Douglas Anderson 69ef4a192b drm: Document the power requirements for DP AUX transfers ... 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 bacbab58f0 ("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.Ia8651894026707e4fa61267da944ff739610d180@changeid Signed-off-by: Douglas Anderson <dianders@chromium.org> Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org> Reviewed-by: Lyude Paul <lyude@redhat.com> Link: https://patchwork.freedesktop.org/patch/msgid/20220509161733.v2.1.Ia8651894026707e4fa61267da944ff739610d180@changeid		2022-05-19 17:22:27 -07:00
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acpi	ACPI: bus: Eliminate acpi_bus_get_device()	2022-04-05 19:49:26 +02:00
asm-generic	bug: Have __warn() prototype defined unconditionally	2022-04-26 10:59:57 +02:00
clocksource
crypto	Merge branch 'linus' of git://git.kernel.org/pub/scm/linux/kernel/git/herbert/crypto-2.6	2022-03-21 16:02:36 -07:00
drm	drm: Document the power requirements for DP AUX transfers	2022-05-19 17:22:27 -07:00
dt-bindings	drm/rockchip: Add VOP2 driver	2022-05-04 14:05:47 +02:00
keys
kunit
kvm
linux	fbdev: Restart conflicting fb removal loop when unregistering devices	2022-05-13 13:48:28 +02:00
math-emu
media
memory	memory: renesas-rpc-if: Fix HF/OSPI data transfer in Manual Mode	2022-04-21 17:00:24 +02:00
misc
net	bluetooth pull request for net:	2022-04-28 10:18:51 +02:00
pcmcia
ras	mm/memory-failure.c: fix race with changing page compound again	2022-03-22 15:57:07 -07:00
rdma
scsi	scsi: iscsi: Fix NOP handling during conn recovery	2022-04-11 22:09:35 -04:00
soc	drm for 5.18-rc1	2022-03-24 16:19:43 -07:00
sound	ASoC: Fixes for v5.18	2022-04-19 17:26:01 +02:00
target	scsi: target: Add iscsi/cpus_allowed_list in configfs	2022-03-14 23:40:36 -04:00
trace	NFSD bug fixes for 5.18-rc:	2022-04-12 14:23:19 -10:00
uapi	Linux 5.18-rc5	2022-05-03 16:08:48 +10:00
vdso
video
xen	xen/grant-table: remove readonly parameter from functions	2022-03-15 20:34:40 -05:00