Re: [PATCH v7 6/6] drm/msm: iommu: Replace runtime calls with runtime suppliers

[Robin Murphy <robin.murphy@arm.com>]

On 15/02/18 04:17, Tomasz Figa wrote:
[...]
>> Could you elaborate on what kind of locking you are concerned about?
>> As I explained before, the normally happening fast path would lock
>> dev->power_lock only for the brief moment of incrementing the runtime
>> PM usage counter.
> 
> My bad, that's not even it.
> 
> The atomic usage counter is incremented beforehands, without any
> locking [1] and the spinlock is acquired only for the sake of
> validating that device's runtime PM state remained valid indeed [2],
> which would be the case in the fast path of the same driver doing two
> mappings in parallel, with the master powered on (and so the SMMU,
> through device links; if master was not powered on already, powering
> on the SMMU is unavoidable anyway and it would add much more latency
> than the spinlock itself).

We now have no locking at all in the map path, and only a per-domain 
lock around TLB sync in unmap which is unfortunately necessary for 
correctness; the latter isn't too terrible, since in "serious" hardware 
it should only be serialising a few cpus serving the same device against 
each other (e.g. for multiple queues on a single NIC).

Putting in a global lock which serialises *all* concurrent map and unmap 
calls for *all* unrelated devices makes things worse. Period. Even if 
the lock itself were held for the minimum possible time, i.e. trivially 
"spin_lock(&lock); spin_unlock(&lock)", the cost of repeatedly bouncing 
that one cache line around between 96 CPUs across two sockets is not 
negligible.

> [1] http://elixir.free-electrons.com/linux/v4.16-rc1/source/drivers/base/power/runtime.c#L1028
> [2] http://elixir.free-electrons.com/linux/v4.16-rc1/source/drivers/base/power/runtime.c#L613
> 
> In any case, I can't imagine this working with V4L2 or anything else
> relying on any memory management more generic than calling IOMMU API
> directly from the driver, with the IOMMU device having runtime PM
> enabled, but without managing the runtime PM from the IOMMU driver's
> callbacks that need access to the hardware. As I mentioned before,
> only the IOMMU driver knows when exactly the real hardware access
> needs to be done (e.g. Rockchip/Exynos don't need to do that for
> map/unmap if the power is down, but some implementations of SMMU with
> TLB powered separately might need to do so).

It's worth noting that Exynos and Rockchip are relatively small 
self-contained IP blocks integrated closely with the interfaces of their 
relevant master devices; SMMU is an architecture, implementations of 
which may be large, distributed, and have complex and wildly differing 
internal topologies. As such, it's a lot harder to make 
hardware-specific assumptions and/or be correct for all possible cases.

Don't get me wrong, I do ultimately agree that the IOMMU driver is the 
only agent who ultimately knows what calls are going to be necessary for 
whatever operation it's performing on its own hardware*; it's just that 
for SMMU it needs to be implemented in a way that has zero impact on the 
cases where it doesn't matter, because it's not viable to specialise 
that driver for any particular IP implementation/use-case.

Robin.


*AFAICS it still makes some sense to have the get_suppliers option as 
well, though - the IOMMU driver does what it needs for correctness 
internally, but the external consumer doing something non-standard can 
can grab and hold the link around multiple calls to short-circuit that.