", I was not talking about >>> adding an additional printk; as mentioned, even changing the numbers in >>> UTS_VERSION can hide the issue. I diffed a working and a broken kernel >>> image, and the UTS_VERSION is really the only difference. I have no idea >>> how to explain this. >> >> If _any_ change may hide the problem, that will make it impossible >> to determine if any attempted fix actually works, regardless of what >> debugging method you use. >> >> FWIW, you could still use the boot console to debug the problem by >> disabling the regular command-line console. >> >> Regards, >> Peter Hurley > > Hi, > it seems Peter was on the right track. With some help from Ralf, I was able > to narrow down the issue a bit, and I'm fairly sure the hang happens > somewhere in autoconfig(). > > autoconfig_16550a() is doing all kinds of weird checks to detect different > hardware by writing a lot of register values which are documented as > reserved in the AR7242 datasheet (there's a leaked version going around > that can be easily googled...), no idea if any of those are problematic. > Just setting UPF_FIXED_TYPE as suggested by Peter would avoid that code > altogether. That's just a debugging patch and not appropriate for permanent use, the reason being that this uart is _not_ 16550 compatible (or even 16450 compatible). The three options for 8250 driver support for this part are: 1. Similar to the debugging patch, set UPF_FIXED_TYPE but set port type to PORT_8250 instead. This will lose FIFO support so 115K won't be possible and likely neither will 38400. 2. Set UPF_FIXED_TYPE but define a new PORT_* value and add support for this PORT_* value to uart_config array, uapi headers, and anywhere the scratch register is used. 3. As with 2. above but don't set UPF_FIXED_TYPE and add a probe function that detects ports of this type to autoconfig(). I don't recommend this method. This requirement is independent of fixing prom_putchar_ar71xx(). > That being said, I found another minimal change that seems to fix the > issue: prom_putchar_ar71xx() in arch/mips/ath79/early_printk.c only waits > for UART_LSR_THRE, while serial_putc() in > drivers/tty/serial/8250/8250_early.c waits for (UART_LSR_TEMT | > UART_LSR_THRE). Adjusting arch/mips/ath79/early_printk.c in the same way > makes the hangs go away. Go ahead and do this as well. > Maybe the AR7242 doesn't like its serial config > registers being poked while there's still something in the FIFO? Waiting > for UART_LSR_TEMT seems like a good idea anyways to ensure that all > characters have been printed before autoconfig() starts taking things > apart. I agree. > (Why do these two versions of essentially the same code exist anyways?) earlyprintk command-line parameter is arch-specific and predates the arch-independent earlycon support. earlycon requires the arch to either supply a fixmap for the uart i/o address or ioremap() must work by parse_early_param(); note how the mmio in arch/mips/ath79/early_printk.c accesses the mmio space @ KSEG1ADDR(uart base addr)? Can't do that in arch-independent earlycon code. Regards, Peter Hurley
