by Nuno Sá

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On Thu, 2024-05-02 at 20:14 +0100, Jonathan Cameron wrote:
> On Thu, 02 May 2024 13:31:55 +0200
> Nuno Sá <[email protected]> wrote:
>
> > On Mon, 2024-04-29 at 20:40 +0100, Jonathan Cameron wrote:
> > > On Mon, 29 Apr 2024 13:17:42 +0000
> > > "Gradinariu, Ramona" <[email protected]> wrote:
> > >
> > > > > -----Original Message-----
> > > > > From: Nuno Sá <[email protected]>
> > > > > Sent: Monday, April 29, 2024 10:59 AM
> > > > > To: Jonathan Cameron <[email protected]>; Ramona Gradinariu
> > > > > <[email protected]>
> > > > > Cc: [email protected]; [email protected]; linux-
> > > > > [email protected]; [email protected]; [email protected];
> > > > > [email protected]; [email protected];
> > > > > [email protected];
> > > > > Gradinariu, Ramona <[email protected]>; Sa, Nuno
> > > > > <[email protected]>
> > > > > Subject: Re: [PATCH 4/5] iio: adis16480: add support for adis16545/7
> > > > > families
> > > > >
> > > > > [External]
> > > > >
> > > > > On Sun, 2024-04-28 at 16:25 +0100, Jonathan Cameron wrote:
> > > > > > On Tue, 23 Apr 2024 11:42:09 +0300
> > > > > > Ramona Gradinariu <[email protected]> wrote:
> > > > > >
> > > > > > > The ADIS16545 and ADIS16547 are a complete inertial system that
> > > > > > > includes a triaxis gyroscope and a triaxis accelerometer.
> > > > > > > The serial peripheral interface (SPI) and register structure
> > > > > > > provide a
> > > > > > > simple interface for data collection and configuration control.
> > > > > > >
> > > > > > > These devices are similar to the ones already supported in the
> > > > > > > driver,
> > > > > > > with changes in the scales, timings and the max spi speed in burst
> > > > > > > mode.
> > > > > > > Also, they support delta angle and delta velocity readings in
> > > > > > > burst
> > > > > > > mode, for which support was added in the trigger handler.
> > > > > > >
> > > > > > > Signed-off-by: Nuno Sá <[email protected]>
> > > > > >
> > > > > > What is Nuno's relationship to this patch? You are author and the
> > > > > > sender
> > > > > > which is fine, but in that case you need to make Nuno's involvement
> > > > > > explicit.
> > > > > > Perhaps a Co-developed-by or similar is appropriate?
> > > > > >
> > > > > > > Signed-off-by: Ramona Gradinariu <[email protected]>
> > > > > > A few comments inline. Biggest one is I'd like a clear statement of
> > > > > > why you
> > > > > > can't do a burst of one type, then a burst of other. My guess is
> > > > > > that the
> > > > > > transition is very time consuming? If so, that is fine, but you
> > > > > > should be
> > > > > > able
> > > > > > to let available_scan_masks handle the disjoint channel sets.
> > > > >
> > > > > Yeah, the burst message is a special spi transfer that brings you all
> > > > > of the
> > > > > channels data at once but for the accel/gyro you need to explicitly
> > > > > configure
> > > > > the chip to either give you the "normal vs "delta" readings. Re-
> > > > > configuring the
> > > > > chip and then do another burst would destroy performance I think. We
> > > > > could
> > > > > do
> > > > > the manual readings as we do in adis16475 for chips not supporting
> > > > > burst32.
> > > > > But
> > > > > in the burst32 case those manual readings should be minimal while in
> > > > > here we
> > > > > could have to do 6 of them which could also be very time consuming...
> > > > >
> > > > > Now, why we don't use available_scan_masks is something I can't really
> > > > > remember
> > > > > but this implementation goes in line with what we have in the
> > > > > adis16475
> > > > > driver.
> > > > >
> > > > > - Nuno Sá
> > > > >
> > > >
> > > > Thank you Nuno for all the additional explanations.
> > > > Regarding the use of available_scan_masks, the idea is to have any
> > > > possible
> > > > combination for accel, gyro, temp and timestamp channels or delta angle,
> > > > delta
> > > > velocity, temp and timestamp channels. There are a lot of combinations
> > > > for
> > > > this and it does not seem like a good idea to write them all manually.
> > > > That is
