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Tue, 13 Feb 2024 07:01:24 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=canonical.com; s=20210705; t=1707807688; bh=Iel6Dzu6unAVG4BrbtyD/lNPC5LjCFHUF/MgsE3DfU8=; h=Message-ID:Date:MIME-Version:Subject:To:References:Cc:From: In-Reply-To:Content-Type; b=e+8ER6X99Qs2nu0CoHgeCEQbd+ebPdDmwIgr04l/DtiV0n6Ncmd2D12izkxRqnyck asM0s8ZS2mi+/A9xto7kZp0kXtgeqxgJ2h06WJGJKrQreviqk2ZqwzQEdOclPBEFVV W2Us+MgoX68TIGIToJ4cYi7Xx4vtu+J/lbx4uf8MphLAcz8qUcCYkVeo/tzTqRfjMq fV3bqbWa8JuU64rB/NfJEhGKLPE7FL+DQoOKs7mL3kJ4d2qqv9iCaPxTnobKbGrTi4 ES5cy9Cx4btxy8i0IWzLx11xtzzthdO4poM7cuIdUxSTZNo7Y4hFnfo5H5XU4iRJ/k 8VTOh88mNYVjQ== Message-ID: Date: Tue, 13 Feb 2024 15:01:20 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 2/2] fbcon: Defer console takeover for splash screens to first switch Content-Language: en-US To: Daniel Vetter References: <20240206101100.25536-1-daniel.van.vugt@canonical.com> <20240206101100.25536-2-daniel.van.vugt@canonical.com> <26893900-2d0d-4624-94f1-c4aa88386e9c@canonical.com> <88f3ff47-8c4b-424c-bf83-1570882cbb54@amd.com> <2a153cb6-4ece-4a37-9711-abdf4ba0ad50@canonical.com> Cc: Mario Limonciello , andy.whitcroft@canonical.com, Greg Kroah-Hartman , Helge Deller , Jani Nikula , Danilo Krummrich , linux-fbdev@vger.kernel.org, dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org From: Daniel van Vugt In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 9/2/24 18:58, Daniel Vetter wrote: > On Thu, Feb 08, 2024 at 09:16:50AM +0800, Daniel van Vugt wrote: >> On 8/2/24 04:21, Mario Limonciello wrote: >>> On 2/7/2024 03:51, Daniel Vetter wrote: >>>> On Wed, Feb 07, 2024 at 10:03:10AM +0800, Daniel van Vugt wrote: >>>>> On 6/2/24 23:41, Mario Limonciello wrote: >>>>>> On 2/6/2024 08:21, Daniel Vetter wrote: >>>>>>> On Tue, Feb 06, 2024 at 06:10:51PM +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. So check the >>>>>>>> command line for the expectation of userspace splash and if present then >>>>>>>> extend the deferral until after the first switch. >>>>>>>> >>>>>>>> V2: Added Kconfig option instead of hard coding "splash". >>>>>>>> >>>>>>>> Closes: https://bugs.launchpad.net/bugs/1970069 >>>>>>>> Cc: Mario Limonciello >>>>>>>> Signed-off-by: Daniel van Vugt >>>>>>>> --- >>>>>>>>    drivers/video/console/Kconfig    | 13 +++++++++++ >>>>>>>>    drivers/video/fbdev/core/fbcon.c | 38 ++++++++++++++++++++++++++++++-- >>>>>>>>    2 files changed, 49 insertions(+), 2 deletions(-) >>>>>>>> >>>>>>>> diff --git a/drivers/video/console/Kconfig b/drivers/video/console/Kconfig >>>>>>>> index bc31db6ef7..a6e371bfb4 100644 >>>>>>>> --- a/drivers/video/console/Kconfig >>>>>>>> +++ b/drivers/video/console/Kconfig >>>>>>>> @@ -138,6 +138,19 @@ config FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER >>>>>>>>          by the firmware in place, rather then replacing the contents with a >>>>>>>>          black screen as soon as fbcon loads. >>>>>>>>    +config FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER_CONDITION >>>>>>>> +    string "Framebuffer Console Deferred Takeover Condition" >>>>>>>> +    depends on FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER >>>>>>>> +    default "splash" >>>>>>>> +    help >>>>>>>> +      If enabled this defers further the framebuffer console taking over >>>>>>>> +      until the first console switch has occurred. And even then only if >>>>>>>> +      text has been output, and only if the specified parameter is found >>>>>>>> +      on the command line. This ensures fbcon does not interrupt userspace >>>>>>>> +      splash screens such as Plymouth which may be yet to start rendering >>>>>>>> +      at the time of the first console output. "splash" is the simplest >>>>>>>> +      distro-agnostic condition for this that Plymouth checks for. >>>>>>> >>>>>>> Hm this seems a bit strange since a lot of complexity that no one needs, >>>>>>> also my impression is that it's rather distro specific how you want this >>>>>>> to work. So why not just a Kconfig option that lets you choose how much >>>>>>> you want to delay fbcon setup, with the following options: >>>>>>> >>>>>>> - no delay at all >>>>>>> - dely until first output from the console (which then works for distros >>>>>>>     which set a log-level to suppress unwanted stuff) >>>>>>> - delay