On 07/06/2014 12:29 AM, Jean Delvare wrote:
Hi Guenter,
On Sat, 05 Jul 2014 11:48:55 -0700, Guenter Roeck wrote:
On 07/05/2014 11:20 AM, Jean Delvare wrote:
The main problem I see is that for I2C_SMBUS_BLOCK_DATA reads, the chip
first returns the number of data bytes (as opposed to
I2C_SMBUS_I2C_BLOCK_DATA where the controller decides how many bytes it
wants to read.) There is no way the i2c-stub driver can guess that byte
count, as it depends on the chip it is supposed to emulate (and might
even change dynamically, at least in theory.)
We could have limited support for that, but that would require extra
module parameters to specify the block size for every register offset
on which SMBus block reads can be attempted. This also assumes that these
block sizes are static. And as you found out, that may also require
allocating extra memory for every such register offset.
I 'solved' the problem so far by only returning smbus block data after
it was written into a specific command. This way it is all dynamic.
This is smart :-) But this assumes that block size is the same the same
in both directions for a given register offset. This also assumes that
block sizes do not change over time. But anyway, i2c-stub is bound to
have such limitations, it can only emulate simple devices.
But another difficulty is also that when SMBus block reads enter the
game, the usual read/write symmetry tends to disappear. Often the
registers you read with SMBus block read commands are also readable and
writable at individual register addresses. i2c-stub has no way to know
that. Drivers would typically use SMBus block reads for performance
reason, but byte writes for convenience. So drivers operating on top of
i2c-stub would get confused in no time.
All these issues have so far convinced me that adding support for SMBus
block read/writes to i2c-stub wasn't worth it. That being said, if you
have a specific chip in mind that could be supported easily, I have no
objection.
The one I needed it for right now was lineage-pem; it was useful for me
since I don't have easy access to the HW anymore. Of course, problem with
that is what you pointed out ... with my change in the i2c-stub driver,
the lm93 driver now assumes that it can execute block commands. The only
way I see around that would be a module parameter.
Actually there already is one, named "functionality". For now it can
only be used to disable commands, because all supported commands were
enabled by default. It would be trivial to separate the full command
set from the default command set. That way SMBus block commands can be
supported but not enabled by default.
That would have to be
one which selects if the smbus block command should be treated similar
to the i2c block command
Do you know of any device actually behaving that way?
No, I thought the lm93 would do that, but as you probably know it is much
more complicated than that; I don't think it would be easy to simulate,
so I won't even try.
or as individual command blocks. I'll play around
with it some more. I'll send a patch if I find a solution which works for
both lm93 and lineage-pem and is not too complicated.
The change I suggested to the "functionality" parameter should do.
Agreed.
Guenter
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