Re: [PATCH v2 1/2] Input: mms114 - use smbus functions whenever possible

[Andi Shyti <andi@etezian.org>]

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 But this means that the i2c in the kernel is wrong (or outdated),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