Re: [RFC PATCH v2 03/15] arm64: mm: change IOMMU notifier action to attach DMA ops

[Lorenzo Pieralisi]

Hi Marek,

On Tue, Jun 21, 2016 at 09:53:20AM +0200, Marek Szyprowski wrote:
> Hi Robin,
> 
> 
> On 2016-06-17 11:27, Robin Murphy wrote:
> >Hi Lorenzo,
> >
> >I think this patch makes sense even independent of the rest of the
> >series, one nit inline notwithstanding.
> >
> >Marek; I'm curious as to whether this could make the workaround in
> >722ec35f7 obsolete as well, or are all the drivers also bound
> >super-early in the setup you had there?
> 
> Yes, this will solve that problem too. I will also hide some possible
> deferred probe issues, because the moment at which IOMMU is activated
> will be postponed. The only drawback with this approach is the fact
> that is drivers won't be allowed to do any dma-mapping operations on
> devices, which they don't own. This should not be a big issue, but
> this was the reason to setup IOMMU on device add instead of driver
> bind.
> 
> While at it, please make sure that the case of failed client driver
> probe will be handled properly. IOMMU might do some operations while
> setting up and if the client driver fails to probe (for whatever
> reason, might be a deferred probe too), those operation has to be
> undone. However the current code of the driver core won't call any
> notifier (like BUS_NOTIFY_UNBOUND_DRIVER or whatever else) in such
> case.

Isn't Andy's commit 14b6257a5f3d enough ? Is that what you had in
mind ?

> Long time ago I used BUS_NOTIFY_BIND_DRIVER based approach for my
> Exynos IOMMU patches and had to extend bus core with such patch:
> https://patchwork.kernel.org/patch/4678181/ to properly cleanup
> after failed client driver probe and avoid leaking resources. Please
> read the discussion, because some changes were requested to it.

It looks like commit 14b6257a5f3d ("device core: add
BUS_NOTIFY_DRIVER_NOT_BOUND notification") does what you
are requesting, please let me know if that's enough.

I will revert the changes in 722ec35f7 and fold them in the
new version along with Robin's suggestions.

Thanks,
Lorenzo

> 
> 
> Best regards
> Marek Szyprowski, PhD
> Samsung R&D Institute Poland
> 
> >
> >On 07/06/16 14:30, Lorenzo Pieralisi wrote:
> >>Current bus notifier in ARM64 (__iommu_attach_notifier)
> >>attempts to attach dma_ops to a device on BUS_NOTIFY_ADD_DEVICE
> >>action notification.
> >>
> >>This causes issues on ACPI based systems, where PCI devices
> >>can be added before the IOMMUs the devices are attached to
> >>had a chance to be probed, causing failures on attempts to
> >>attach dma_ops in that the domain for the respective IOMMU
> >>may not be set-up yet by the time the bus notifier is run.
> >>
> >>Devices dma_ops do not require to be set-up till the matching
> >>device drivers are probed. This means that instead of running
> >>the notifier attaching dma_ops to devices (__iommu_attach_notifier)
> >>on BUS_NOTIFY_ADD_DEVICE action, it can be run just before the
> >>device driver is bound to the device in question (on action
> >>BUS_NOTIFY_BIND_DRIVER) so that it is certain that its IOMMU
> >>group and domain are set-up accordingly at the time the
> >>notifier is triggered.
> >>
> >>This patch changes the notifier action upon which dma_ops
> >>are attached to devices and defer it to driver binding time,
> >>so that IOMMU devices have a chance to be probed and to register
> >>their bus notifiers before the dma_ops attach sequence for a
> >>device is actually carried out.
> >>
> >>Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@xxxxxxx>
> >>Cc: Will Deacon <will.deacon@xxxxxxx>
> >>Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> >>Cc: Robin Murphy <robin.murphy@xxxxxxx>
> >>---
> >>  arch/arm64/mm/dma-mapping.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >>diff --git a/arch/arm64/mm/dma-mapping.c b/arch/arm64/mm/dma-mapping.c
> >>index c566ec8..79b0882 100644
> >>--- a/arch/arm64/mm/dma-mapping.c
> >>+++ b/arch/arm64/mm/dma-mapping.c
> >>@@ -848,7 +848,7 @@ static int __iommu_attach_notifier(struct
> >>notifier_block *nb,
> >>  {
> >>      struct iommu_dma_notifier_data *master, *tmp;
> >>
> >>-    if (action != BUS_NOTIFY_ADD_DEVICE)
> >>+    if (action != BUS_NOTIFY_BIND_DRIVER)
> >
> >With this, you can also get rid of the priority setting and big
> >fat explanatory comment in register_iommu_dma_ops_notifier().
> >
> >Robin.
> >
> >>          return 0;
> >>
> >>      mutex_lock(&iommu_dma_notifier_lock);
> >>
> >
> >
> >
>