[ale] Meet the Raspberry Pi 3 Model B+

[Solomon Peachy]

On Wed, Mar 14, 2018 at 09:26:39AM -0700, Alex Carver via Ale wrote:
> Define flaky?  The LAN9514 (in the older Pi) is not an OTG controller to
> start.  It's just the USB hub with an Ethernet PHY attached to one
> internal port.  The newer controller has six downstream ports.  The
> datasheet isn't available yet from Microchip.

The RPi family's SoC only has one USB port connected to a DesignWare OTG 
USB controller that's infamously flaky when multiple (or sometimes even 
only one) higher-bandwidth USB device is plugged in, causing one device 
or the other to get starved for bandwidth.

The LAN9514 is actually a 5-port USB hub, with an ethernet controller 
wired into one of those ports.

I know of several printer models that fail to work with the RPi2 or RPi3 
unless something is plugged into the RPi's onboard ethernet port, 
ironically becasue simply having a link up on the LAN9514 is enough to 
create enough chattiness on the (shared) bus to prevent the controller 
from DOSing the printer with USB2 PING flow control messages every 
350-500ns.  A typical PC USB controller implements a backoff, settling 
at around a 4us period.  There's apparently no direct way to program the 
DWC OTG controller to implement something similar...

(And yes, there are multiple open bugs filed with the RPi kernel 
 folks.  Ironically the newer RPis are worse, not better, due to faster
 internal busses..)

 - Solomon
-- 
Solomon Peachy			       pizza at shaftnet dot org
Coconut Creek, FL                          ^^ (email/xmpp) ^^
Quidquid latine dictum sit, altum videtur.
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