[i2c] i2c-nforce patch adding a check for read/writes of >=31 bytes

[Jean Delvare]

Hi Al,

On Tue, 15 May 2007 11:09:23 -0700, Al Borchers wrote:
> Jean Delvare wrote:
> >>                                   Unfortunately, the MCP51 I2C host controller
> >>does hang if the device responds to a block read with a length of more than
> >>32 bytes.  The driver can save us from that--though it is a question
> >>whether working around this hardware bug risks worse side effects.
> > 
> > 
> > The driver cannot actually save us from that with guaranteed success.
> > Hans-Frieder's workaround was based on the assumption that an SMBus
> > block read at a given address always returns the same size. This might
> > not be the case.
> > 
> > Anyway, I don't expect it to happen in practice. That would be a bug in
> > the target SMBus chip that would make it effectively no longer SMBus
> > compliant. So the MCP51 implementation still looks better than what the
> > nForce2-4 had, even if it's still not perfect.
> 
> This bug hurts us frequently in development when debugging new device
> firmware.
> 
> Our devices use fixed block lengths--so that is not a problem for us,
> anyway.

I was trying to explain why that workaround wasn't acceptable upstream.
Of course, if it helps you, feel tree to do that locally.

> > The SMBus specification is pretty clear that blocks larger than 32
> > bytes aren't supported, so it is expected that the nForce SMBus master
> > doesn't support greater values. The Linux implementation sized its
> > structures in consequence, so we wouldn't support that either.
> > 
> > If you want to transfer larger blocks at once, then you're out of the
> > SMBus scope and back to the I2C realm. That it, use i2c_transfer() and
> > an I2C-compliant bus, instead of i2c_smbus_*().
> 
> We follow the SMBus spec and transfer 32 bytes or less at a time.  We use
> block transfers for strings, 4 byte values, and firmware images
> (in 32 byte chunks).
> 
> My point is that there are people who use SMBus block transfers and
> want to see support for it in the I2C drivers.

Alright. As I said before, I would welcome a patch to the i2c-nforce2
driver adding SMBus block transaction supports for the MCP51 and later
chips.

-- 
Jean Delvare