On Wed, 22 May 2024 12:01:21 +0000 "Gradinariu, Ramona" <Ramona.Gradinariu@xxxxxxxxxx> wrote: > > > > 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). > > I did not get a chance to look at this again until now. Unfortunately, a > memcpy will not work. > The current implementation is as follows: > /* The lower register data is sequenced first */ > st->data[i++] = buffer[2 * bit + offset + 3]; > st->data[i++] = buffer[2 * bit + offset + 2]; Ah. That's horrible... :( Thanks for pointing that out! > > The device first sends the 16LSB, then the next 16MSB in big endian > format. > > So then I wonder, can we keep the same implementation logic? The code > is implemented in the same manner for adis16475 driver which uses the > same channels data packing approach. Not much choice and given the need to handle a mixed endian stream you might as well do the packing here as well. So sure, keep the code as you have it. > > > > > 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. > > > > Jonathan > > >
