linux-kernel - Re: Kernel driver vs libusb performance

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Date:	Fri, 11 Jan 2013 05:41:18 -0800
From:	Greg KH <gregkh@...uxfoundation.org>
To:	Ozan Çağlayan <ozancag@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Kernel driver vs libusb performance

On Fri, Jan 11, 2013 at 11:53:36AM +0200, Ozan Çağlayan wrote:
> Hi,
> 
> I have a device which I'm currently accessing using libusb. It's
> basically a HID compliant USB device. It reports 32bytes of data with
> a time resolution of 128Hz, e.g. I have to read and store 32bytes each
> 1/128 second. One performance drawback is that those 32bytes chunks
> are encrypted with AES so once I receive them through the USB endpoint
> I first decrypt them.
> 
> So 128Hz is quite a low polling frequency and can be handled in a
> single-thread using a modern CPU, but I'm planning to run this loop on
> a Raspberry Pi or namely low-end, cheap embedded processors. I'm also
> using Python which is significantly slow on Raspberry Pi.

Ah, my sympathies, USB on a rpi is horrible, and you will have all sorts
of nasty issues, no matter how you end up doing this (in the kernel or
userspace).

> So I wonder whether writing a kernel driver which decrypts the packets
> using in-kernel crypto API and then exposing them through a character
> device node would bring a performance gain at all. At least on the
> userspace side I may get rid of all the bus searching, crypto key
> generation, fetching and decrypting boilerplate, am I in the wrong
> way?

Test it out and see.  I really don't think that user vs. kernel space is
going to be the issue here, I think you are going to see some very real
problems with the hardware itself in this area.

Best of luck,

greg k-h
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