linux-kernel - Re: [PATCH V11 09/22] LoongArch: Add boot and setup routines

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Date:   Sat, 21 May 2022 15:37:30 +0800
From:   Huacai Chen <chenhuacai@...il.com>
To:     Javier Martinez Canillas <javierm@...hat.com>
Cc:     Ard Biesheuvel <ardb@...nel.org>,
        Huacai Chen <chenhuacai@...ngson.cn>,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Airlie <airlied@...ux.ie>,
        Jonathan Corbet <corbet@....net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Xuefeng Li <lixuefeng@...ngson.cn>,
        Yanteng Si <siyanteng@...ngson.cn>,
        Guo Ren <guoren@...nel.org>, Xuerui Wang <kernel@...0n.name>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        linux-efi <linux-efi@...r.kernel.org>
Subject: Re: [PATCH V11 09/22] LoongArch: Add boot and setup routines

Hi, Javier,

On Sat, May 21, 2022 at 3:07 PM Javier Martinez Canillas
<javierm@...hat.com> wrote:
>
> Hello Huacai,
>
> On 5/21/22 03:40, Huacai Chen wrote:
> > Hi, Javier,
>
> [snip]
>
> >>>> Conversely, if the sysfb_init() is executed first then the platform device
> >>>> will be registered and latter when the driver's init register the driver
> >>>> this will match the already registered device.
> >>> Yes, you are right, my consideration is too complex. The only real
> >>> problem is a harmless error "efifb: a framebuffer is already
> >>> registered" when both efifb and the native display driver are
> >>> built-in.
> >>>
> >>
> >> But this shouldn't be a problem if you drop your register_gop_device() that
> >> registers an "efi-framebuffer", since sysfb would either register a platform
> >> device "simple-framebufer" or "efi-framebuffer", but never both. Those are
> >> mutually exclusive.
> >>
> >> I think what's happening now is that sysfb is registering a "simple-framebuffer"
> >> but your register_gop_device() function is also registering an "efi-framebuffer".
> > No, I have already removed register_gop_device(). Now my problem is like this:
> > 1, efifb (or simpledrm) is built-in;
> > 2, a native display driver (such as radeon) is also built-in.
> >
>
> Ah, I see. The common configuration is for the firmware-provide framebuffer
> drivers ({efi,simple}fb,simpledrm,etc) to be built-in and native drivers to
> be built as a module.
>
> > Because efifb, radeon and sysfb are all in device_initcall() level,
> > the order in practise is like this:
> >
> > efifb registered at first, but no "efi-framebuffer" device yet.
> > radeon registered later, and /dev/fb0 created.
> > sysfb_init() comes at last, it registers "efi-framebuffer" and then
> > causes the error "efifb: a framebuffer is already registered".
>
> Yes, this is problem because only conflicting framebuffers and associated
> devices are unregistered when a real driver is registered, but no devices
> that have not matched with drivers and registered framebuffers or disable
> devices to be registered later.
>
> I proposed the following patch series but the conclusion was that this has
> to be fixed in a more general way:
>
> https://lore.kernel.org/lkml/20220511112438.1251024-1-javierm@redhat.com/
>
> > make sysfb_init() to be subsys_initcall_sync() can avoid this.
> >
>
> Right, now I understand your problem and you are correct that this will
> avoid it. But I believe is just papering over the issue, the problem is
> that if a native fbdev or DRM driver probed, then sysfb (or any other
> platform code) should not register a device to match a driver that will
> attempt to use a firmware-provided framebuffer.
>
> A problem with moving to subsys_initcall_sync() is that this will delay
> more when a display is available in the system, and just to cope up with
> a corner case (as mentioned the common case is native drivers as module).
OK, your method seems better, but I think moving to
subsys_initcall_sync() can make the screen display as early as
possible.

Huacai
>  --
> Best regards,
>
> Javier Martinez Canillas
> Linux Engineering
> Red Hat
>