RE: [PATCH 1/1] iommu/vt-d: Add MTL to quirk list to skip TE disabling

[Tian, Kevin]

> From: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> Sent: Thursday, November 16, 2023 10:23 AM
> 
> From: "Abdul Halim, Mohd Syazwan"
> <mohd.syazwan.abdul.halim@xxxxxxxxx>
> 
> The VT-d spec requires (10.4.4 Global Command Register, TE field) that:
> 
> Hardware implementations supporting DMA draining must drain any in-flight
> DMA read/write requests queued within the Root-Complex before
> completing
> the translation enable command and reflecting the status of the command
> through the TES field in the Global Status register.

this talks about 'enable'...

> 
> Unfortunately, some integrated graphic devices fail to do so after some
> kind of power state transition. As the result, the system might stuck in
> iommu_disable_translation(), waiting for the completion of TE transition.

...while this fixes 'disable'. wrong citation?

> @@ -5080,7 +5080,7 @@ static void quirk_igfx_skip_te_disable(struct
> pci_dev *dev)
>  	ver = (dev->device >> 8) & 0xff;
>  	if (ver != 0x45 && ver != 0x46 && ver != 0x4c &&
>  	    ver != 0x4e && ver != 0x8a && ver != 0x98 &&
> -	    ver != 0x9a && ver != 0xa7)
> +	    ver != 0x9a && ver != 0xa7 && ver != 0x7d)
>  		return;
> 

this fix alone is fine, but I found this quirk overall is not cleanly handled.

Basically it sets iommu_skip_te_disable as true, leading to early return
in iommu_disable_translation():

	if (iommu_skip_te_disable && iommu->drhd->gfx_dedicated &&
	   (cap_read_drain(iommu->cap) || cap_write_drain(iommu->cap)))
		return;

However the caller of iommu_disable_translation() is not aware of this
quirk and continues as if the iommu is disabled. IMHO this is problematic
w/o meeting the caller's assumption.

e.g. kdump and suspend. We may want to abort those paths early in case
of such quirk...