Re: [PATCH v3 1/3] Kconfig : Remove HAS_IOMEM dependency for Graphics support

From: Christian Borntraeger <borntraeger@de.ibm.com>
To: Farhan Ali <alifm@linux.vnet.ibm.com>, Cornelia Huck <cohuck@redhat.com>
Cc: Thomas Huth <thuth@redhat.com>,
	linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org,
	linux-fbdev@vger.kernel.org, geert@linux-m68k.org,
	tomi.valkeinen@ti.com, b.zolnierkie@samsung.com
Subject: Re: [PATCH v3 1/3] Kconfig : Remove HAS_IOMEM dependency for Graphics support
Date: Thu, 22 Feb 2018 08:34:13 +0100	[thread overview]
Message-ID: <2af46f6f-d225-9802-c6f9-98099fb2c2c8@de.ibm.com> (raw)
In-Reply-To: <2170dce6-4456-3ab6-d07c-2169d99c4173@linux.vnet.ibm.com>

On 02/21/2018 05:39 PM, Farhan Ali wrote:
> 
> 
> On 02/21/2018 07:11 AM, Christian Borntraeger wrote:
>>
>>
>> On 02/21/2018 01:07 PM, Cornelia Huck wrote:
>> [...]
>>>>> But if you need to enable PCI to get IOMEM, I wonder why this patch here
>>>>> is needed at all? The Graphics menu / VT dummy console should be
>>>>> available in the config if IOMEM is enabled anyway?
>>>>
>>>> That is a good question. With CONFIG_PCI=y I can select virtio-gpu and dummy-console.
>>>> IIRC the issue was that with patch 3 we can have the situation where we have
>>>> CONFIG_VT = y and CONFIG_DUMMY_CONSOLE=n and this will crash early during boot as
>>>> conswitchp is NULL.
> 
> Yes, VT layer initializes very early in the boot process and looks for a console. If it can't find any it will crash. I believe that was the whole point of having the dummy console.
> 
>>>
>>> So in practice, CONFIG_VT depends on "there's a console available, even
>>> if it's only the dummy one"?
>>
>> Yes. Maybe we should simple move dummy_console outside of drivers/video/console/Kconfig
>> into something that is always available.
>>
> 
> I agree, but where should it go? consoles are kinda tightly tied to video/Graphics Support.
> 
>>>
>>>> This patches goal was to always enable dummy console even without
>>>> PCI, but it obviously fails to do so.
>>>
> This patch should enable the dummy console even without PCI but we won't have DRM/Virtio GPU.

All right. This patch then makes sure that VT does not crash since it guarantees
that DUMMY_CONSOLE is available. If the user want to have virtio-gpu it needs to
enable PCI due to the dependencies, but this is a different issue that we can fix
later.
So the patch is fine as long as you take care of these things

- add missing HAS_IOMEM dependency to VGA_CONSOLE

- extend "if HAS_IOMEM" in drivers/video/Kconfig to cover config items
  previously covered by this dependency (it should start before config
  HAVE_FB_ATMEL and end after config HDMI)

as requested by Bartlomiej 