On 9/24/23 19:10, Jonathan Cameron wrote:
On Sun, 24 Sep 2023 17:07:26 +0100
Jonathan Cameron <jic23@xxxxxxxxxx> wrote:
On Fri, 22 Sep 2023 14:17:49 +0300
Matti Vaittinen <mazziesaccount@xxxxxxxxx> wrote:
When IIO goes through the available scan masks in order to select the
best suiting one, it will just accept the first listed subset of channels
which meets the user's requirements. This works great for most of the
drivers as they can sort the list of channels in the order where
the 'least costy' channel selections come first.
It may be that in some cases the ordering of the list of available scan
masks is not thoroughly considered. We can't really try outsmarting the
drivers by selecting the smallest supported subset - as this might not
be the 'least costy one' - but we can at least try searching through the
list to see if we have an exactly matching mask. It should be sane
assumption that if the device can support reading only the exact
channels user is interested in, then this should be also the least costy
selection - and if it is not and optimization is important, then the
driver could consider omitting setting the 'available_scan_mask' and
doing demuxing - or just omitting the 'costy exact match' and providing
only the more efficient broader selection of channels.
Signed-off-by: Matti Vaittinen <mazziesaccount@xxxxxxxxx>
Whilst I fully agree with the reasoning behind this, I'd rather we
did an audit of drivers to find any that have a non logical order
(one came up today in review) and fix them up.
A quick and dirty grep didn't find it to be a common problem, at least
partly as most users of this feature only provide one valid mask.
The few complex corners I found appear to be fine with the expected
shortest sequences first.
Defending against driver bugs is losing game if it makes the core
code more complex to follow by changing stuff in non debug paths.
One option might be to add a trivial check at iio_device_register()
that we don't have scan modes that are subsets of modes earlier in the list.
These lists are fairly short so should be cheap to run.
That would incorporate ensuring exact matches come earlier by default.
BTW I'd have sent these as a separate series as there is potential that
this will distract from or slow down the driver + not all the CC list
will care about this core cleanup.
I was not so worried about the driver being postponed. I was prepared to
suggest to merging a subset of the patches if need be - while I can
continue work with the rest of the series ;)
What comes to people being interested in the core-changes Vs. people
being interested in the driver changes - I'd expect the core changes to
concern much wider audience than the driver changes. But yes, knowing
the amount of mails people go through, limiting the recipient to most
relevant ones never hurts. Besides, I think there is no
conflicts/dependencies as driver changes don't change core/tools, and
core/tool changes don't touch the driver so splitting this to two series
should be trivial. Will do that for next version.
Yours,
-- Matti
--
Matti Vaittinen
Linux kernel developer at ROHM Semiconductors
Oulu Finland
~~ When things go utterly wrong vim users can always type :help! ~~
