Re: [PATCH] PCI: Prevent LS7A Bus Master clearing on kexec

[Bjorn Helgaas <helgaas@kernel.org>]

On Fri, Jul 26, 2024 at 05:28:29PM +0800, Huacai Chen wrote:
> This is similar to commit 62b6dee1b44aa23b39355 ("PCI/portdrv: Prevent
> LS7A Bus Master clearing on shutdown"), which prevents LS7A Bus Master
> clearing on kexec.
> 
> Only skip Bus Master clearing on bridges because endpoint devices still
> need it.

I think we need some explanation here and a hint in the comment below
about why this is needed.

I guess the point is to work around the LS7A defect that clearing
PCI_COMMAND_MASTER prevents MMIO requests from going downstream, and
we may need to do that even after .shutdown(), e.g., to print console
messages?

And in this case we rely on .shutdown() for the downstream devices to
disable interrupts and DMA?

s/62b6dee1b44aa23b39355/62b6dee1b44a/

> Signed-off-by: Ming Wang <wangming01@loongson.cn>
> Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
> ---
>  drivers/pci/pci-driver.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/pci-driver.c b/drivers/pci/pci-driver.c
> index f412ef73a6e4..b7d3a4d8532f 100644
> --- a/drivers/pci/pci-driver.c
> +++ b/drivers/pci/pci-driver.c
> @@ -517,7 +517,7 @@ static void pci_device_shutdown(struct device *dev)
>  	 * If it is not a kexec reboot, firmware will hit the PCI
>  	 * devices with big hammer and stop their DMA any way.
>  	 */
> -	if (kexec_in_progress && (pci_dev->current_state <= PCI_D3hot))
> +	if (kexec_in_progress && !pci_is_bridge(pci_dev) && (pci_dev->current_state <= PCI_D3hot))
>  		pci_clear_master(pci_dev);
>  }
>  
> -- 
> 2.43.5
>