Re: [9fans] PDP11 (Was: Re: what heavy negativity!)

[cinap_lenrek@felloff.net]

> The big one is USB.  disk/radio->kernel->user-space-usbd->kernel->application.
> Four copies.

that sounds wrong.

usbd is not involved in the data transfer. it mainly is just responsible to
enumerating devices and instantiating drivers and registering the endpoints
in devusb. after that you access the endpoint files from devusb which goes
directly to the kernel. devusb also allows you to create a alias for a
endpoint file which then appears directly under /dev. usb audio uses this
mechanism. the usb driver just activates the device and provides the ctl/volume
files, while audio data is handled by the kernel's devusb.

on another remark regarding zero copy. the reason plan9 drivers are small comes
from NOT doing these "optimizations". identity mapping the low part of memory
in the kernel avoids alot of trouble and allows you to get DMA capable memory
with just wrapping a pointer in PADDR(va). no page lists needed. no MMU tricks
needed in the drivers. you can use any kernel memory va for DMA... even your
kernel stack! its never paged out. you can be sure it is not changed while the
device looks at it ect. do not underestimate the impact of this "simplification".

linux block layer is broken in that regard btw. it just hands user pages into
the drivers without making sure they do not change while the i/o is in flight,
which results in all kinds of false-negatives when you actually start verifying
your raid arrays as different snapshots in time got written out to the raid
members. they know about this and ignore it because benchmarks are more important.

--
cinap