Re: [PATCH] i2c/busses: (mpc) Add support for SMBUS_READ_BLOCK_DATA

[Guenter Roeck <guenter.roeck-IzeFyvvaP7pWk0Htik3J/w@public.gmane.org>]

On Tue, 2011-11-15 at 14:02 -0500, Tabi Timur-B04825 wrote:
> On Tue, Nov 15, 2011 at 2:54 AM, Jean Delvare <khali-PUYAD+kWke1g9hUCZPvPmw@public.gmane.org> wrote:
> 
> > This needs careful testing (which I can't do.)
> 
> I can test these patches, so please CC me on them.
> 
Ok, I'll copy you on the next version of the patch. Turns out I have a
board as well, so I can at least do some basic testing.

> > There may have been a
> > reason why the read was done after the writes. Swapping the commands
> > may be the wrong thing to do. The dummy read earlier in this function
> > suggests that maybe changes to CCR do not take effect until you read
> > from (or write to) the DR register.
> 
> Unfortunately, I don't know enough about the I2C protocol to answer
> this, but I can provide this description on the DR register from the
> reference manual:
> 
>          Transmission starts when an address and the R/W bit are
> written to the data register and the I2C interface
>          performs as the master. A data transfer is initiated when
> data is written to the I2CDR. The most significant bit
>          is sent first in both cases. In master receive mode, reading
> the data register allows the read to occur, but also
>          allows the I2C module to receive the next byte of data on the
> I2C interface. In slave mode, the same function
>          is available after it is addressed. Note that in both master
> receive and slave receive modes, the very first read
>          is always a dummy read.
> 
> > Can't the above be rewritten to keep the order of the commands as it
> > was before? AFAICS it would only take one or two extra tests.
> 
> I think it's very important to keep the order as-is, if at all
> possible.  This driver is very old and the hardware is used on all of
> our SOCs, dating back several years.  It would be a nightmare to test
> it all over again.
> 
I'll see if I can work around that. 1-byte block reads may be tricky,
though, but maybe I can get that working as well.

Thanks,
Guenter