[PATCH] iommu: Better document the IOMMU_PRIV flag

[Sricharan]

Hi Robin,

>-----Original Message-----
>From: linux-arm-kernel [mailto:linux-arm-kernel-bounces at lists.infradead.org] On Behalf Of Robin Murphy
>Sent: Friday, January 27, 2017 5:53 PM
>To: will.deacon at arm.com
>Cc: iommu at lists.linux-foundation.org; sricharan at codeaurora.org; linux-arm-kernel at lists.infradead.org
>Subject: [PATCH] iommu: Better document the IOMMU_PRIV flag
>
>This is a fairly subtle thing - let's make sure it's described as
>clearly as possible to avoid potential misunderstandings.
>
>Signed-off-by: Robin Murphy <robin.murphy at arm.com>
>---
>
>Having another look through the IOMMU_PRIV series, I wasn't convinced
>that the original comment was really all that helpful - I'm happy for
>this to be squashed in if you like.
>
>Robin.
>
> include/linux/iommu.h | 11 +++++++----
> 1 file changed, 7 insertions(+), 4 deletions(-)
>
>diff --git a/include/linux/iommu.h b/include/linux/iommu.h
>index 69e2417a2965..3c830e153069 100644
>--- a/include/linux/iommu.h
>+++ b/include/linux/iommu.h
>@@ -32,10 +32,13 @@
> #define IOMMU_NOEXEC	(1 << 3)
> #define IOMMU_MMIO	(1 << 4) /* e.g. things like MSI doorbells */
> /*
>- * This is to make the IOMMU API setup privileged
>- * mapppings accessible by the master only at higher
>- * privileged execution level and inaccessible at
>- * less privileged levels.
>+ * Where the bus hardware includes a privilege level as part of its access type
>+ * markings, and certain devices are capable of issuing transactions marked as
>+ * either 'supervisor' or 'user', the IOMMU_PRIV flag requests that the other
>+ * given permission flags only apply to accesses at the higher privilege level,
>+ * and that unprivileged transactions should have as little access as possible.
>+ * This would usually imply the same permissions as kernel mappings on the CPU,
>+ * if the IOMMU page table format is equivalent.
>  */

Agree, gives much more insight.

Regards,
  Sricharan