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Message-ID: <2434f050-b82c-03e6-ee8f-8c8799119815@infradead.org>
Date:   Tue, 8 Feb 2022 12:45:38 -0800
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Subject: Re: Kconfig CONFIG_FB dependency regression
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To:     Thinh Nguyen <Thinh.Nguyen@synopsys.com>,
        Arnd Bergmann <arnd@kernel.org>
Cc:     Fabio Estevam <festevam@gmail.com>,
        Kees Cook <keescook@chromium.org>,
        Daniel Vetter <daniel.vetter@ffwll.ch>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "dri-devel@lists.freedesktop.org" <dri-devel@lists.freedesktop.org>,
        John Youn <John.Youn@synopsys.com>,
        Bing Yuan <Bing.Yuan@synopsys.com>,
        "linux-mediatek@lists.infradead.org" 
        <linux-mediatek@lists.infradead.org>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>
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From:   Randy Dunlap <rdunlap@infradead.org>
In-Reply-To: <992f01cc-eb0c-b503-f9b4-4a037c6cf67a@synopsys.com>
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Hi--

On 2/8/22 12:10, Thinh Nguyen wrote:
> Randy Dunlap wrote:
>>
>>
>> On 2/3/22 19:21, Thinh Nguyen wrote:
>>> Arnd Bergmann wrote:
>>>> On Thu, Feb 3, 2022 at 12:55 AM Thinh Nguyen <Thinh.Nguyen@synopsys.com> wrote:
>>>>> Arnd Bergmann wrote:
>>>>>> On Wed, Feb 2, 2022 at 1:14 AM Thinh Nguyen <Thinh.Nguyen@synopsys.com> wrote:
>>>>>>> Fabio Estevam wrote:
>>>>>>
>>>>>> CONFIG_FB should not normally be needed for booting, so unless
>>>>>> you have a graphical application in your initramfs that requires the /dev/fb0
>>>>>> device to work, it is not supposed to make a difference.
>>>>>>
>>>>>
>>>>> I'm not sure, but it seems like the setup we have isn't the only one
>>>>> that needed it. Fabio also noted that the imx_v6_v7_defconfig also needs
>>>>> to have CONFIG_FB set.
>>>>
>>>> No, that one is different: the change for imx_v6_v7_defconfig was
>>>> done because they actually use a framebuffer console on some devices,
>>>> so the patch just adds the symbol to enable the drivers they are using.
>>>>
>>>> This is expected with my original patch that doesn't implicitly enable
>>>> the framebuffer layer any more. What is not expected is for the kernel
>>>> to hang during boot as you reported for your unidentified platform.
>>>>
>>>>>> Are there any other differences in your .config before and after the patch?
>>>>>> It's possible that you use some other driver that in turn depends on
>>>>>> CONFIG_FB. Does your machine have any graphical output device?
>>>>>> If yes, which driver do you use?
>>>>>
>>>>> I don't have the answer to those questions yet. Need more investigation.
>>>>> I'm new to this particular test setup.
>>>>
>>>> Do you mean you don't know if there is a screen attached to the system?
>>>>
>>>
>>> It does have a graphical output device, but I didn't check what it is or
>>> what driver is driving it. I just notice that after the reported commit,
>>> something stopped working.
>>>
>>>>>>
>>>>>> You may also want to make sure that you have 9d6366e743f3 ("drm:
>>>>>> fb_helper: improve CONFIG_FB dependency") in your kernel, which
>>>>>> fixes a minor problem with my original patch.
>>>>>>
>>>>>
>>>>> The issue also occurs in mainline, which has your minor fix commit
>>>>> above. The revert isn't clean for the latest kernel version. I also have
>>>>> to revert some of the changes along with CONFIG_FB. The revert looks
>>>>> more like this for the latest kernel:
>>>>>
>>>>> diff --git a/drivers/gpu/drm/Kconfig b/drivers/gpu/drm/Kconfig
>>>>> index b1f22e457fd0..7cbc733a8569 100644
>>>>> --- a/drivers/gpu/drm/Kconfig
>>>>> +++ b/drivers/gpu/drm/Kconfig
>>>>> @@ -118,8 +118,9 @@ config DRM_DEBUG_MODESET_LOCK
>>>>>
>>>>>  config DRM_FBDEV_EMULATION
>>>>>         bool "Enable legacy fbdev support for your modesetting driver"
>>>>> -       depends on DRM_KMS_HELPER
>>>>> -       depends on FB=y || FB=DRM_KMS_HELPER
>>>>> +       depends on DRM
>>>>> +       select DRM_KMS_HELPER
>>>>> +       select FB
>>>>>         select FB_CFB_FILLRECT
>>>>>         select FB_CFB_COPYAREA
>>>>>         select FB_CFB_IMAGEBLIT
>>>>>
>>>>>
>>>>>
>>>>> I attached the configs for kernel v5.17-rc1. The "bad" config is without
>>>>> any revert, the "good" config is with the change above.
>>>>
>>>> Looking at the config, I see that this is for an x86 machine,
>>>> and you have the FB_EFI driver and EFI_EARLYCON enabled.
>>>>
>>>> What I suspec is going on is that you are looking at a screen rather
>>>> than a serial console, and the kernel doesn't actually hang but you
>>>> just don't see any more messages after the DRM driver takes
>>>> over from EFI_EARLYCON because there is no console driver.
>>>>
>>>> In this case, what you see is the intended behavior, not a bug.
>>>> If you want a graphical console in your system, you need to
>>>> enable the support for this in your config.
>>>>
>>>
>>> It sounds like that's the case. Unfortunately I'm not familiar with this
>>> subsystem to say that's what happening. If there's nothing actually
>>> broken from review, we can ignore this email thread.
>>
>> Hi,
>> I don't know of anything that is broken...
>>
>> I am curious how CONFIG_FB_EFI came to be set when going from bad.config to
>> good.config.  Can you explain that?
>>
> 
> I just use the change above and "make" with olddefconfig option. Is it
> not expected?

Maybe I am not doing the same as you. If I take your previous bad.config
with kernel v5.17-rc2 and use your Kconfig patch:

diff --git a/drivers/gpu/drm/Kconfig b/drivers/gpu/drm/Kconfig
index b1f22e457fd0..7cbc733a8569 100644
--- a/drivers/gpu/drm/Kconfig
+++ b/drivers/gpu/drm/Kconfig
@@ -118,8 +118,9 @@ config DRM_DEBUG_MODESET_LOCK

 config DRM_FBDEV_EMULATION
        bool "Enable legacy fbdev support for your modesetting driver"
-       depends on DRM_KMS_HELPER
-       depends on FB=y || FB=DRM_KMS_HELPER
+       depends on DRM
+       select DRM_KMS_HELPER
+       select FB
        select FB_CFB_FILLRECT
        select FB_CFB_COPYAREA
        select FB_CFB_IMAGEBLIT

and then run
'make olddefconfig', I see:

# CONFIG_FB_EFI is not set

which is what I would expect to see.


thanks.
-- 
~Randy