Re: [PATCH v6 3/5] fbdev: Disable sysfb device registration when removing conflicting FBs

[Zack Rusin]

On Tue, 2022-06-07 at 20:23 +0200, Javier Martinez Canillas wrote:
> The platform devices registered by sysfb match with firmware-based DRM or
> fbdev drivers, that are used to have early graphics using a framebuffer
> provided by the system firmware.
> 
> DRM or fbdev drivers later are probed and remove all conflicting framebuffers,
> leading to these platform devices for generic drivers to be unregistered.
> 
> But the current solution has a race, since the sysfb_init() function could
> be called after a DRM or fbdev driver is probed and request to unregister
> the devices for drivers with conflicting framebuffes.
> 
> To prevent this, disable any future sysfb platform device registration by
> calling sysfb_disable(), if a driver requests to remove the conflicting
> framebuffers.
> 
> Suggested-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
> Signed-off-by: Javier Martinez Canillas <javierm@xxxxxxxxxx>
> Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
> ---
> 
> Changes in v6:
> - Move the sysfb_disable() before the remove conflicting framebuffers
>   loop (Daniel Vetter).
> 
> Changes in v5:
> - Move the sysfb_disable() call at conflicting framebuffers again to
>   avoid the need of a DRIVER_FIRMWARE capability flag.
> - Add Daniel Vetter's Reviewed-by tag again since reverted to the old
>   patch that he already reviewed in v2.
> 
> Changes in v3:
> - Call sysfb_disable() when a DRM dev and a fbdev are registered rather
>   than when conflicting framebuffers are removed (Thomas Zimmermann).
> - Call sysfb_disable() when a fbdev framebuffer is registered rather
>   than when conflicting framebuffers are removed (Thomas Zimmermann).
> - Drop Daniel Vetter's Reviewed-by tag since patch changed a lot.
> 
> Changes in v2:
> - Explain in the commit message that fbmem has to unregister the device
>   as fallback if a driver registered the device itself (Daniel Vetter).
> - Also explain that fallback in a comment in the code (Daniel Vetter).
> - Don't encode in fbmem the assumption that sysfb will always register
>   platform devices (Daniel Vetter).
> - Add a FIXME comment about drivers registering devices (Daniel Vetter).
> 
>  drivers/video/fbdev/core/fbmem.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
> index 2fda5917c212..e0720fef0ee6 100644
> --- a/drivers/video/fbdev/core/fbmem.c
> +++ b/drivers/video/fbdev/core/fbmem.c
> @@ -19,6 +19,7 @@
>  #include <linux/kernel.h>
>  #include <linux/major.h>
>  #include <linux/slab.h>
> +#include <linux/sysfb.h>
>  #include <linux/mm.h>
>  #include <linux/mman.h>
>  #include <linux/vt.h>
> @@ -1764,6 +1765,17 @@ int remove_conflicting_framebuffers(struct apertures_struct *a,
>  		do_free = true;
>  	}
>  
> +	/*
> +	 * If a driver asked to unregister a platform device registered by
> +	 * sysfb, then can be assumed that this is a driver for a display
> +	 * that is set up by the system firmware and has a generic driver.
> +	 *
> +	 * Drivers for devices that don't have a generic driver will never
> +	 * ask for this, so let's assume that a real driver for the display
> +	 * was already probed and prevent sysfb to register devices later.
> +	 */
> +	sysfb_disable();
> +
>  	mutex_lock(&registration_lock);
>  	do_remove_conflicting_framebuffers(a, name, primary);
>  	mutex_unlock(&registration_lock);

Hi, Javier.

