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

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Date:   Thu, 22 Feb 2018 08:34:13 +0100
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Farhan Ali <alifm@...ux.vnet.ibm.com>,
        Cornelia Huck <cohuck@...hat.com>
Cc:     Thomas Huth <thuth@...hat.com>, linux-kernel@...r.kernel.org,
        linux-s390@...r.kernel.org, linux-fbdev@...r.kernel.org,
        geert@...ux-m68k.org, tomi.valkeinen@...com,
        b.zolnierkie@...sung.com
Subject: Re: [PATCH v3 1/3] Kconfig : Remove HAS_IOMEM dependency for Graphics
 support



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