Printf hangs when internal buffer of driver is temporarily full

[Philip Oberstaller <Philip.Oberstaller@septentrio.com>]

Hi,

I'm writing a driver for the Exar's XR20M1280 SPI-based serial port
using kernel version 3.4.17.

If a string, which is considerably longer than the driver's internal buffer
(512 bytes), is written with printf/fprintf to the dedicated port, then the
function call hangs after the driver's write_room function returns zero,
which happens when the driver's internal buffer is temporarily full.
I would have expected that the TTY layer or the libc implementation of
printf would requery write_room automatically and thus would notice
after a while that there is space again, but this doesn't seem to happen.
Eventually the printf function returns after a very long timeout but without
that the remaining characters have been printed.

I based my code mainly on the ifx6x60-driver and I looked at other similar
drivers as well, but I can't see that they are using a different strategy.

In a dated driver book it was suggested that write_room and write should
always return at least one. I could use busy waiting in the write function
until there is space for at least one character, but I hope that there is
a better solution around?!

Thanks,
Philip

[Please CC me, I'm not on the list]

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