[Letux-kernel] GPS

Andreas Kemnade andreas at kemnade.info
Sun Nov 18 22:51:09 CET 2018


Hi,

just to avoid some ruffled feathers.
I have a patch set for the gps sirf driver on my boiling plate making
it work without wakeup line. It was surprisingly easy. Well, I have
more distance to the topic. I have not contributed to our w2sg0004
driver, was just taking part in the discussion why such a thing shuold
be in the kernel.
It does not have all the bells-and-whistles. But if that really goes
upstream, that can be added later.

So why did I do this?
I run tests analysing power consumption behavior from the lowest ground
with as least as possible loaded drivers It is the hardest test for
drivers. They really have to clearly request their ressources. I found
the hdq problem by doing that.
And often I am wondering why the current is too high just because the
gps is active.
If I am booting without serdev gps entries in the device tree, I can
just disable the gps from userspace by accessing gpio145, well, not so
nice but works just for the testing purpose.
If I am booting with serdev gps entries in dtb and a kernel with serdev
support, I do not have that possibiliy but I can load the gnss_w2sg0004
Unfortunately, it keeps the serdev open all the time even without any
user of it (gnss-sirf does not do that). If uart autosuspend is enabled
this leads to an additonal load of around 35mA if the system is idle
otherwise.
Ok, next idea. modprobe, rmmod. Big surpise. The GPS seem to get
turned *on* by rmmod! That is a really interesting topic, where
I also have a separate fix for on my boiling plate. I will come back
with that later.
As Nikolaus did not shout "I will fix it soon!", it seems it is up to me
to fix it and I guess he does not suffer from boredom. 
That means, I have to dig into the code, understand it. That
is the same for both drivers.
For the gnss-sirf I have the advantage, that more eyes will look at
patches and it will perhaps be accepted (after some fixes) or we have a
solid proof that our own driver is needed.
On the other hand the gnss-w2sg0004 has all the bells-and-whistles but
any ways to have more pm with always-opened serdev will result in rarely
used ways, so the chances are higher that it might get broken because
maintainers do not have that in their mind while reviewing patches.
Especially that there is an out-of-tree user of it.

Regards,
Andreas
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