Re: [PATCH] Fix failure of simpledrm probe when trying to grab FB from the EFI-based Framebuffer

[Javier Martinez Canillas <javierm@redhat.com>]

Andrew Worsley <amworsley@gmail.com> writes:

Hello Andrew,

> On Sat, 11 Nov 2023 at 20:10, Javier Martinez Canillas
> <javierm@redhat.com> wrote:
>>
> ....
>> > On Sat, 11 Nov 2023 at 15:30, Andrew Worsley <amworsley@gmail.com> wrote:
>> >>
>> >>    The simpledrm.c does not call aperture_remove_conflicting_devices() in it's probe
>> >>    function as the drivers/video/aperture.c documentation says it should. Consequently
>> >>    it's request for the FB memory fails.
>> >>
>>
>> The current behaviour is correct since aperture_remove_conflicting_devices()
>> is for native drivers to remove simple framebuffer devices such as simpledrm,
>> simplefb, efifb, etc.
>
> The efifb is the driver that has "grabbed" the FB earlier
>
> Here's a debug output with a dump_stack() call in the devm_aperture_acquire()
> % grep --color -A14 -B4 devm_aperture_acquire ~/dmesg2.txt
> [    0.055752] efifb: framebuffer at 0xbd58dc000, using 16000k, total 16000k
> [    0.055755] efifb: mode is 2560x1600x32, linelength=10240, pages=1
> [    0.055758] efifb: scrolling: redraw
> [    0.055759] efifb: Truecolor: size=2:10:10:10, shift=30:20:10:0
> [    0.055771] devm_aperture_acquire: dump stack for debugging
> [    0.055775] CPU: 2 PID: 1 Comm: swapper/0 Tainted: G S

I see. This is the problem then. Your platform then is using a Device Tree
that contains a "simple-framebuffer" node but also doing a UEFI boot and
providing an EFI GOP table that is picked by the Linux EFI stub on boot.

[...]

>>
>> >> ...
>> >> [    3.085302] simple-framebuffer bd58dc000.framebuffer: [drm] *ERROR* could not acquire memory range [??? 0xffff6e1d8629d580-0x2a5000001a7 flags 0x0]: -16
>> >> [    3.086433] simple-framebuffer: probe of bd58dc000.framebuffer failed with error -16
>> >> ...
>> >>
>>
>> This is -EBUSY. What is your kernel configuration? Can you share it please.
>
> Attached - hope that's acceptable...
>
>

Thanks a lot for providing this. It was very helpful to understand the issue.

[...]

>>
>> I would rather try to understand what is going on in your setup and why
>> the acquire is returning -EBUSY.
>>
>
> Ok - thanks - let me know where to go from here.
>

I think that what we should do instead is to prevent both the EFI GOP and
"simple-framebuffer" to provide a system framebuffer information and the
kernel to register two devices (a "simple-framebuffer" by the OF core and
an "efi-framebuffer" by the sysfb infrastructure).

In my opinion, the DTB is the best source of truth on an DT platform and
so is the sysfb that should be disabled if there's a "simple-framebuffer"
DT node found.

Can you please try the following (untested) patch?

From 7bf4a7917962c24c9f15aaf6e798db9d652c6806 Mon Sep 17 00:00:00 2001
From: Javier Martinez Canillas <javierm@redhat.com>
Date: Sun, 12 Nov 2023 11:06:22 +0100
Subject: [PATCH] of/platform: Disable sysfb if a simple-framebuffer node is
 found

Some DT platforms use EFI to boot and in this case the EFI Boot Services
may register a EFI_GRAPHICS_OUTPUT_PROTOCOL handle, that will later be
queried by the Linux EFI stub to fill the global struct screen_info data.

The data is used by the Generic System Framebuffers (sysfb) framework to
add a platform device with platform data about the system framebuffer.

But if there is a "simple-framebuffer" node in the DT, the OF core will
also do the same and add another device for the system framebuffer.

This could lead for example, to two platform devices ("simple-framebuffer"
and "efi-framebuffer") to be added and matched with their corresponding
drivers. So both efifb and simpledrm will be probed, leading to following:

[    0.055752] efifb: framebuffer at 0xbd58dc000, using 16000k, total 16000k
[    0.055755] efifb: mode is 2560x1600x32, linelength=10240, pages=1
[    0.055758] efifb: scrolling: redraw
[    0.055759] efifb: Truecolor: size=2:10:10:10, shift=30:20:10:0
...
[    3.295896] simple-framebuffer bd58dc000.framebuffer: [drm] *ERROR*
could not acquire memory range [??? 0xffff79f30a29ee40-0x2a5000001a7
flags 0x0]: -16
[    3.298018] simple-framebuffer: probe of bd58dc000.framebuffer
failed with error -16

To prevent the issue, make the OF core to disable sysfb if there is a node
with a "simple-framebuffer" compatible. That way only this device will be
registered and sysfb would not attempt to register another one using the
screen_info data even if this has been filled.

Reported-by: Andrew Worsley <amworsley@gmail.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
---
 drivers/of/platform.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/drivers/of/platform.c b/drivers/of/platform.c
index f235ab55b91e..a9fd91e6a6df 100644
--- a/drivers/of/platform.c
+++ b/drivers/of/platform.c
@@ -621,8 +621,21 @@ static int __init of_platform_default_populate_init(void)
 		}
 
 		node = of_get_compatible_child(of_chosen, "simple-framebuffer");
-		of_platform_device_create(node, NULL, NULL);
-		of_node_put(node);
+		if (node) {
+			/*
+			 * Since a "simple-framebuffer" device is already added
+			 * here, disable the Generic System Framebuffers (sysfb)
+			 * to prevent it from registering another device for the
+			 * system framebuffer later (e.g: using the screen_info
+			 * data that may had been filled as well).
+			 *
+			 * This can happen for example on DT systems that do EFI
+			 * booting and may provide a GOP table to the EFI stub.
+			 */
+			sysfb_disable();
+			of_platform_device_create(node, NULL, NULL);
+			of_node_put(node);
+		}
 
 		/* Populate everything else. */
 		of_platform_default_populate(NULL, NULL, NULL);
-- 
2.41.0

-- 
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat