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.

--

Neil Williams
=============
neil.williams@linaro.org
http://www.linux.codehelp.co.uk/
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