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

Fri Dec 27 10:25:05 UTC 2019

Hi Boris,

On 24.12.2019 11:03, Boris Brezillon wrote:
> On Tue, 24 Dec 2019 10:49:36 +0100
> Boris Brezillon <boris.brezillon at collabora.com> wrote:
>> On Tue, 24 Dec 2019 10:44:22 +0100
>> Boris Brezillon <boris.brezillon at collabora.com> wrote:
>>> 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:
>>>>> 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:
>>>>>>> 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.
>> 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.

Right, I've just checked it and indeed the display chain outputs nothing 
after such 'fix'. To get it finally working I've replaced 
drm_bridge_chain_*() operations for exynos_dsi 'out_bridge' by a direct 
calls. I will submit a patch in a few minutes. I hope that such 
workaround can be used now to fix the regression until a better solution 
is agreed.

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland