linux-kernel - Re: [linux-sunxi] Re: [PATCH v6 08/13] drm/sun4i: add support for Allwinner DE2 mixers

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Date:   Fri, 5 May 2017 11:40:17 +0800
From:   Chen-Yu Tsai <wens@...e.org>
To:     Icenowy Zheng <icenowy@...c.io>
Cc:     Maxime Ripard <maxime.ripard@...e-electrons.com>,
        Rob Herring <robh+dt@...nel.org>, Chen-Yu Tsai <wens@...e.org>,
        linux-clk <linux-clk@...r.kernel.org>,
        devicetree <devicetree@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        linux-sunxi <linux-sunxi@...glegroups.com>
Subject: Re: [linux-sunxi] Re: [PATCH v6 08/13] drm/sun4i: add support for
 Allwinner DE2 mixers

On Fri, May 5, 2017 at 12:52 AM,  <icenowy@...c.io> wrote:
> 在 2017-05-04 21:05，Maxime Ripard 写道：
>>
>> On Thu, May 04, 2017 at 07:48:53PM +0800, Icenowy Zheng wrote:
>>>
>>> Allwinner have a new "Display Engine 2.0" in their new SoCs, which comes
>>> with mixers to do graphic processing and feed data to TCON, like the old
>>> backends and frontends.
>>>
>>> Add support for the mixer on Allwinner V3s SoC; it's the simplest one.
>>>
>>> Currently a lot of functions are still missing -- more investigations
>>> are needed to gain enough information for them.
>>>
>>> Signed-off-by: Icenowy Zheng <icenowy@...c.io>
>>> ---
>>> Changes in v6:
>>> - Rebased on wens's multi-pipeline patchset.
>>> Changes in v5:
>>> - Changed some code alignment.
>>> - Request real 32-bit DMA (prepare for 64-bit SoCs).
>>> Changes in v4:
>>> - Killed some dead code according to Jernej.
>>>
>>>  drivers/gpu/drm/sun4i/Kconfig       |  10 +
>>>  drivers/gpu/drm/sun4i/Makefile      |   3 +
>>>  drivers/gpu/drm/sun4i/sun8i_layer.c | 140 +++++++++++++
>>>  drivers/gpu/drm/sun4i/sun8i_layer.h |  36 ++++
>>>  drivers/gpu/drm/sun4i/sun8i_mixer.c | 394
>>> ++++++++++++++++++++++++++++++++++++
>>>  drivers/gpu/drm/sun4i/sun8i_mixer.h | 137 +++++++++++++
>>>  6 files changed, 720 insertions(+)
>>>  create mode 100644 drivers/gpu/drm/sun4i/sun8i_layer.c
>>>  create mode 100644 drivers/gpu/drm/sun4i/sun8i_layer.h
>>>  create mode 100644 drivers/gpu/drm/sun4i/sun8i_mixer.c
>>>  create mode 100644 drivers/gpu/drm/sun4i/sun8i_mixer.h
>>>

[...]

>>> diff --git a/drivers/gpu/drm/sun4i/sun8i_mixer.c
>>> b/drivers/gpu/drm/sun4i/sun8i_mixer.c
>>> new file mode 100644
>>> index 000000000000..e216b84d5bb2
>>> --- /dev/null
>>> +++ b/drivers/gpu/drm/sun4i/sun8i_mixer.c
>>> @@ -0,0 +1,394 @@

[...]

>>> +       list_add_tail(&mixer->engine.list, &drv->engine_list);
>>
>>
>> You didn't call INIT_LIST_HEAD on that list.
>
>
> So didn't the sun4i_backend driver...
>
> I think the mixer->engine.list only means an item in the
> engine_list, and the drv->engine_list is initialized in the
> sun4i_drv source code.

I read [1] that if the item is subsequently added to a list,
you could omit the INIT_LIST_HEAD call.

Makes sense, though you have to be sure you aren't doing anything
else with the list element.

ChenYu

[1] https://isis.poly.edu/kulesh/stuff/src/klist/