Re: [PATCH V2 5/5] iommu: Document usage of "/sys/kernel/iommu_groups/<grp_id>/type" file

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

Hi,

On 2020/2/17 5:57, Sai Praneeth Prakhya wrote:
> The default domain type of an iommu group can be changed using file
> "/sys/kernel/iommu_groups/<grp_id>/type". Hence, document it's usage and
> more importantly spell out it's 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>
> Signed-off-by: Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>
> ---
>   .../ABI/testing/sysfs-kernel-iommu_groups          | 29 ++++++++++++++++++++++
>   1 file changed, 29 insertions(+)
> 
> diff --git a/Documentation/ABI/testing/sysfs-kernel-iommu_groups b/Documentation/ABI/testing/sysfs-kernel-iommu_groups
> index 017f5bc3920c..808a9507b281 100644
> --- a/Documentation/ABI/testing/sysfs-kernel-iommu_groups
> +++ b/Documentation/ABI/testing/sysfs-kernel-iommu_groups
> @@ -33,3 +33,32 @@ 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:		February 2020
> +KernelVersion:	v5.6
> +Contact:	Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>
> +Description:	Lets the user know the type of default domain in use by iommu
> +		for this group. A privileged user could request kernel to change

Let the users know the default domain type of the IOMMU group by
reading this file. A privileged user could request to change it by
writing to this file.

> +		the group type by writing to this file. Presently, only three
> +		types are supported
> +		1. DMA: All the DMA transactions from the devices in this group
> +			are translated by the iommu.
> +		2. identity: All the DMA transactions from the devices in this
> +			     group are *not* translated by the iommu.
> +		3. auto: Kernel decides one among DMA/identity mode and hence
> +			 when the user reads the file he would never see "auto".

Just out of curiosity, when could a user reads "auto" from this file?

> +			 This is just a write only value.
> +		Note:
> +		-----
> +		A group type could be modified only when *all* the devices in

group's default domain type (not group type).

> +		the group are not binded to any device driver. So, the user has

bound

> +		to first unbind the appropriate drivers and then change the
> +		default domain type.
> +		Caution:
> +		--------
> +		Unbinding a device driver will take away the drivers 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.
> 

Best regards,
baolu
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