until first vt-switch. In that case I don't think we also need to >>>>>>>     delay for first output, since vt switch usually means you'll get first >>>>>>>     output right away (if it's a vt terminal) or you switch to a different >>>>>>>     graphical console (which will keep fbcon fully suppressed on the drm >>>>>>>     side). >>>>>>> >>>>> >>>>> I had similar thoughts, and had prototyped some of this already. But in the end >>>>> it felt like extra complexity there was no demand for. >>>> >>>> For me this one is a bit too complex, since if you enable the vt switch >>>> delay you also get the output delay on top. That seems one too much and I >>>> can't come up with a use-case where you actually want that. So just a >>>> choice of one or the other or none feels cleaner. >> >> Remember the output "delay" goes to zero if any putc has occurred prior to >> registration (see dummycon.c). >> >> My current reason for using both is that in theory it prevents fbcon from >> taking over *earlier* than it did before, in case there was never any output. >> But I don't think there is "never any output" by the time you've tried to >> manually VT switch so maybe that's unnecessary. > > Yeah, but I'm not sure that's like a choice anyone needs, just these > three: > > - no delay > - wait until first output, and set debuglevel appropriately (what fedora > and other distros do) > - wait until first vt switch (what ubuntu wants) Come to think of it, an enum or Kconfig "choice" isn't necessary if I change the default for CONFIG_FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER_CONDITION to blank/disabled. Then the default behaviour is unchanged and you've still got the choice of all three modes: - No delay: fbcon=nodefer - Wait until first output: Already happens when CONFIG_FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER_CONDITION is absent from the command line. - Wait until first VT switch: Would only happen if compiled with CONFIG_FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER_CONDITION and its value is present on the command line. P.S. I suspect Fedora and other distros are not immune to the problem, just make it less likely by starting Plymouth super early on simpledrm. Certainly I am finding some models of laptop emit kernel messages that would still pass under a reduced loglevel. > > I don't ever expect fedora to just enable this, because they have > something that works. Plus many distros are moving away from CONFIG_VT and > all the in-kernel consoles anyway, so they want this even less. > > So if just the delay to first vt-switch is enough for you, I think it's > best we just implement that. The entire vt switch/ownership rules with drm > and fbdev and all that is already really complex and in many cases it's > impossible to tell what's accidental cargo-culted behaviour and what is > actually required. That's why I'd prefer we exactly implement what you > need in this area, nothing more, nothing less. > > And from the testing you discuss below it sounds like you don't need both > delays? > > Cheers, Sima > >>>>> If you would like to specify the preferred Kconfig design then I can implement >>>>> it. Though I don't think there is an enumeration type. It could also be a >>>>> runtime enumeration (deferred_takeover) controlled by fbcon=something. >>>> >>>> There's a choice in Kconfig, see e.g. kernel/Kconfig.hz for an example. >> >> Thanks! >> >>>> >>>>>> IIUC there is an "automatic" VT switch that happens with Ubuntu GRUB + Ubuntu >>>>>> kernels. >>>>>> >>>>>> Why? >>>>>> >>>>>> Couldn't this also be suppressed by just not doing that? >>>>> >>>>> I have not seen any automatic VT switches in debugging but now that you mention >>>>> it I was probably only debugging on drm-misc-next and not an Ubuntu kernel. >>>> >>>> Hm but I don't see how the output delay would paper over a race (if there >>>> is one) reliable for this case? If you do vt switch for boot splash or >>>> login screen and don't yet have drm opened for display or something like >>>> that, then fbcon can sneak in anyway ... >> >> There is no VT switch according to my logs, so there is no race with the >> patchset. The only race that occurs is without this patchset, which is what's >> being fixed here. >> >>> >>> Ubuntu has had in (at least some) kernels: >>> >>> https://git.launchpad.net/~ubuntu-kernel/ubuntu/+source/linux/+git/unstable/commit/?id=320cfac8ef31 >>> >>> I'm unsure if it's still there today, but maybe it would be best if the author >>> (Andy) could enlighten us?  