On 4 July 2016 at 15:41, Remi Duraffort remi.duraffort@linaro.org wrote:
Hello Neil,
There are some hints in the documentation but it can largely only be hints as it depends on how that instance is going to be used.
The details will depend on:
0: how many workers are connected to each master 1: how many devices are connected to each worker 2: how many users / expected number of test jobs per day/hour etc. 3: what kinds of devices need to be supported - devices which can only be tested over USB will impact on your choice as many servers only have 1 or 2 USB host ports and USB hubs are problematic, even when powered.
For the usb hubs, ST boards does have an external power supply so we don't need the power-over-usb. Except this power supply issue (not applicable for ST boards), is it reliable to use a usb hub?
The problems with USB hubs and the reason for requiring power to the hub (as well as to the board) is that during a board reset, the enumeration on the dispatcher / serial console server will fail / return a different UID if the hub loses power in between. The second problem is that no matter what the hub says on the box or on the hub, the actual chipset used inside will vary unpredictably and some chipsets just randomly reset. The third problem is that LAVA is testing prototype boards which may or may not have bugs in their own USB stacks (hardware and software). Put all these together and devices which only provide serial over USB can be very troublesome. It's not to power the device over USB, it's to power the hub during device reboots / disconnects. The more of these powered USB hubs get involved between the device and the dispatcher, the worse the problem becomes. Hence, if your dispatcher only has 2 USB host ports and is meant to talk serial to a dozen devices which can only provide serial over USB, things go wrong at random intervals. The Cambridge lab are experimenting with custom powered USB hubs from Cambrionix for these reasons.