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 10:44:22 +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.