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

[Dmitry Torokhov <dmitry.torokhov@gmail.com>]

On Mon, Oct 21, 2019 at 07:39:56PM +0300, Andi Shyti wrote:
> > > > 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 But this means that the i2c in the kernel is wrong (or outdated),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.

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.

Thanks.

-- 
Dmitry