When doing DP AUX transfers there are two actors that need to be powered in order for the DP AUX transfer to work: the DP source and the DP sink. Commit bacbab58f09d ("drm: Mention the power state requirement on side-channel operations") added some documentation saying that the DP source is required to power itself up (if needed) to do AUX transfers. However, that commit doesn't talk anything about the DP sink.

For full fledged DP the sink isn't really a problem. It's expected that if an external DP monitor isn't plugged in that attempting to do AUX transfers won't work. It's also expected that if a DP monitor is plugged in (and thus asserting HPD) then AUX transfers will work.

When we're looking at eDP, however, things are less obvious. Let's add some documentation about expectations. Here's what we'll say:

1. We don't expect the DP AUX transfer function to power on an eDP panel. If an eDP panel is physically connected but powered off then it makes sense for the transfer to fail.

2. We'll document that the official way to power on a panel is via the bridge chain, specifically by making sure that the panel's prepare function has been called (which is called by panel_bridge_pre_enable()). It's already specified in the kernel doc of drm_panel_prepare() that this is the way to power the panel on and also that after this call "it is possible to communicate with any integrated circuitry via a command bus."

3. We'll also document that for code running in the panel driver itself that it is legal for the panel driver to power itself up however it wants (it doesn't need to officially call drm_panel_pre_enable()) and then it can do AUX bus transfers. This is currently the way that edp-panel works when it's running atop the DP AUX bus.

NOTE: there was much discussion of all of this in response to v1 [1] of this patch. A summary of that is: * With the Intel i195 driver, apparently eDP panels do get powered up. We won't forbid this but it is expected that code that wants to run on a variety of platforms should ensure that the drm_panel's prepare() function has been called. * There is at least a reasonable amount of agreement that the transfer() functions itself shouldn't be responsible for powering the panel. It's proposed that if we need the DP AUX dev nodes to be robust for eDP that the code handling the DP AUX dev nodes could handle powering the panel by ensuring that the panel's prepare() call was made. Potentially drm_dp_aux_dev_get_by_minor() could be a good place to do this. This is left as a future exercise. Until that's fixed the DP AUX dev nodes for eDP are probably best just used for debugging. * If a panel could be in PSR and DP AUX via the dev node needs to be reliable then we need to be able to pull the panel out of PSR. On i915 this is also apparently handled as part of the transfer() function.

[1] https://lore.kernel.org/r/20220503162033.1.Ia8651894026707e4fa61267da944ff73...

Signed-off-by: Douglas Anderson dianders@chromium.org Reviewed-by: Dmitry Baryshkov dmitry.baryshkov@linaro.org Reviewed-by: Lyude Paul lyude@redhat.com --- Hopefully I've resolved everything here to everyone's satisfaction. There's no crazy hurry here. I'll give this a week or so and then land it on drm-misc if there is no additional discussion.

Changes in v2: - Updated wording as per discussion on v1. - Updated commit message as per discussion on v1. - Add pointer to v1 discussion for future reference.

include/drm/display/drm_dp_helper.h | 16 +++++++++++++--- 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/include/drm/display/drm_dp_helper.h b/include/drm/display/drm_dp_helper.h index dca40a045dd6..dc3c02225fcf 100644 --- a/include/drm/display/drm_dp_helper.h +++ b/include/drm/display/drm_dp_helper.h @@ -370,9 +370,19 @@ struct drm_dp_aux { * helpers assume this is the case. * * Also note that this callback can be called no matter the - * state @dev is in. Drivers that need that device to be powered - * to perform this operation will first need to make sure it's - * been properly enabled. + * state @dev is in and also no matter what state the panel is + * in. It's expected: + * - If the @dev providing the AUX bus is currently unpowered then + * it will power itself up for the transfer. + * - If we're on eDP (using a drm_panel) and the panel is not in a + * state where it can respond (it's not powered or it's in a + * low power state) then this function may return an error, but + * not crash. It's up to the caller of this code to make sure that + * the panel is powered on if getting an error back is not OK. If a + * drm_panel driver is initiating a DP AUX transfer it may power + * itself up however it wants. All other code should ensure that + * the pre_enable() bridge chain (which eventually calls the + * drm_panel prepare function) has powered the panel. */ ssize_t (*transfer)(struct drm_dp_aux *aux, struct drm_dp_aux_msg *msg);