Re: [Patch V8 3/3] iommu: Document usage of "/sys/kernel/iommu_groups/<grp_id>/type" file

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

Hi Will,

On 11/18/20 9:51 PM, Will Deacon wrote:
> On Fri, Sep 25, 2020 at 12:06:20PM -0700, Ashok Raj wrote:
>> From: Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>
>>
>> The default domain type of an iommu group can be changed by writing to
>> "/sys/kernel/iommu_groups/<grp_id>/type" file. Hence, document it's usage
>> and more importantly spell out its limitations.
>>
>> Cc: Christoph Hellwig <hch@lst.de>
>> Cc: Joerg Roedel <joro@8bytes.org>
>> Cc: Ashok Raj <ashok.raj@intel.com>
>> Cc: Will Deacon <will.deacon@arm.com>
>> Cc: Lu Baolu <baolu.lu@linux.intel.com>
>> Cc: Sohil Mehta <sohil.mehta@intel.com>
>> Cc: Robin Murphy <robin.murphy@arm.com>
>> Cc: Jacob Pan <jacob.jun.pan@linux.intel.com>
>> Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
>> Signed-off-by: Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>
>> ---
>>   .../ABI/testing/sysfs-kernel-iommu_groups          | 30 ++++++++++++++++++++++
>>   1 file changed, 30 insertions(+)
>>
>> diff --git a/Documentation/ABI/testing/sysfs-kernel-iommu_groups b/Documentation/ABI/testing/sysfs-kernel-iommu_groups
>> index 017f5bc3920c..effde9d23f4f 100644
>> --- a/Documentation/ABI/testing/sysfs-kernel-iommu_groups
>> +++ b/Documentation/ABI/testing/sysfs-kernel-iommu_groups
>> @@ -33,3 +33,33 @@ Description:    In case an RMRR is used only by graphics or USB devices
>>   		it is now exposed as "direct-relaxable" instead of "direct".
>>   		In device assignment use case, for instance, those RMRR
>>   		are considered to be relaxable and safe.
>> +
>> +What:		/sys/kernel/iommu_groups/<grp_id>/type
>> +Date:		September 2020
>> +KernelVersion:	v5.10
> 
> ^^ Please can you update these two lines?

Sure.

> 
>> +Contact:	Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>
>> +Description:	Let the user know the type of default domain in use by iommu
>> +		for this group. A privileged user could request kernel to change
>> +		the group type by writing to this file. Presently, only three
>> +		types are supported
>> +		1. DMA: All the DMA transactions from the device in this group
>> +			are translated by the iommu.
>> +		2. identity: All the DMA transactions from the device in this
>> +			     group are *not* translated by the iommu.
>> +		3. auto: Change to the type the device was booted with. When the
>> +			 user reads the file he would never see "auto". This is
> 
> Can we avoid assuming gender here and just use "they" instead of "he", please?
> Same thing for the "Caution" note below.

Yes, absolutely.

> 
>> +			 just a write only value.
> 
> I can't figure out from this description what string is returned to
> userspace in the case that the group is configured as  blocked or unmanaged.

This series only enables switching a default domain in use between DMA
and IDENTITY. Other cases will result in write failures.

> 
>> +		Note:
>> +		-----
>> +		A group type could be modified only when
> 
> s/could be/may be/
> 
>> +		1. The group has *only* one device
>> +		2. The device in the group is not bound to any device driver.
>> +		   So, the user must first unbind the appropriate driver and
>> +		   then change the default domain type.
>> +		Caution:
>> +		--------
>> +		Unbinding a device driver will take away the driver's control
>> +		over the device and if done on devices that host root file
>> +		system could lead to catastrophic effects (the user might
>> +		need to reboot the machine to get it to normal state). So, it's
>> +		expected that the user understands what he is doing.
> 
> Thanks,
> 
> Will

Best regards,
baolu
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