Hi Robin, On 22/07/2016:11:03:14 AM, Robin Murphy wrote: > On 22/07/16 05:08, Pratyush Anand wrote: > > On 21/07/2016:02:49:36 PM, Geoff Levand wrote: > >> On Thu, 2016-07-21 at 11:50 +0100, Robin Murphy wrote: > >>> The Exynos UART (drivers/tty/serial/samsung.c) is one which comes to > >>> mind as definitely existing, and on arm64 systems to boot. The TX > >>> register is at offset 0x20 there. > >> > >> Here's what I came up with. > >> > >> > >> + struct data {const char *name; int tx_offset;}; > >> + static const struct data ok_list[] = { > >> + /* {"armada-3700-uart", ?}, */ > >> + {"exynos4210-uart", 0x20}, > >> + /* {"ls1021a-lpuart", ?}, */ > >> + /* {"meson-uart", ?}, */ > >> + /* {"mt6577-uart", ?}, */ > >> + {"ns16550", 0}, > >> + {"ns16550a", 0}, > >> + {"pl011", 0}, > >> + {NULL, 0} > >> + }; > > > > sinc functionality is just to debug the scenario when something goes wrong in > > purgatory. IMHO, it should be disabled by default. So, why not to keep it as > > simple as possible. Its a low level debugging mainly for developer, so user > > should know the absolute address. Therefore, I think no need to parse earlycon > > or earlyprintk from command line. Whatever user passes in --port can be treated > > as address of TX register. If TX offset is 0x20, then user can pass --port as > > base+0x20. Additionally, we can pass TX register width as well. So what about > > something like "--port=0x1c020000,1" where 0x1c020000 is TX register address and > > 1 says about it's width in bytes. > > I don't think even that is worthwhile, since without any polling it > still relies on the UART having FIFOs, someone having already enabled > the FIFOs, the FIFOs being deep enough and/or the output being short > enough. In short, it's fragile enough that I'm not convinced it's even > useful as a debug option. I suggest we simply copy the purgatory console > implementation from, say, ARM or Alpha. May be I am missing, but deep TX FIFO should not be an issue. Whatever, we write in TX register, that will go to the port eventually. However, I do agree that short FIFO could be an issue and overflow is quite possible in that case. I had been trying to convince Geoff to take [1], which will help to resolve it. [1] https://github.com/pratyushanand/kexec-tools/commit/0d49d420810f070bcff9c96e377477d96f909d11 ~Pratyush > > Robin. > > > > > ~Pratyush > >