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

[Baolu Lu <baolu.lu@linux.intel.com>]

On 2023/11/16 11:27, Tian, Kevin wrote:
>> From: Lu Baolu <baolu.lu@linux.intel.com>
>> Sent: Thursday, November 16, 2023 10:23 AM
>>
>> From: "Abdul Halim, Mohd Syazwan"
>> <mohd.syazwan.abdul.halim@intel.com>
>>
>> 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