On Thu 2018-11-29 19:29:24, Luis Chamberlain wrote:
On Wed, Nov 28, 2018 at 11:36:20AM -0800, Brendan Higgins wrote:
A number of test features need to do pretty complicated string printing where it may not be possible to rely on a single preallocated string with parameters.
So provide a library for constructing the string as you go similar to C++'s std::string.
Hrm, what's the potential for such thing actually being eventually generically useful for printk folks, I wonder? Petr?
printk() is a bit tricky:
+ It should work in any context. Any additional lock adds risk of a deadlock. Especially the NMI and scheduler contexts are problematic. There are problems with any other code that might be called from console drivers and calls printk() under a lock.
+ It should work also when the system is out of memory. Especially atomic context is problematic because we could not wait for memory reclaim or swap.
+ We also do to the best effort to get the message out on the console. It is important when the system is about to die. Any extra buffering layer might cause delay and avoid seeing the message.
From this point of views, this API is not generally usable with printk().
Now, the question is how many of the above fits also for unit testing. At least, you might need to be careful when allocating memory in atomic context.
BTW: There are more existing printk APIs: Well, I admit the they are not easily reusable in unit testing:
+ printk() is old, crappy code, complicated with all the cornercases and consoles.
+ include/linux/seq_buf.h is simple buffering. It is used primary for sysfs output. It might be usable if you add support for loglevel and use big enough buffer. I quess that you should flush the buffer regularly anyway.
+ trace_printk() uses lockless per-CPU buffers. It currently does not support loglevels. But it might be pretty interesting choice as well.
I do not say that you have to use one of the existing API. But you might consider them if you encouter any problems and maintaining your variant gets complicated.
Best Regards, Petr