Re: [PATCH v2 2/5] em28xx: respect the message size constraints for i2c transfers

[Sascha Sommer <saschasommer@freenet.de>]

Hello,

Am Sat, 22 Dec 2012 22:07:46 -0200
schrieb Mauro Carvalho Chehab <mchehab@redhat.com>:

> Em Sun, 16 Dec 2012 19:23:28 +0100
> Frank Schäfer <fschaefer.oss@googlemail.com> escreveu:
> 
> > The em2800 can transfer up to 4 bytes per i2c message.
> > All other em25xx/em27xx/28xx chips can transfer at least 64 bytes
> > per message.
> > 
> > I2C adapters should never split messages transferred via the I2C
> > subsystem into multiple message transfers, because the result will
> > almost always NOT be the same as when the whole data is transferred
> > to the I2C client in a single message.
> > If the message size exceeds the capabilities of the I2C adapter,
> > -EOPNOTSUPP should be returned.
> > 
> > Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
> > ---
> >  drivers/media/usb/em28xx/em28xx-i2c.c |   44
> > ++++++++++++++------------------- 1 Datei geändert, 18 Zeilen
> > hinzugefügt(+), 26 Zeilen entfernt(-)
> > 
> > diff --git a/drivers/media/usb/em28xx/em28xx-i2c.c
> > b/drivers/media/usb/em28xx/em28xx-i2c.c index 44533e4..c508c12
> > 100644 --- a/drivers/media/usb/em28xx/em28xx-i2c.c
> > +++ b/drivers/media/usb/em28xx/em28xx-i2c.c
> > @@ -50,14 +50,18 @@ do
> > {							\ } while
> > (0) 
> >  /*
> > - * em2800_i2c_send_max4()
> > - * send up to 4 bytes to the i2c device
> > + * em2800_i2c_send_bytes()
> > + * send up to 4 bytes to the em2800 i2c device
> >   */
> > -static int em2800_i2c_send_max4(struct em28xx *dev, u8 addr, u8
> > *buf, u16 len) +static int em2800_i2c_send_bytes(struct em28xx
> > *dev, u8 addr, u8 *buf, u16 len) {
> >  	int ret;
> >  	int write_timeout;
> >  	u8 b2[6];
> > +
> > +	if (len < 1 || len > 4)
> > +		return -EOPNOTSUPP;
> > +
> 
> Except if you actually tested it with all em2800 devices, I think that
> this change just broke it for em2800.
> 
> Maybe Sascha could review this patch series on his em2800 devices.
> 
> Those devices are limited, and just like other devices (cx231xx for
> example), the I2C bus need to split long messages, otherwise the I2C
> devices will fail.
> 

The only device that I own is the Terratec Cinergy 200 USB.
Unfortunately I left it in my parents house so I won't be able to
test the patch within the next two weeks. I don't know if any of the
other devices ever transfered more than 4 bytes but as far as I
remember the windows driver of the cinergy 200 usb did not do this.
The traces obtained from it were the only information I had during
development. On first sight, the splitting code looks wrong.

Regards

Sascha