Zero-copy irq-driven data

[Ricardo Ribalda <ribalda@chromium.org>]

Hello

I have just started using io_uring so please bear with me.

I have a device that produces data at random time and I want to read
it with the lowest latency possible and hopefully zero copy.

In userspace:

I have a sqe with a bunch of io_uring_prep_read_fixed and when they
are ready I process them and push them again to the sqe, so it always
has operations.

In kernelspace:

I have implemented the read() file operation in my driver. The data
handling follows this loop:

loop():
 1) read() gets called by io_uring
 2) save the userpointer and the length into a structure
 3) go to sleep
 4) get an IRQ from the device, with new data
 5) dma/copy the data to the user
 6) wake up read() and return

I guess at this point you see my problem.... What happens if I get an
IRQ between 6 and 1?
Even if there are plenty of read_operations waiting in the sqe, that
data will be lost. :(


So I guess what I am asking is:

A) Am I doing something stupid?

B) Is there a way for a driver to call a callback when it receives
data and push it to a read operation on the cqe?

C) Can I ask the io_uring to call read() more than once if there are
more read_operations in the sqe?

D) Can the driver inspect what is in the sqe, to make an educated
decision of delaying the irq handling for some cycles if there are
more reads pending?


Thanks!
-- 
Ricardo Ribalda