Hi Dmitry,
On Wed, 30 Mar 2022 at 19:04, Sankeerth Billakanti quic_sbillaka@quicinc.com wrote:
The panel-edp driver modes needs to be validated differently from DP because the link capabilities are not available for EDP by
that time.
Signed-off-by: Sankeerth Billakanti quic_sbillaka@quicinc.com
This should not be necessary after
https://patchwork.freedesktop.org/patch/479261/?series=101682&rev=1.
Could you please check?
The check for DP_MAX_PIXEL_CLK_KHZ is not necessary anymore but
we
need to return early for eDP because unlike DP, eDP context will not have the information about the number of lanes and link clock.
So, I will modify the patch to return after the DP_MAX_PIXEL_CLK_KHZ
check if is_eDP is set.
I haven't walked through all the relevant code but something you said above sounds strange. You say that for eDP we don't have info about the number of lanes? We _should_.
It's certainly possible to have a panel that supports _either_ 1 or 2 lanes but then only physically connect 1 lane to it. ...or you could have a panel that supports 2 or 4 lanes and you only connect 1 lane. See, for instance, ti_sn_bridge_parse_lanes. There we assume 4 lanes but if a "data-lanes" property is present then we can use that to know that fewer lanes are physically connected.
It's also possible to connect more lanes to a panel than it supports. You could connect 2 lanes to it but then it only supports 1. This case needs to be handled as well...
I was referring to the checks we do for DP in dp_bridge_mode_valid. We check if the Link bandwidth can support the pixel bandwidth. For an external DP connection, the Initial DPCD/EDID read after cable connection will return the sink capabilities like link rate, lane count and bpp information that are used to we filter out the unsupported
modes from the list of modes from EDID.
For eDP case, the dp driver performs the first dpcd read during bridge_enable. The dp_bridge_mode_valid function is executed before bridge_enable and hence does not have the full link or the sink capabilities information like external DP connection, by then.
It sounds to me like we should emulate the HPD event for eDP to be handled earlier than the get_modes()/prepare() calls are attempted. However this might open another can of worms.
For DP, the HPD handler mainly initiates link training and gets the EDID.
Before adding support for a separate eDP panel, we had discussed about this internally and decided to emulate eDP HPD during enable(). Main reason being, eDP power is guaranteed to be on only after bridge_enable(). So, eDP link training can happen and sustain only after bridge_enable().
Emulating HPD before/during get_modes will not have any effect because:
1. get_modes() will go to panel's get_modes() function to power on read EDID
2. panel power will be turned off after get_modes(). Panel power off will reset every write transaction in DPCD. anyway invalidating link training
3. mode_valid will land in dp driver but panel will not be powered on at that time and we cannot do aux transfers or DPCD read writes.
So, we need to proceed with the reported mode for eDP.
Well... Even if during the first call to get_modes() the DPCD is not read, during subsequent calls the driver has necessary information, so it can proceed with all the checks, can't it?
get_modes() currently does not land in DP driver. It gets executed in panel bridge. But the mode_valid() comes to DP driver to check the controller compatibility.
-- With best wishes Dmitry
Thank you, Sankeerth