> > > > why adis16480_update_scan_mode is used for checking the correct channels
> > > > selection.
> > >
> > > If you are using bursts, the data is getting read anyway - which is the
> > > main
> > > cost here. The real question becomes what are you actually saving by
> > > supporting all
> > > the combinations of the the two subsets of channels in the pollfunc?
> > > Currently you have to pick the channels out and repack them, if pushing
> > > them all
> > > looks to me like a mempcy and a single value being set (unconditionally).
> >
> > > Then it's a question of what the overhead of the channel demux in the core
> > > is.
> > > What you pass out of the driver via iio_push_to_buffers*()
> > > is not what ends up in the buffer if you allow the IIO core to do data
> > > demuxing
> > > for you - that is enabled by providing available_scan_masks. At buffer
> > > start up the demux code computes a fairly optimal set of copies to repack
> > > the incoming data to match with what channels the consumer (here probably
> > > the kfifo on the way to userspace) is expecting.
> > >
> > > That demux adds a small overhead but it should be small as long
> > > as the channels wanted aren't pathological (i.e. every other one).
> > >
> > > Advantage is the driver ends up simpler and in the common case of turn
> > > on all the channels (why else did you buy a device with those measurements
> > > if you didn't want them!) the demux is zerocopy so effectively free which
> > > is not going to be the case for the bitmap walk and element copy in the
> > > driver.
> > >
> >
> > Maybe my younger me was smarter but reading again the validation of the scan
> > mask
> > code (when available_scan_masks is available), I'm not sure why we're not
> > using them.
> > I think that having one mask with delta values + temperature and another one
> > with
> > normal + temperature would be enough for what we want in here. The code in
> > adis16480_update_scan_mode() could then be simpler I think.
> >
> > Now, what I'm still not following is the straight memcpy(). I may be missing
> > something but the demux code only appears to kick in when we have compound
> > masks
> > resulting of multiple buffers being enabled. So I'm not seeing how we can
> > get away
> > without picking the channels and place them correctly in the buffer passed
> > to IIO?
>
> It runs whenever the enabled mask requested (the channels that are enabled) is
> different from the active_scan_mask. It only sends channels in one
> direction if there is only one user but it only sends the ones enabled by that
> consumer.
> It's called unconditionally from iio_enable_buffers()
>
> That iterates over all enabled buffers (often there is only 1)
>
> and then checks if the active scan mask is the same as the one for this
> buffer.
> https://elixir.bootlin.com/linux/v6.9-rc6/source/drivers/iio/industrialio-buffer.c#L1006
>
> The setup for whether to find a superset from available_scan_masks is here
> https://elixir.bootlin.com/linux/v6.9-rc6/source/drivers/iio/industrialio-buffer.c#L928
>
> Key is that if it happens to match, then we don't actually do anything in the
> demux.
> It just gets passed straight on through. That does the fast path you mention
> below.

Ahh got it... May failure was not realizing that iio_scan_mask_match() returns
the available masks so I was assuming that the bitmap_equal() check would only
differ when multiple buffers are enabled.

Oh well, I think then this should work... I'm not sure it will be more
performant for the case where we don't enable all the channels because the demux
is a linked list which is far from being a performance friend (maybe we can do
some trials with something like xarray...). Still, for this to work the buffer
we pass into IIO has to be bigger than it needs to be (so bigger memset())
because, AFAIU, we will have to pass on all the scan elements and, as I said,
the burst data will either have delta or normal measurements (not both). I guess
the code will still look simpler so if there's no visible difference in
performance I'm fine with the change. I guess Ramona can give it a try to see
how it looks like.

- Nuno Sá
>