Hi Dmitry, > > > > > On Sun, Oct 20, 2019 at 11:28:55PM +0300, Andi Shyti wrote: > > > > > > The exchange of data to and from the mms114 touchscreen never > > > > > > exceeds 256 bytes. In the worst case it goes up to 80 bytes in > > > > > > the interrupt handler while reading the events. > > > > > > > > > > > > > > > > i2c_smbus_read_i2c_block_data() is actually limited to > > > > > I2C_SMBUS_BLOCK_MAX = 32. > > > > > > > > oh sorry, I don't know how I slipped on this. > > > > > > > > But this means that the i2c in the kernel is wrong (or outdated), > > > > smbus specifies 256 bytes of data[*]. I might have relied on the > > > > specification more than the code. > > > > > > > > I guess SMBUS needs some update. > > > > > > You are right. It seems like that part of the specification was changed > > > with SMBus version 3.0 [1]: > > > > > > D.6 255 Bytes in Process Call: > > > The maximum number of bytes allowed in the Block Write-Block Read > > > Process Call (Section 6.5.8) was increased from 32 to 255. > > > > > > [1]: http://www.smbus.org/specs/SMBus_3_0_20141220.pdf > > > > yes :) > > > > OK, then I would ask Dmitry to hold on this patch I will try to > > update the smbus properly. > > 3.0 is from 2014, so we can't simply update the limits. And we need to > handle the case where device connected to a controller that does not > implement 3.0 standard. actually I don't see why, given that devices that were sending 32bytes will keep sending 32bytes and in any case I still haven't seen a controller that is strictly compliant to SMBUS 2. The mms114 device is a good example (and I think most of the touchscreens don't really care of the 32byte limit). In any case, I agree that I can't simply update the I2C_SMBUS_BLOCK_MAX because for sure I might have forgotten some cases and I'm currently looking how to do it. I have a few ideas but no one is good. I planned to send an RFC sometimes soon in order to kickstart some discussion. > If regmap is too much work then as a stop gap we could maybe only > convert write functions and mention why read needs to be custom. regmap is not too much work but I don't have the device with me, I might get one at some point, but can't do anything right now. Perhaps, for now you can take Stephan's patches and I would update everything once I get the device. They are not mutually exclusive, anyway. Thanks, Andi
