On Wed, 2011-01-05 at 20:02 -0200, Mauro Carvalho Chehab wrote:
> Em 05-01-2011 19:51, Jean Delvare escreveu:
> > Hi Mauro,
> >
> > On Wed, 05 Jan 2011 15:34:28 -0200, Mauro Carvalho Chehab wrote:
> >> Hi Jean,
> >>
> >> Thanks for your acks for patches 1 and 2. I've already applied the patches
> >> on my tree and at linux-next. I'll try to add the acks on it before sending
> >> upstream.
> >
> > If you can't, it's fine. I merely wanted to show my support to Andy's
> > work, I don't care if I'm not counted as a reviewer for these small
> > patches.
>
> Ok. So, it is probably better to keep it as-is, to avoid rebasing and having
> to wait for a couple days at linux-next before sending the git pull request.
>
> >
> >> Em 05-01-2011 12:45, Jean Delvare escreveu:
> >>> From a purely technical perspective, changing client->addr in the
> >>> probe() function is totally prohibited.
> >>
> >> Agreed. Btw, there are some other hacks with client->addr abuse on some
> >> other random places at drivers/media, mostly at the device bridge code,
> >> used to test if certain devices are present and/or to open some I2C gates
> >> before doing some init code. People use this approach as it provides a
> >> fast way to do some things. On several cases, the amount of code for
> >> doing such hack is very small, when compared to writing a new I2C driver
> >> just to do some static initialization code. Not sure what would be the
> >> better approach to fix them.
> >
> > Hard to tell without seeing the exact code. Ideally,
> > i2c_new_dummy() would cover these cases: you don't need to write an
> > actual driver for the device, it's perfectly OK to use the freshly
> > instantiated i2c_client from the bridge driver directly. Alternatively,
> > i2c_smbus_xfer() or i2c_transfer() can be used for one-time data
> > exchange with any slave on the bus as long as you know what you're
> > doing (i.e. you know that no i2c_client will ever be instantiated for
> > this slave.)
> >
> > If you have specific cases you don't know how to solve, please point me
> > to them and I'll take a look.
>
> You can take a look at saa7134-cards.c, for example. saa7134_tuner_setup()
> has several examples. It starts with this one:
>
> switch (dev->board) {
> case SAA7134_BOARD_BMK_MPEX_NOTUNER:
> case SAA7134_BOARD_BMK_MPEX_TUNER:
> /* Checks if the device has a tuner at 0x60 addr
> If the device doesn't have a tuner, TUNER_ABSENT
> will be used at tuner_type, avoiding loading tuner
> without needing it
> */
> dev->i2c_client.addr = 0x60;
> board = (i2c_master_recv(&dev->i2c_client, &buf, 0) < 0)
> ? SAA7134_BOARD_BMK_MPEX_NOTUNER
> : SAA7134_BOARD_BMK_MPEX_TUNER;
>
> In this specific case, it is simply a probe for a device at address 0x60, but
> there are more complex cases there, with eeprom reads and/or some random init
> that happens before actually attaching some driver at the i2c address.
> It is known to work, but it sounds like a hack.
The cx18 driver has a function scope i2c_client for reading the EEPROM,
and there's a good reason for it. We don't want to register the EEPROM
with the I2C system and make it visible to the rest of the system,
including i2c-dev and user-space tools. To avoid EEPROM corruption,
it's better keep communication with EEPROMs to a very limited scope.
Regards,
Andy
> Cheers,
> Mauro
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