On Sunday 03 August 2014 16:47:17 Geert Uytterhoeven wrote: > On Sun, Aug 3, 2014 at 2:42 PM, Jean Delvare <jdelvare@xxxxxxx> wrote: > > 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. Thanks for explanation and datasheet. I wrote that code without docs, just observing what card's BIOS does. "DRIVE_EN" name was just a guess. The bit is set always because other code (especially X11) can mess with the HW behind our back. > Gr{oetje,eeting}s, > > Geert > > -- > Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- > geert@xxxxxxxxxxxxxx > > In personal conversations with technical people, I call myself a hacker. > But when I'm talking to journalists I just say "programmer" or something > like that. -- Linus Torvalds -- Ondrej Zary -- To unsubscribe from this list: send the line "unsubscribe linux-fbdev" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
