[Qemu-devel] [PATCH 00/12] RFC: usb: input pipelining support and other speedups

[Hans de Goede]

This patchset started out as adding support for pipelining of input bulk
transfers, although input pipelining is a bit tricky to do, there are 2 good
reasons to do it:

1) Performance, I expect this to ie significantly speed-up reading from USB
mass storage devices.

2) Currently reading from a USB<->serial converter, ie a pl2303 based one is
unreliable when the serial transport is not using flowcontrol. The problem
is that 115200 bps takes aprox 200 64 bytes packets / sec, which means we
must not exceed a roundtip time average of 5 ms over a period of 20 ms. If
within any 20 ms period we exceed that round trip time average the internal
buffer of the pl2303 will likely overflow. Input pipelining greatly improve
things here as: it combines the previous 4 packets submitted per device-driver
level read into 1 big read, reducing the number of packets / sec by a factor 4,
and it allows to have multiple reads outstanding.

I had working on this on my mind for a while because of 1), and 2) has made
me spend the necessary time to get this working now. Even though 2 was my
main reason to start working on this now, most of my testing sofar has
focussed on 1, resulting in some more speedup patches and a total improvement
of the read speed from a cheap USB thumb drive from 4.4 MB/s to 10.8 MB/s or
a factor of 2.4 !

The reason this patch-set is currently still RFC is because I need to run more
tests wrt 2). Despite the RFC status reviews are very much welcome now!

Regards,

Hans