On 02/05/18 15:48, Rob Herring wrote:
On Wed, May 2, 2018 at 6:40 AM, Robin Murphy <robin.murphy@xxxxxxx> wrote:
On 01/05/18 22:31, Rob Herring wrote:
Deferred probe will currently wait forever on dependent devices to probe,
but sometimes a driver will never exist. It's also not always critical for
a driver to exist. Platforms can rely on default configuration from the
bootloader or reset defaults for things such as pinctrl and power domains.
This is often the case with initial platform support until various drivers
get enabled. There's at least 2 scenarios where deferred probe can render
a platform broken. Both involve using a DT which has more devices and
dependencies than the kernel supports. The 1st case is a driver may be
disabled in the kernel config. The 2nd case is the kernel version may
simply not have the dependent driver. This can happen if using a newer DT
(provided by firmware perhaps) with a stable kernel version.
Unfortunately, this change breaks with modules as we have no way of
knowing when modules are done loading. One possibility is to make this
opt in or out based on compatible strings rather than at a subsystem
level.
Ideally this information could be extracted automatically somehow. OTOH,
maybe the lists are pretty small. There's only a handful of subsystems
that can be optional, and then only so many drivers in those that can be
modules (at least for pinctrl, many drivers are built-in only).
Ooh, this is exactly what I wanted for of_iommu_xlate(), and would be much
nicer than the current bodge using system_state that I ended up with. The
context there is very similar - if the device has a parent IOMMU then we
want to wait for that to probe first if possible, but with a deadline such
that if it doesn't show up then we can go ahead and make progress without it
(on the assumption that DMA ops can fall back to CMA). The modules problem
doesn't currently apply to IOMMU drivers either, although we do use a
special of_device_id table for detecting built-in drivers via
OF_IOMMU_DECLARE() to avoid deferring at all when we know it would be
pointless - a more generic solution for that could certainly be useful too.
Ah, so you kept the IOMMU_OF_DECLARE() but it does nothing but define
a table. We already have the driver match table which should pretty
much be the same data, so it would be better if we could use that if
possible. If we used MODULE_DEVICE_TABLE somehow, we could avoid
modifying lots of drivers. Though many built-in only drivers omit
that. The other problem is it would become a large set of tables to
search thru because it is global. That would probably end up slower
than just deferring. So we need something like
<subsystem>_DEVICE_TABLE() to have per subsystem tables. Then this
function in this patch would need to be told which table to use.
However, this is all really just an optimization to avoid deferring at
all and could be addressed later. Is there any data on how much time
you save avoiding deferring? This has come up in the past and I don't
think it is much.
In the of_iommu case it's not actually an optimisation, but dodges a big
problem with the self-contained implementation - if we blindly defer on
a not-yet-present IOMMU until all built-in drivers have had a chance to
register themselves naturally, then by the time we could safely assume
the relevant IOMMU driver *isn't* built-in, all the clients (which may
include the boot device) can already be stuck on the deferred probe list
with nothing left to kick it and make progress again. It's quite
possible I could have done better, I just wasn't very familiar with the
driver core at the time, and repurposing the magic table instead of
entirely removing it was by far the easiest way forward.
With this patch we would have a guarantee that the deferred list gets at
least one kick after the deadline for waiting has passed, so in theory
we could then just use the regular driver matching mechanisms to see
whether a given IOMMU node can possibly probe or not. I'm not sure we
could get rid of the driver introspection aspect entirely, since it
might be the case that the IOMMU itself has dependencies and winds up on
the deferred list behind one or more of its clients, in which case we'd
still want them to keep waiting even after the deadline is nominally up.
I've also been thinking about if we could use MODULE_DEVICE_TABLE to
provide a list compatible strings from modules as a whitelist of
devices to keep deferring probe on. That would require building
modules to build the kernel which I don't think would work. I think my
conclusion is that the cases we care about may be short enough to just
manually maintain such a list.
FWIW we did get rid of the equivalent table completely for the arm64
ACPI code, but that only has to support 2 types of IOMMU so just
evaluates the respective driver config symbols directly with
IS_BUILTIN(). Sadly that method really can't scale to DT with multiple
compatible strings per driver...
I guess there's also the possibility that a single driver may want
multiple behaviours, if e.g. if SoC variants A and B have some identical
peripherals but slightly different pinctrl/IOMMU/etc. hardware such that
A has workable default behaviour and can be treated as optional, whereas
B absolutely must be controlled by the kernel for the consumers to
function properly, and they *should* defer forever otherwise. I think
that would pretty much demand some sort of explicitly-curated
white/blacklist setup at the subsystem or driver level.
Robin.
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