[lkml]   [2019]   [Dec]   [5]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [PATCH v2 4/8] iommu/arm-smmu: Add split pagetables for Adreno IOMMU implementations
On Wed, Dec 04, 2019 at 04:44:59PM +0000, Robin Murphy wrote:
> On 22/11/2019 11:31 pm, Jordan Crouse wrote:
> >Add implementation specific support to enable split pagetables for
> >SMMU implementations attached to Adreno GPUs on Qualcomm targets.
> >
> >To enable split pagetables the driver will set an attribute on the domain.
> >if conditions are correct, set up the hardware to support equally sized
> >TTBR0 and TTBR1 regions and programs the domain pagetable to TTBR1 to make
> >it available for global buffers while allowing the GPU the chance to
> >switch the TTBR0 at runtime for per-context pagetables.
> >
> >After programming the context, the value of the domain attribute can be
> >queried to see if split pagetables were successfully programmed. The
> >domain geometry will be updated so that the caller can determine the
> >start of the region to generate correct virtual addresses.
> Why is any of this in impl? It all looks like perfectly generic
> architectural TTBR1 setup to me. As long as DOMAIN_ATTR_SPLIT_TABLES is
> explicitly an opt-in for callers, I'm OK with them having to trust that
> SEP_UPSTREAM is good enough. Or, even better, make the value of
> DOMAIN_ATTR_SPLIT_TABLES not a boolean but the actual split point, where the
> default of 0 would logically mean "no split".

(apologies if you get multiple copies of this email, I have tickets in with the
CAF IT folks).

I made it impl specific because my impression from the previous conversations
was that setting up the T0 space but leaving TTBR0 un-programmed was a silly
thing that was unique to the Adreno GPU. I don't mind moving it to the generic
code since that saves us from some silly compatible string games.

I like the idea of DOMAIN_ATTR_SPLIT_TABLES returning the split point but would
we want to allow the user to try to specific a desired split point ahead of
time? It is my impression that we only have a handful of valid SEP values and
I'm not sure what the right response would be if the user specified an incorrect

So far I've not found a use for anything except SEP_UPSTREAM but I have the
extreme luxury of a SMMU with an actual 49 bit IAS.

New patchset coming soon.


The Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
a Linux Foundation Collaborative Project

 \ /
  Last update: 2019-12-05 16:52    [W:0.077 / U:0.448 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site