On Sun, Oct 29, 2023 at 08:21:21PM +0200, ariel marcovitch wrote: > Greetings! > > While using a usb-serial device as console, I've noticed some > significant gaps in the kernel logs it receives. > > The problem can be reproduced in qemu like this (the kernel is a > x86_64_defconfig with usb-serial enabled and with the ftdi_sio driver > enabled): > qemu-system-x86_64 -m 4G -kernel arch/x86_64/boot/bzImage -usb -device > usb-serial,chardev=ser -chardev pty,id=ser -append 'console=ttyUSB0' > (this will create a `pts` device that will connect to the other end of > the emulated usb-serial) > > Then the logs look something like this: > [ 1.006459] SELinux: Initializing. > [ 1.011620] Mount-cache hash table entries: 8192 (order: 4, 65536 > bytes, li[ 2.315341] ACPI: \_SB_.LNKD: Enabled at IRQ 11 > > This probably happens because of the code in > `usb_serial_generic_write` which tries to insert the data into the > fifo: > count = kfifo_in_locked(&port->write_fifo, buf, count, &port->lock); > Because added indications for when the result is less than expected > and it showed significant losses. > The return value is silently ignored in `usb_console_write` > Also making the fifo bigger in `setup_port_bulk_out` helped (I made it > 10 times bigger and there were no losses) > > The reason so much data is written at a short time is because > usb-serial is initialized rather late, and when it is registered as a > console, all the logs until this point are written to it. > > I'm not sure what the solution should be. Maybe we need to check > whether the write in `console_emit_next_record` was successful and not > increase the seq counter in this case. > > Any suggestions? Please realize that usb-serial console was the result of me loosing a drunken bet. It's amazing it works at all. For "fake" devices like this, that use the generic usb-serial code, yes, you will have overruns and other problems, that's just part of how it works (i.e. not well.) For something like qemu, please use a real console, like the serial port (i.e. a fake serial port), not the fake usb-serial port. So this is "working as designed" in that it wasn't designed at all and again, it is a miracle any data is flowing there at all :) sorry, greg k-h
