Re: [RFC 2/6] x86: provide platform-devices for boot-framebuffers

[David Herrmann]

Hi

On Wed, Jun 26, 2013 at 10:49 PM, Stephen Warren <swarren@xxxxxxxxxxxxx> wrote:
> On 06/24/2013 04:27 PM, David Herrmann wrote:
>> The current situation regarding boot-framebuffers (VGA, VESA/VBE, EFI) on
>> x86 causes troubles when loading multiple fbdev drivers. The global
>> "struct screen_info" does not provide any state-tracking about which
>> drivers use the FBs. request_mem_region() theoretically works, but
>> unfortunately vesafb/efifb ignore it due to quirks for broken boards.
>>
>> Avoid this by creating a "platform-framebuffer" device with a pointer
>> to the "struct screen_info" as platform-data. Drivers can now create
>> platform-drivers and the driver-core will refuse multiple drivers being
>> active simultaneously.
>>
>> We keep the screen_info available for backwards-compatibility. Drivers
>> can be converted in follow-up patches.
>>
>> Apart from "platform-framebuffer" devices, this also introduces a
>> compatibility option for "simple-framebuffer" drivers which recently got
>> introduced for OF based systems. If CONFIG_X86_SYSFB is selected, we
>> try to match the screen_info against a simple-framebuffer supported
>> format. If we succeed, we create a "simple-framebuffer" device instead
>> of a platform-framebuffer.
>> This allows to reuse the simplefb.c driver across architectures and also
>> to introduce a SimpleDRM driver. There is no need to have vesafb.c,
>> efifb.c, simplefb.c and more just to have architecture specific quirks
>> in their setup-routines.
>>
>> Instead, we now move the architecture specific quirks into x86-setup and
>> provide a generic simple-framebuffer. For backwards-compatibility (if
>> strange formats are used), we still allow vesafb/efifb to be loaded
>> simultaneously and pick up all remaining devices.
>
>> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
>
>> +config X86_SYSFB
>> +     bool "Mark VGA/VBE/EFI FB as generic system framebuffer"
>> +     help
>> +       Firmwares often provide initial graphics framebuffers so the BIOS,
>> +       bootloader or kernel can show basic video-output during boot for
>> +       user-guidance and debugging. Historically, x86 used the VESA BIOS
>> +       Extensions and EFI-framebuffers for this, which are mostly limited
>> +       to x86. However, a generic system-framebuffer initialization emerged
>> +       recently on some non-x86 architectures.
>
> After this patch has been in the kernel a while, that very last won't
> really be true; simplefb won't have been introduced recently. Perhaps
> just delete that one sentence?

It rather belongs in the commit message, right. I will rephrase that.

>> +       This option, if enabled, marks VGA/VBE/EFI framebuffers as generic
>> +       framebuffers so the new generic system-framebuffer drivers can be
>> +       used on x86.
>> +
>> +       This breaks any x86-only driver like efifb, vesafb, uvesafb, which
>> +       will not work if this is selected.
>
> Doesn't that imply that some form of conflicts or depends ! statement
> should be added here?

There is no real conflict here. You still can use vesafb/... with this
option, but they will not pick up the device. I intend to fix these up
to use "platform-framebuffer" devices instead of globally binding to
"struct screen_info". This way, framebuffers either end up as
simple-framebuffers or platform-framebuffers. This option selects
which device they end up as.

As all non-compatible framebuffers (with incompatible pixel-formats)
always end up as "platform-framebuffer", it still makes sense to use
vesafb as fallback. Hence, I'd not introduce any "conflicts"
dependency here.
Maybe I should rephrase the warning to something that makes clear that
if this option is selected, you need simplefb.c or simpledrm to make
use of these devices.

>> diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
>
>> +obj-y                                        += sysfb.o
>
> I suspect that should be obj-$(CONFIG_X86_SYSFB) += sysfb.o.

No. This patch tries to solve two things: First of all, every
system-framebuffer now gets a "platform-framebuffer" platform-device.
Iff X86_SYSFB is selected, it additionally tries to parse the
framebuffer information as "simple-framebuffer". If it succeeds, it
created a "simple-framebuffer" object, if it doesn't, a fallback
"platform-framebuffer" is provided.

This series is missing vesafb/efifb/.. patches, which should now bind
to "platform-framebuffer" devices instead of using "struct
screen_info" directly. I intend to add these in the next round.

>> diff --git a/arch/x86/kernel/sysfb.c b/arch/x86/kernel/sysfb.c
>
>> +#ifdef CONFIG_X86_SYSFB
>
> Rather than ifdef'ing the body of this file, why not create a dummy
> static inline version of add_sysfb() and put that into a header file
> that users include. There should be a header file to prototype the
> function anyway. That way, you avoid all of the ifdefs and static inline
> functions in this file.
>
>> +static bool parse_mode(const struct screen_info *si,
>> +                    struct simplefb_platform_data *mode)
>
>> +                     strlcpy(mode->format, f->name, sizeof(mode->format));
>
> Per my comments about the type of mode->format, I think that could just be:
>
> mode->format = f->name;
>
> ... since formats[] (i.e. f) isn't initdata.
>
>> +#else /* CONFIG_X86_SYSFB */
>> +
>> +static bool parse_mode(const struct screen_info *si,
>> +                    struct simplefb_platform_data *mode)
>> +{
>> +     return false;
>> +}
>> +
>> +static int create_simplefb(const struct screen_info *si,
>> +                        const struct simplefb_platform_data *mode)
>> +{
>> +     return -EINVAL;
>> +}
>> +
>> +#endif /* CONFIG_X86_SYSFB */
>
> Following on from my ifdef comment above, I believe those versions of
> those functions will always cause add_sysfb() to return -ENODEV, so you
> may as well provide a static inline for add_sysfb() instead.

No. add_sysfb() is supposed to always succeed. However, if
parse_mode/create_simplefb fail, it creates a "platform-framebuffer"
as fallback. I don't see any way to avoid these ifdefs. Considering
the explanation above, could you elaborate how you think this should
work?

Thanks
David
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