Virtualization - Jul 2021 - [PATCH v11] i2c: virtio: add a virtio i2c frontend driver

On 01-07-21, 21:24, Wolfram Sang wrote:
> 
> > I just noticed this now, but this function even tries to send data
> > partially, which isn't right. If the caller (i2c device's
driver)
> > calls this for 5 struct i2c_msg instances, then all 5 need to get
> > through or none.. where as we try to send as many as possible here.
> > 
> > This looks broken to me. Rather return an error value here on success,
> > or make it complete failure.
> > 
> > Though to be fair I see i2c-core also returns number of messages
> > processed from i2c_transfer().
> > 
> > Wolfram, what's expected here ? Shouldn't all message transfer
or
> > none?
> 
> Well, on a physical bus, it can simply happen that after message 3 of 5,
> the bus is stalled, so we need to bail out.
Right, and in that case the transfer will have any meaning left? I believe it
needs to be fully retried as the requests may have been dependent on each other.
> Again, I am missing details of a virtqueue, but I'd think it is
> different. If adding to the queue fails, then it probably make sense to
> drop the whole transfer.
Exactly my point.
> Of course, it can later happen on the physical bus of the host, though,
> that the bus is stalled after message 3 of 5, and I2C_RDWR will bail
> out.
Basically we fail as soon as we know something is not right, correct?

-- 
viresh

On 2021/7/2 12:55, Viresh Kumar wrote:
> On 01-07-21, 21:24, Wolfram Sang wrote:
>>> I just noticed this now, but this function even tries to send data
>>> partially, which isn't right. If the caller (i2c device's
driver)
>>> calls this for 5 struct i2c_msg instances, then all 5 need to get
>>> through or none.. where as we try to send as many as possible here.
>>>
>>> This looks broken to me. Rather return an error value here on
success,
>>> or make it complete failure.
>>>
>>> Though to be fair I see i2c-core also returns number of messages
>>> processed from i2c_transfer().
>>>
>>> Wolfram, what's expected here ? Shouldn't all message
transfer or
>>> none?
>> Well, on a physical bus, it can simply happen that after message 3 of
5,
>> the bus is stalled, so we need to bail out.
> Right, and in that case the transfer will have any meaning left? I believe
it
> needs to be fully retried as the requests may have been dependent on each
other.
>
>> Again, I am missing details of a virtqueue, but I'd think it is
>> different. If adding to the queue fails, then it probably make sense to
>> drop the whole transfer.
> Exactly my point.
>
This is not efficient. If adding the ith request to the queue fails, we 
can still send

the requests before it. We don't need to know if it fails here (adding 
to the queue)

or there (later in the host). The "master_xfer" just need to return 
final number of

messages successfully processed.