Re: [PATCH] firmware/sysfb: Avoid simple-framebuffer behind DT PCI host bridges

[Jennifer Berringer <jberring@redhat.com>]

Hi,

On Sat, Nov 07, 2026 at 04:46:40PM +0800, Han Gao via B4 Relay wrote:
> From: Han Gao <gaohan@iscas.ac.cn>
> 
> With SYSFB_SIMPLEFB, sysfb registers a simple-framebuffer device that
> claims the framebuffer memory when it is added. On DT systems, the PCI
> host bridge may probe later and then fails to claim its memory window.
> On QEMU RISC-V virt with EDK2, a bochs-display and pci-host-generic
> built as a module:
> 
>   [    0.703813] pci-host-generic 30000000.pci: resource collision: [mem 0x40000000-0x7fffffff] conflicts with BOOTFB [mem 0x40000000-0x403e7fff]
> 
> In that case, register an efi-framebuffer device instead. It claims no
> memory before it probes, and commit 64c8a0cd0a53 ("efi/arm: Defer probe
> of PCIe backed efifb on DT systems") makes it wait for the host bridge.
> 
> Tested on a HiFive Unmatched with EDK2 and a PCIe graphics card, and on
> QEMU.
> 
> Fixes: 8633ef82f101 ("drivers/firmware: consolidate EFI framebuffer setup for all arches")
> Cc: stable@vger.kernel.org
> Signed-off-by: Han Gao <gaohan@iscas.ac.cn>
> ---

This issue also affects UltraRISC DP1000 boards such as Milk-V Titan.
For that system, a2d4a2d2479b ("clk: ultrarisc: Register DP1000 clock
provider earlier") was recently added to clk-next and can serve as a
workaround to make its pci driver be able to initialize before sysfb,
but only if both the clock and pci drivers are built in rather than
built as modules.

I confirmed that your patch resolves the issue on Milk-V Titan even when
building with CONFIG_PCIE_ULTRARISC=m, making a PCIe graphics card able
to initialize and function without issue. CC'd the ultrarisc developers.

Tested-by: Jennifer Berringer <jberring@redhat.com> # Milk-V Titan

> In QEMU, efifb takes over with SYSFB_SIMPLEFB and efidrm without it.
> With pci-host-generic built in, simpledrm is still used.
> ---
>  drivers/firmware/efi/sysfb_efi.c | 16 ++++++++++++++++
>  drivers/firmware/sysfb.c         |  8 +++++++-
>  include/linux/sysfb.h            |  6 ++++++
>  3 files changed, 29 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/firmware/efi/sysfb_efi.c b/drivers/firmware/efi/sysfb_efi.c
> index 2338de73c62c..5c2cb87178ff 100644
> --- a/drivers/firmware/efi/sysfb_efi.c
> +++ b/drivers/firmware/efi/sysfb_efi.c
> @@ -421,6 +421,22 @@ __init void sysfb_apply_efi_quirks(struct screen_info *si)
>  		dmi_check_system(efifb_dmi_swap_width_height);
>  }
>  
> +/* Returns true if the EFI framebuffer is in a DT PCI host bridge window */
> +__init bool sysfb_efifb_behind_dt_pci(const struct screen_info *si)
> +{
> +	struct device_node *np;
> +
> +	if (si->orig_video_isVGA != VIDEO_TYPE_EFI || !IS_ENABLED(CONFIG_PCI))
> +		return false;
> +
> +	np = find_pci_overlap_node();
> +	if (!np)
> +		return false;
> +
> +	of_node_put(np);
> +	return true;
> +}
> +
>  __init void sysfb_set_efifb_fwnode(const struct screen_info *si, struct platform_device *pd)
>  {
>  	if (si->orig_video_isVGA == VIDEO_TYPE_EFI && IS_ENABLED(CONFIG_PCI)) {
> diff --git a/drivers/firmware/sysfb.c b/drivers/firmware/sysfb.c
> index 8833582c1883..8853a74560b9 100644
> --- a/drivers/firmware/sysfb.c
> +++ b/drivers/firmware/sysfb.c
> @@ -164,8 +164,14 @@ static __init int sysfb_init(void)
>  		goto unlock_mutex;
>  	}
>  
> -	/* try to create a simple-framebuffer device */
> +	/*
> +	 * Try to create a simple-framebuffer device. It claims the memory
> +	 * when it is added, so a DT PCI host bridge that probes later would
> +	 * fail. Use an efi-framebuffer device there; it waits for the bridge.
> +	 */
>  	compatible = sysfb_parse_mode(si, &mode);
> +	if (compatible && !parent && sysfb_efifb_behind_dt_pci(si))
> +		compatible = false;
>  	if (compatible) {
>  		pd = sysfb_create_simplefb(si, &mode, parent);
>  		if (!IS_ERR(pd))
> diff --git a/include/linux/sysfb.h b/include/linux/sysfb.h
> index ed23d6516223..b136fdebc746 100644
> --- a/include/linux/sysfb.h
> +++ b/include/linux/sysfb.h
> @@ -95,6 +95,7 @@ static inline bool sysfb_handles_screen_info(void)
>  
>  extern struct efifb_dmi_info efifb_dmi_list[];
>  void sysfb_apply_efi_quirks(struct screen_info *si);
> +bool sysfb_efifb_behind_dt_pci(const struct screen_info *si);
>  void sysfb_set_efifb_fwnode(const struct screen_info *si, struct platform_device *pd);
>  
>  #else /* CONFIG_EFI */
> @@ -103,6 +104,11 @@ static inline void sysfb_apply_efi_quirks(struct screen_info *si)
>  {
>  }
>  
> +static inline bool sysfb_efifb_behind_dt_pci(const struct screen_info *si)
> +{
> +	return false;
> +}
> +
>  static inline void sysfb_set_efifb_fwnode(const struct screen_info *si,
>  					  struct platform_device *pd)
>  {
> 
> ---
> base-commit: 6628e7d4c6b25c8d18f1345e3a20d21829aae7e4
> change-id: 20261107-sysfb-dt-pci-efifb-d73b08ded4b2
> 
> Best regards,
> --  
> Han Gao <gaohan@iscas.ac.cn>
>