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

[Boris Brezillon <boris.brezillon@collabora.com>]

On Tue, 24 Dec 2019 10:49:36 +0100
Boris Brezillon <boris.brezillon@collabora.com> wrote:

> On Tue, 24 Dec 2019 10:44:22 +0100
> Boris Brezillon <boris.brezillon@collabora.com> wrote:
> 
> > On Tue, 24 Dec 2019 10:16:49 +0100
> > Andrzej Hajda <a.hajda@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@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@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@collabora.com>
> > > >>>>>> Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
> > > >>>>>> Reviewed-by: Laurent Pinchart <laurent.pinchart@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.  
> 
> Okay, I think I figured it out: drm_bridge_chain_xx() helpers use
> encoder->bridge_chain as their list head, and you'll never hit the 'elem
> is list head' condition since we moved all elems from
> encoder->bridge_chain to exynos_dsi->bridge_chain. The only way this
> can work is if we stop using the helpers and implement our own list
> iterators.

Just to make it clear, calling INIT_LIST_HEAD(encoder->bridge_chain)
doesn't really fix the bug, it just prevents the hang (infinite loop)
and turn all drm_bridge_chain_xx() calls into NOPs.