linux-kernel - Re: [PATCH] ARM: exynos_defconfig: Disable simplefb support

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Date:	Fri, 11 Sep 2015 16:16:01 +0900
From:	Krzysztof Kozlowski <k.kozlowski@...sung.com>
To:	Javier Martinez Canillas <javier@....samsung.com>,
	linux-kernel@...r.kernel.org
Cc:	Michael Turquette <mturquette@...libre.com>,
	Kukjin Kim <kgene@...nel.org>,
	Thierry Reding <treding@...dia.com>,
	Anand Moon <linux.amoon@...il.com>,
	Russell King <linux@....linux.org.uk>,
	linux-samsung-soc@...r.kernel.org,
	Lukasz Majewski <l.majewski@...sung.com>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] ARM: exynos_defconfig: Disable simplefb support

On 11.09.2015 16:07, Javier Martinez Canillas wrote:
> Hello Krzysztof,
> 
> On 09/11/2015 07:01 AM, Krzysztof Kozlowski wrote:
>> On 10.09.2015 22:42, Javier Martinez Canillas wrote:
>>> The simplefb driver allows the kernel to render on a pre-allocated
>>> buffer that's been initialized by firmware before the kernel boots.
>>>
>>> This option was enabled to have display working on the Exynos5250
>>> Snow Chromebook by commit da9d0fbf5e9a ("ARM: exynos: defconfig
>>> update") since proper DRM/KMS support did not exist at that time.
>>>
>>> But now that the Exynos DRM driver has support for this hardware,
>>> there is no need to have simplefb enabled. In fact, if a user has
>>> a u-boot that injects the simplefb dev node to the FDT before pass
>>> it to the kernel, display won't be properly initialized and only a
>>> blank screen will be shown since there isn't a proper handoff from
>>> the simplefb driver to the Exynos DRM driver.
>>>
>>> Signed-off-by: Javier Martinez Canillas <javier@....samsung.com>
>>>
>>> ---
>>>
>>>  arch/arm/configs/exynos_defconfig | 1 -
>>>  1 file changed, 1 deletion(-)
>>
>> Seems logical. None of the boards use simple-framebuffer compatible
>> anyway. I understand that on Snow simplefb was needed along with change
>> in Uboot like this one:
>> https://chromium.googlesource.com/chromiumos/third_party/u-boot/+/refs/changes/58/49358/2
>>
> 
> Exactly but you won't see the dev node with the "simple-framebuffer"
> compatible string in the DTS since is the bootloader that adds this
> device node to the FDT before passing it to the kernel.
> 
> The bootloader shouldn't mangle the FDT (with the exception of the
> memory and choosen/bootargs nodes) but simplefb is just a hack to
> re-use the display HW initialization made by the bootloader.
> 
>> and now none of Exynos boards use simplefb anymore?
>>
> 
> Yes, there are no other Exynos boards using simplefb besides Snow
> that I'm aware of but since Exynos DRM is working well on this board
> from v4.0, there is no need for it anymore.

OK,
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@...sung.com>

> 
> In fact, as explained in the commit message, it could do more harm
> than good since users that are still booting with a u-boot that adds
> the simplefb device node, only get a blank screen since the simplefb
> driver is probed, creates a console and later the Exynos DRM probes
> and re-initializes the HW creating its own console, causing this issue.
> 
> I got several reports of users that says that mainline stop booting for
> them but is just that they didn't get display working. Disabling simplefb
> makes display to work again so maybe this is even -rc material and should
> go to stable # v4.0+

You know, it is only a defconfig. The issue is there regardless of
change in defconfig. I am not convinced that defconfig problems are
worth backporting. multi_v7 has it enabled anyway.

Maybe the EXYNOS_DRM should have some anti-dependency on SIMPLE_FB? But
on the other hand the issue is actually caused by hacks in bootloader...


Best regards,
Krzysztof


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