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On some DP monitors we found that setting the wrong flip state on the AUX channel could cause the monitor to stop asserting HotPlug Detect (HPD). Setting the right flip state caused these monitors to start asserting HotPlug Detect again. Here's what we believe was happening: * We'd plug in the monitor and we'd see HPD assert * We'd quickly see HPD deassert * The kernel would try to init the type C PHY but would init it in USB mode (because there was a peripheral there but no HPD) * Because the kernel never set the flip mode properly we'd never see the HPD come back. With this change, we'll still see HPD disappear (we don't think there's anything we can do about that), but then it will come back. Overall we can say that it's sane to set the AUX channel flip state even when HPD is not asserted. NOTE: to make this change possible, I needed to do a bit of cleanup to the tcphy_dp_aux_calibration() function so that it doesn't ever clobber the FLIP state. This made it very obvious that a line of code documented as "setting bit 12" also did a bunch of other magic, undocumented stuff. For now I'll just break out the bits and add a comment that this is black magic and we'll try to document tcphy_dp_aux_calibration() better in a future CL. ALSO NOTE: the old function used to write a bunch of hardcoded values in _some_ cases instead of doing a read-modify-write. One could possibly assert that these could have had (beneficial) side effects and thus with this new code (which always does read-modify-write) we could have a bug. We shouldn't need to worry, though, since in the old code tcphy_dp_aux_calibration() was always called following the de-assertion of "reset" the the type C PHY. ...so the type C PHY was always in default state. TX_ANA_CTRL_REG_1 is documented to be 0x0 after reset. This was also confirmed by printk. Suggested-by: Shawn Nematbakhsh <shawnn@chromium.org> Reviewed-by: Chris Zhong <zyw@rock-chips.com> Signed-off-by: Douglas Anderson <dianders@chromium.org> Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com> |
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Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.