Re: [Qemu-devel] [PATCH 1/2] omap_i2c: Clear SBD bit in STAT register on DATA read

[Andreas Färber]

Am 13.12.2012 15:45, schrieb Peter Maydell:
> On 12 December 2012 06:29, Andreas Färber <address@hidden> wrote:
>> After reading a single-byte I2C response such as the tmp105's response
>> to 0x01 0x00, the SBD status bit would not get reset on next read, still
>> indicating validity of only a single byte. Clear it on next word read.
> 
> This doesn't seem to correspond to what the OMAP1510 manual describes
> as the condition for this bit to be zeroed:

I don't have the manual, just Andrzej's omap_i2c code. n800/n810 seems
to be OMAP2420 btw.

> "This bit is cleared to 0 by the core when the local host reads the
> I2C_IV register if INTCODE is register access ready."

    case 0x0c:  /* I2C_IV */
        if (s->revision >= OMAP2_INTR_REV)
            break;

And our s->revision == OMAP2_INTR_REV (0x34), so reading IV is a no-op.
It reads as related to interrupt handling, which I was otherwise not
touching on.

There was no other comment saying "SBD" anywhere or touching "1 << 15"
of s->stat, so to me it seems nothing resets this bit today.

I don't get any hit for "SBD" in the Linux driver either, i2c-omap.c in
torvalds/linux.git seems to unconditionally read both bytes in
omap_i2c_receive_data() if the device has a 16-bit register. CC'ing the
Linux OMAP maintainer.

I could try to simply ignore SBD and rely on my own counting. I already
moved it to its own libqos source file last night.

> The manual also says for I2C_DATA:
> "In case of an odd number of bytes received to read, the upper byte of
> the last access always reads as 0x00. The local host must check the SBD
> status bit in I2C_STAT register to flush this null byte."
> 
> ...which to a naive reading implies that the high byte has
> to be jammed to all-zeroes until I2C_STAT is read, but that
> seems a little implausible.

To me that just says I need to check SBD to decide whether the high byte
last read is valid.

Andreas

> (interestingly the SBD bit is always 0 for omap3 because the data
> FIFO is 8 bits wide and so data is always read byte at at time)
> 
> -- PMM