linux-kernel - Re: [PATCH v4 2/2] fbcon: Defer console takeover for splash screens to first switch

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Message-ID: <46329386-c779-4e9c-80b0-c0f01e51d0a5@amd.com>
Date: Thu, 22 Feb 2024 10:25:27 -0600
From: Mario Limonciello <mario.limonciello@....com>
To: Maxime Ripard <mripard@...nel.org>,
 Daniel van Vugt <daniel.van.vugt@...onical.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Helge Deller <deller@....de>, Daniel Vetter <daniel@...ll.ch>,
 Thomas Hellström <thomas.hellstrom@...ux.intel.com>,
 Jani Nikula <jani.nikula@...el.com>,
 "linux-fbdev@...r.kernel.org" <linux-fbdev@...r.kernel.org>,
 "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 2/2] fbcon: Defer console takeover for splash screens
 to first switch

On 2/22/2024 05:08, Maxime Ripard wrote:
> Hi Daniel,
> 
> On Mon, Feb 19, 2024 at 05:02:34PM +0800, Daniel van Vugt wrote:
>> Until now, deferred console takeover only meant defer until there is
>> output. But that risks stepping on the toes of userspace splash screens
>> as console messages may appear before the splash screen.
>>
>> This becomes more likely the later the splash screen starts, but even
>> systems whose splash exists in initrd may not be not immune because they
>> still rely on racing against all possible kernel messages that might
>> trigger the fbcon takeover. And those kernel messages are hardware
>> dependent so what boots silently on one machine may not be so quiet on
>> the next. We also want to shield users from seeing warnings about their
>> hardware/firmware that they don't always have the power to fix themselves,
>> and may not be deemed worthy of fixing by the vendor.
>>
>> So now we check the command line for the expectation of userspace splash
>> (CONFIG_FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER_CONDITION) and if present
>> then defer fbcon's takeover until the first console switch. In the case
>> of Plymouth, its value would typically be "splash". This keeps the boot
>> experience clean and silent so long as the command line requests so.
>>
>> Closes: https://bugs.launchpad.net/bugs/1970069
>> Cc: Mario Limonciello <mario.limonciello@....com>
>> Signed-off-by: Daniel van Vugt <daniel.van.vugt@...onical.com>

I did test this series on an Ubuntu userspace and it works as you 
suggest it should.

Tested-by: Mario Limonciello <mario.limonciello@....com>

> 
> It's not clear to me why we should want to make it an option? If one
> strategy is better than the other, and I guess the new one is if you
> consider it fixes a bug and bothered to submit it upstream, why not just
> get rid of the old one entirely?
> 
> I guess my question is: why do we want the choice, and what are the
> tradeoff each strategy brings?
> 
> Maxime

The reason for choice is that it keys off a kernel command line 
parameter that is inconsistent across distributions.

For example Ubuntu uses "splash", Fedora used "rhgb" etc.

Even the plymouth userspace maintains a list for it's behaviors of what 
parameters to look for to start at bootup.  So the obvious alternative 
is to clone that list in the kernel.