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

[Baolu Lu]

On 2023/11/16 11:27, Tian, Kevin wrote:
> > 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?

Right. It's confusing. I will change it to below.

"
...before switching address translation on or off and reflecting the
status of the command through the TES field in the Global Status
register.
"

> > @@ -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...

I can see your point.

This fix is just to add a new device model to the established quirk
list. All devices (including the new one) in this quirk list have
undergone thorough verification. Therefor, I'd like to keep it as-is.

We can refine the quirk implementation in a separated patch series with
sufficient consideration and verification. Does this work for you?

Best regards,
baolu