Any idea why that didn't go upstream? >>> >>> I had thought that tied with a automatic VT switch that was trying to hide >>> fbcon as well. >> >> I checked the current Ubuntu 24.04 kernel yesterday and there is no VT switch >> (anymore). The vc_num stays at zero until you do a manual VT switch. This seems >> to be true for both drm-misc-next and Ubuntu kernels. >> >> There is still vt.handoff=7 on the command line for Ubuntu, but I'm not sure it >> has a function anymore. Maybe it was primarily for legacy BIOS? Andy can confirm. >> >>> >>>> >>>> Cheers, Sima >>>>> >>>>> - Daniel >>>>> >>>>>> >>>>>>> I think you could even reuse the existing >>>>>>> CONFIG_FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER for this, and then just >>>>>>> compile-time select which kind of notifier to register (well plus the >>>>>>> check for "splash" on the cmdline for the vt switch one I guess). >>>>>>> >>>>>>> Thoughts? >>>>>>> >>>>>>> Cheers, Sima >>>>>>> >>>>>>> >>>>>>>> + >>>>>>>>    config STI_CONSOLE >>>>>>>>        bool "STI text console" >>>>>>>>        depends on PARISC && HAS_IOMEM >>>>>>>> diff --git a/drivers/video/fbdev/core/fbcon.c >>>>>>>> b/drivers/video/fbdev/core/fbcon.c >>>>>>>> index 63af6ab034..98155d2256 100644 >>>>>>>> --- a/drivers/video/fbdev/core/fbcon.c >>>>>>>> +++ b/drivers/video/fbdev/core/fbcon.c >>>>>>>> @@ -76,6 +76,7 @@ >>>>>>>>    #include /* For counting font checksums */ >>>>>>>>    #include >>>>>>>>    #include >>>>>>>> +#include >>>>>>>>      #include "fbcon.h" >>>>>>>>    #include "fb_internal.h" >>>>>>>> @@ -3358,6 +3359,26 @@ static int fbcon_output_notifier(struct >>>>>>>> notifier_block *nb, >>>>>>>>          return NOTIFY_OK; >>>>>>>>    } >>>>>>>> + >>>>>>>> +#ifdef CONFIG_FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER_CONDITION >>>>>>>> +static int initial_console; >>>>>>>> +static struct notifier_block fbcon_switch_nb; >>>>>>>> + >>>>>>>> +static int fbcon_switch_notifier(struct notifier_block *nb, >>>>>>>> +                 unsigned long action, void *data) >>>>>>>> +{ >>>>>>>> +    struct vc_data *vc = data; >>>>>>>> + >>>>>>>> +    WARN_CONSOLE_UNLOCKED(); >>>>>>>> + >>>>>>>> +    if (vc->vc_num != initial_console) { >>>>>>>> +        dummycon_unregister_switch_notifier(&fbcon_switch_nb); >>>>>>>> +        dummycon_register_output_notifier(&fbcon_output_nb); >>>>>>>> +    } >>>>>>>> + >>>>>>>> +    return NOTIFY_OK; >>>>>>>> +} >>>>>>>> +#endif >>>>>>>>    #endif >>>>>>>>      static void fbcon_start(void) >>>>>>>> @@ -3370,7 +3391,16 @@ static void fbcon_start(void) >>>>>>>>          if (deferred_takeover) { >>>>>>>>            fbcon_output_nb.notifier_call = fbcon_output_notifier; >>>>>>>> -        dummycon_register_output_notifier(&fbcon_output_nb); >>>>>>>> +#ifdef CONFIG_FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER_CONDITION >>>>>>>> +        if (cmdline_find_option_bool(boot_command_line, >>>>>>>> +              CONFIG_FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER_CONDITION)) { >>>>>>>> +            initial_console = fg_console; >>>>>>>> +            fbcon_switch_nb.notifier_call = fbcon_switch_notifier; >>>>>>>> +            dummycon_register_switch_notifier(&fbcon_switch_nb); >>>>>>>> +        } else >>>>>>>> +#endif >>>>>>>> +            dummycon_register_output_notifier(&fbcon_output_nb); >>>>>>>> + >>>>>>>>            return; >>>>>>>>        } >>>>>>>>    #endif >>>>>>>> @@ -3417,8 +3447,12 @@ void __exit fb_console_exit(void) >>>>>>>>    { >>>>>>>>    #ifdef CONFIG_FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER >>>>>>>>        console_lock(); >>>>>>>> -    if (deferred_takeover) >>>>>>>> +    if (deferred_takeover) { >>>>>>>>            dummycon_unregister_output_notifier(&fbcon_output_nb); >>>>>>>> +#ifdef CONFIG_FRAMEBUFFER_CONSOLE_DEFERRED_TAKEOVER_CONDITION >>>>>>>> +        dummycon_unregister_switch_notifier(&fbcon_switch_nb); >>>>>>>> +#endif >>>>>>>> +    } >>>>>>>>        console_unlock(); >>>>>>>>          cancel_work_sync(&fbcon_deferred_takeover_work); >>>>>>>> --  >>>>>>>> 2.43.0 >>>>>>>> >>>>>>> >>>>>> >>>> >>> >