Hi Geert,

On Sun, 3 Aug 2014 16:47:17 +0200, Geert Uytterhoeven wrote:

On Sun, Aug 3, 2014 at 2:42 PM, Jean Delvare [off-list ref] wrote:

quoted

I am looking at the DDC / I2C implementation in that driver and I'm not
sure I understand how it works. Specifically I am wondering about bit
DDC_DRIVE_EN. It is set unconditionally in s3fb_ddc_setscl() and
s3fb_ddc_setsda() and never cleared explicitly. If this bit is a
regular bit then I don't understand why it is not just set at driver
initialization time. Or is this bit self-clearing and/or not-sticking?

The reason why I am asking is that I don't think the code in these
functions is completely correct. For the I2C protocol, 1 is the natural
state of both lines and the devices on the bus should only ever pull
the lines low to force a 0 state. They should never force a 1 state, as
this breaks some protocol features (specifically arbitration and clock
stretching.) My interpretation of the current code is that the lines
are forced to both 0 and 1 by the master, which is not correct. But as
I don't know how DDC_DRIVE_EN works, I'm not sure, and if it's indeed
broken [1], I'm also not sure how to fix it.

So if you could clarify how bit DDC_DRIVE_EN works, that would be great.

http://www.vgamuseum.info/images/stories/doc/s3/virge.pdf, p. 24-11:

Bit 0 SCW - Serial Clock Write
0 = Pin 205 is driven low
1 = Pin 205 is tri-stated
Pin 205 carries the DDC/I 2 C clock, depending on the operational
mode. When pin 205
is tri-stated, other devices may drive this line. The actual state of
the pin is read via
bit 2 of this register.
Bit 1 SDW - Serial Data Write
0 = Pin 206 is driven low
1 = Pin 206 is tri-stated
Pin 206 carries the DDC/I 2 C data, depending on the operational mode.
When pin 206
is tri-stated, other devices may drive this line. The actual state of
the pin is read via
bit 3 of this register.
Bit 2 SCR - Serial Clock Read (Read Only)
0 = Pin 205 is low
1 = Pin 205 is tri-stated (no device is driving this line)
Bit 3 SDR - Serial Data Read (Read Only)
0 = Pin 206 is low
1 = Pin 206 is tri-stated (no device is driving this line)
Bit 4 SPE - Serial Port Enable
0 = Use of bits 1-0 of this register disabled
1 = Use of bits 1-0 of this register enabled

Still doesn't explain well the real meaning of bit 4, but it does confirm that
writing a 1 to bit 0 or bit 1 doesn't drive the line high.

Very nice, thanks for the documentation. So the driver code looks
correct and I can move on to the remaining two suspect drivers (intelfb
and i740fb.)

-- 
Jean Delvare
SUSE L3 Support