[PATCH v4 04/11] drm/bridge: Make the bridge chain a double-linked list

[Boris Brezillon]

On Tue, 24 Dec 2019 10:16:49 +0100
Andrzej Hajda <a.hajda at samsung.com> wrote:

> On 23.12.2019 10:55, Marek Szyprowski wrote:
> > Hi Boris,
> >
> > On 16.12.2019 16:25, Boris Brezillon wrote:  
> >> On Mon, 16 Dec 2019 16:02:36 +0100
> >> Marek Szyprowski <m.szyprowski at samsung.com> wrote:  
> >>> Hi Boris,
> >>>
> >>> On 16.12.2019 15:55, Boris Brezillon wrote:  
> >>>> On Mon, 16 Dec 2019 14:54:25 +0100
> >>>> Marek Szyprowski <m.szyprowski at samsung.com> wrote:  
> >>>>> On 03.12.2019 15:15, Boris Brezillon wrote:  
> >>>>>> So that each element in the chain can easily access its predecessor.
> >>>>>> This will be needed to support bus format negotiation between elements
> >>>>>> of the bridge chain.
> >>>>>>
> >>>>>> Signed-off-by: Boris Brezillon <boris.brezillon at collabora.com>
> >>>>>> Reviewed-by: Neil Armstrong <narmstrong at baylibre.com>
> >>>>>> Reviewed-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>  
> >>>>> I've noticed that this patch got merged to linux-next as commit
> >>>>> 05193dc38197021894b17239fafbd2eb1afe5a45. Sadly it breaks booting of
> >>>>> Samsung Exynos5250-based Arndale board. Booting stops after following
> >>>>> messages:
> >>>>>
> >>>>> [drm] Exynos DRM: using 14400000.fimd device for DMA mapping operations
> >>>>> exynos-drm exynos-drm: bound 14400000.fimd (ops fimd_component_ops)
> >>>>> exynos-drm exynos-drm: bound 14450000.mixer (ops mixer_component_ops)
> >>>>> exynos-drm exynos-drm: bound 14500000.dsi (ops exynos_dsi_component_ops)
> >>>>> exynos-drm exynos-drm: bound 14530000.hdmi (ops hdmi_component_ops)
> >>>>> [drm] Supports vblank timestamp caching Rev 2 (21.10.2013).
> >>>>> [drm] No driver support for vblank timestamp query.
> >>>>> [drm] Cannot find any crtc or sizes
> >>>>> [drm] Cannot find any crtc or sizes
> >>>>> [drm] Initialized exynos 1.1.0 20180330 for exynos-drm on minor 0
> >>>>>
> >>>>> I will try to debug this and provide more information soon.
> >>>>>     
> >>>> Can you try with this diff applied?  
> >>> This patch doesn't change anything.  
> >> Okay. Can you do a list_for_each_entry() on both encoder->bridge_chain
> >> and dsi->bridge_chain (dump bridge pointers in a pr_info()) before and
> >> after the list_splice_init() call?  
> > encoder->bridge_chain contains only one element. dsi->drive_chain is empty.
> >
> > Replacing that list_splice() with INIT_LIST_HEAD(&encoder->bridge_chain) 
> > fixed the boot issue.

If INIT_LIST_HEAD() worked, I don't understand why replacing the
list_splice() call by a list_splice_init() (which doing a list_splice()
+ INIT_LIST_HEAD()) didn't fix the problem. Are you sure the
list_splice_init() version doesn't work?

> > It looks that this is related with the way the 
> > Exynos DSI handles bridges (in bridge and out brige?). Maybe Andrzej 
> > will give a bit more detailed comment and spread some light on this.  
> 
> 
> Hi Marek, Boris,
> 
> 
> I have not followed latest patches due to high work load, my bad. Marek
> thanks from pointing
> 
> About ExynosDSI bridge handling:
> 
> The order of calling encoder, bridge (and consequently panel) ops
> enforced by DRM core (bridge->pre_enable, encoder->enable,
> bridge->enable) does not fit to ExynosDSI hardware initialization
> sequence, if I remember correctly it does not fit to whole MIPI DSI
> standard (I think similar situation is with eDP). As a result DSI
> drivers must use some ugly workarounds, rely on HW properly coping with
> incorrect sequences, or, as in case of ExynosDSI driver, just avoid
> using encoder->bridge chaining and call bridge ops by itself when suitable.

Yes, that's definitely hack-ish, and I proposed 2 solutions to address
that in previous versions of this patchset, unfortunately I didn't get
any feedback so I went for the less invasive option (keep the hack but
adapt it to the double-linked list changes), which still lead to
regressions :-/.

Just a reminder of my 2 proposals:

1/ implement the bridge_ops->pre_enable/post_disable() hooks so you can
   split your enable/disable logic in 2 parts and make sure things are
   ready when the panel/next bridge tries to send DSI commands
2/ move everything that's needed to send DSI commands out of the
   ->enable() path (maybe in runtime PM resume/suspend hooks) so you
   can call that in the DSI transfer path too

As pointed out by Laurent, #1 doesn't work because some panel drivers
send DSI commands in their ->prepare() hook, and ->pre_enable() methods
are called in reverse order, meaning that the DRM panel bridge driver
would try to issue DSI commands before the DSI host controllers is ready
to send them. I still thing #2 is a good option.

> 
> So proper patch converting to double-linked list should not try to
> splice ExynosDSI private bridge list with with encoder's, encoder's list
> should be always empty, as Marek suggested.

That's exactly what I wanted to do: make the encoder's list empty after
attach() and restore it to its initial state before unregistering
the bridge, except I forgot that list_splice() doesn't call
INIT_LIST_HEAD(). It's still not clear to me why replacing the
list_splice() call by a list_splice_init() didn't work.
Also note that calling INIT_LIST_HEAD() only works if you have one
bridge in the chain, so if we go for that option we need a comment
explaining the limitations of this approach.