linux-kernel - Re: [PATCH v2] drm/mediatek: fix uninitialized symbol

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Message-ID: <CAC=S1njEG2sV28XAWDw8FoQ0p5w=-sU4i+D-ESfapHMQC1ZW3Q@mail.gmail.com>
Date:   Mon, 17 Jul 2023 11:59:38 +0800
From:   Fei Shao <fshao@...omium.org>
To:     CK Hu (胡俊光) <ck.hu@...iatek.com>
Cc:     "p.zabel@...gutronix.de" <p.zabel@...gutronix.de>,
        Nancy Lin (林欣螢) <Nancy.Lin@...iatek.com>,
        "matthias.bgg@...il.com" <matthias.bgg@...il.com>,
        "chunkuang.hu@...nel.org" <chunkuang.hu@...nel.org>,
        "angelogioacchino.delregno@...labora.com" 
        <angelogioacchino.delregno@...labora.com>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mediatek@...ts.infradead.org" 
        <linux-mediatek@...ts.infradead.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "clang-built-linux@...glegroups.com" 
        <clang-built-linux@...glegroups.com>,
        Singo Chang (張興國) 
        <Singo.Chang@...iatek.com>,
        Project_Global_Chrome_Upstream_Group 
        <Project_Global_Chrome_Upstream_Group@...iatek.com>
Subject: Re: [PATCH v2] drm/mediatek: fix uninitialized symbol

Hi CK,

On Fri, Jul 14, 2023 at 5:27 PM CK Hu (胡俊光) <ck.hu@...iatek.com> wrote:
>
> Hi, Nancy:
>
> On Fri, 2023-04-21 at 10:16 +0800, Nancy.Lin wrote:
snip
>
> In line 566, the statement is
>
> if (nodo) {
>         comp_pdev = ...
> }
>
> The comment says that only ovl_adaptoer has no device node, so the
> checking should be
>
> if (type != MTK_DISP_OVL_ADAPTOR) {
>         comp_pdev = ...
> }
>
> and later it would return when type = MTK_DISP_OVL_ADAPTOR,
> so there would be no problem of uninitialized symbol.

That sounds fair, but IIUC what Nancy tries to resolve here is the
false-positive Smatch warning.
How about this: given the `if (node)` block was exclusively added for
ovl_adaptor in [1], plus the init function will immediately return
after that in this case, it should be safe to do the following

```
/* Not all drm components have a DTS device node... */
if (node == NULL)
    return 0;

comp_pdev = of_find_device_by_node(node);
...

if (type == MTK_DISP_AAL ||
...
```

which is equivalent to adding a `node == NULL` check before [1].
This should suppress the Smatch warning because `comp_pdev` will be
(again) unconditionally assigned to something, and the `type ==
MTK_DISP_OVL_ADAPTOR` line can be dropped also (optional?).

[1]: commit 0d9eee9118b7 ("drm/mediatek: Add drm ovl_adaptor sub
driver for MT8195")

Regards,
Fei