Re: [PATCH v2] docs: add PCIe root bus for VGA compat guideline

[Laszlo Ersek]

On 06/12/22 19:32, Kevin Locke wrote:
> PCI Express devices which use legacy VGA compatibility should be placed
> on the Root Complex.  This simplifies ioport access to VGA registers,
> which requires use of a special exception bit to work across PCI(e)
> bridges.  It is also necessary for ioport access to VESA BIOS Extension
> (VBE) registers, which is not forwarded over PCI(e) bridges, even with
> the special exception bit for VGA register access.[1]
> 
> Update the PCI Express Guidelines to add these to the list of devices
> which can be placed directly on the Root Complex.
> 
> Note that the only PCI Express display devices currently supported
> (bochs-display and virtio-gpu-pci) do not offer VGA compatibility.
> Legacy PCI devices (e.g. vga, qxl-vga, virtio-vga) are already
> documented as allowed on the Root Complex by the first item in the list.
> However, this item documents an additional consideration for placing
> devices which was not previously mentioned, and may be relevant for PCIe
> devices offering VGA compatibility in the future.
> 
> [1]: 
> https://mail.coreboot.org/hyperkitty/list/seabios@seabios.org/thread/XG2RN3HKVRDEDTLA2PRELLIENIIH7II7/#XVP3I2KQVZHSTDA4SNVKOITWGRGSDU3F
> 
> Signed-off-by: Laszlo Ersek <lersek@redhat.com>

Please make this a "Suggested-by: Laszlo Ersek <lersek@redhat.com>"
(concerning the text in the patch body).

The commit message looks OK to me, but I'd like Gerd and/or Alex to
approve it.

Thanks!
Laszlo

> Signed-off-by: Kevin Locke <kevin@kevinlocke.name>
> ---
> Changes since v1:
>  * Replace my overly-broad exception for devices requiring ioport access
>    with a list item specifically for PCI Express devices offering VGA
>    Compatibility provided by Laszlo Ersek.
>  * Rewrite the commit message based on my improved understanding of the
>    issue and the improved scope of the change.
> 
> P.S. Let me know if the Signed-off-by tag is not appropriate for either
> of us.  I'm not clear on the etiquette of including someone else's
> sign-off, but also don't want to misrepresent myself as the source of
> your work.
> 
>  docs/pcie.txt | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/docs/pcie.txt b/docs/pcie.txt
> index 89e3502075..59b26817f9 100644
> --- a/docs/pcie.txt
> +++ b/docs/pcie.txt
> @@ -48,13 +48,17 @@ Place only the following kinds of devices directly on the 
> Root Complex:
>          strangely when PCI Express devices are integrated
>          with the Root Complex.
>  
> -    (2) PCI Express Root Ports (ioh3420), for starting exclusively PCI 
> Express
> +    (2) Assigned PCI Express GPUs that offer legacy VGA compatibility, and
> +        that such compatibility is expected of (due to booting with SeaBIOS,
> +        or due to UEFI driver bugs or native OS driver bugs).
> +
> +    (3) PCI Express Root Ports (ioh3420), for starting exclusively PCI 
> Express
>          hierarchies.
>  
> -    (3) PCI Express to PCI Bridge (pcie-pci-bridge), for starting legacy PCI
> +    (4) PCI Express to PCI Bridge (pcie-pci-bridge), for starting legacy PCI
>          hierarchies.
>  
> -    (4) Extra Root Complexes (pxb-pcie), if multiple PCI Express Root Buses
> +    (5) Extra Root Complexes (pxb-pcie), if multiple PCI Express Root Buses
>          are needed.
>  
>     pcie.0 bus
>