This change broke arm64 with vmwgfx. We get a kernel oops at boot (let me know if
you'd like .config or just have us test something directly for you):


 Unable to handle kernel NULL pointer dereference at virtual address
0000000000000008
 Mem abort info:
   ESR = 0x96000004
   EC = 0x25: DABT (current EL), IL = 32 bits
   SET = 0, FnV = 0
   EA = 0, S1PTW = 0
   FSC = 0x04: level 0 translation fault
 Data abort info:
   ISV = 0, ISS = 0x00000004
   CM = 0, WnR = 0
 user pgtable: 4k pages, 48-bit VAs, pgdp=00000001787ee000
 [0000000000000008] pgd=0000000000000000, p4d=0000000000000000
 Internal error: Oops: 96000004 [#1] SMP
 Modules linked in: vmwgfx(+) e1000e(+) nvme ahci(+) xhci_pci drm_ttm_helper ttm
sha256_arm64 sha1_ce nvme_core xhci_pci_renesas aes_neon_bs aes_neon_blk aes>
 CPU: 3 PID: 215 Comm: systemd-udevd Tainted: G     U            5.18.0-rc5-vmwgfx
#12
 Hardware name: VMware, Inc. VBSA/VBSA, BIOS VEFI 12/31/2020
 pstate: 80400005 (Nzcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
 pc : kernfs_find_and_get_ns+0x2c/0x80
 lr : sysfs_unmerge_group+0x30/0x80
 sp : ffff80000b78b3f0
 x29: ffff80000b78b3f0 x28: ffff0000f7970910 x27: 0000000000000002
 x26: ffff0000f7970900 x25: ffff80000abca358 x24: ffff80000936cfa0
 x23: ffff80000ac8f458 x22: 0000000000000000 x21: ffff800009431938
 x20: ffff800009431800 x19: 0000000000000000 x18: 0000000000000000
 x17: 42555300302e7265 x16: 66667562656d6172 x15: 4d006d726f667461
 x14: 6c703d4d45545359 x13: 595342555300302e x12: 726566667562656d
 x11: 00323137323d4d55 x10: 4e51455300726566 x9 : ffff8000085f7400
 x8 : ffff0000f7970980 x7 : 0000000000000000 x6 : 0000000077ffffff
 x5 : 0000000070000000 x4 : ffff80000b78b548 x3 : ffff8000088b0420
 x2 : 0000000000000000 x1 : ffff800009431938 x0 : 0000000000000000
 Call trace:
  kernfs_find_and_get_ns+0x2c/0x80
  sysfs_unmerge_group+0x30/0x80
  dpm_sysfs_remove+0x3c/0x17c
  device_del+0xb0/0x3a0
  platform_device_del.part.0+0x24/0xb0
  platform_device_unregister+0x30/0x50
  sysfb_disable+0x4c/0x80
  remove_conflicting_framebuffers+0x40/0x100
  remove_conflicting_pci_framebuffers+0x128/0x240
  drm_aperture_remove_conflicting_pci_framebuffers+0xb8/0x170
  vmw_probe+0x50/0xd30 [vmwgfx]
  local_pci_probe+0x4c/0xc0
  pci_device_probe+0x1e8/0x230
  really_probe+0x18c/0x3f0
  __driver_probe_device+0x124/0x1c0
  driver_probe_device+0x44/0x140
  __driver_attach+0xe0/0x234
  bus_for_each_dev+0x7c/0xe0
  driver_attach+0x30/0x40
  bus_add_driver+0x158/0x250
  driver_register+0x84/0x140
  __pci_register_driver+0x50/0x5c
  vmw_pci_driver_init+0x44/0x1000 [vmwgfx]
  do_one_initcall+0x50/0x250
  do_init_module+0x50/0x260
  load_module+0x23e4/0x27c0
  __do_sys_finit_module+0xac/0x12c
  __arm64_sys_finit_module+0x2c/0x40
  invoke_syscall+0x78/0x100
  el0_svc_common.constprop.0+0x54/0x184
  do_el0_svc+0x34/0x9c
  el0_svc+0x54/0x1e0
  el0t_64_sync_handler+0xa4/0x130
  el0t_64_sync+0x1a0/0x1a4
 Code: aa0003f3 a9025bf5 aa0103f5 aa0203f6 (f9400400)
 ---[ end trace 0000000000000000 ]---