[PATCH v9 2/4] iommu/dma: Add a helper function to reserve HW MSI address regions for IOMMU drivers

[Shameerali Kolothum Thodi]

> -----Original Message-----
> From: Robin Murphy [mailto:robin.murphy at arm.com]
> Sent: Wednesday, October 18, 2017 1:34 PM
> To: Shameerali Kolothum Thodi <shameerali.kolothum.thodi at huawei.com>;
> Will Deacon <will.deacon at arm.com>
> Cc: lorenzo.pieralisi at arm.com; Gabriele Paoloni
> <gabriele.paoloni at huawei.com>; marc.zyngier at arm.com; linux-
> pci at vger.kernel.org; joro at 8bytes.org; John Garry <john.garry at huawei.com>;
> Guohanjun (Hanjun Guo) <guohanjun at huawei.com>; Linuxarm
> <linuxarm at huawei.com>; linux-acpi at vger.kernel.org; iommu at lists.linux-
> foundation.org; Wangzhou (B) <wangzhou1 at hisilicon.com>;
> sudeep.holla at arm.com; bhelgaas at google.com; linux-arm-
> kernel at lists.infradead.org; devel at acpica.org
> Subject: Re: [PATCH v9 2/4] iommu/dma: Add a helper function to reserve HW
> MSI address regions for IOMMU drivers
> 
> On 16/10/17 17:09, Shameerali Kolothum Thodi wrote:
> > Hi Robin,
> >
> >> -----Original Message-----
> >> From: Will Deacon [mailto:will.deacon at arm.com]
> >> Sent: Friday, October 13, 2017 8:24 PM
> >> To: Shameerali Kolothum Thodi <shameerali.kolothum.thodi at huawei.com>
> >> Cc: lorenzo.pieralisi at arm.com; marc.zyngier at arm.com;
> >> sudeep.holla at arm.com; robin.murphy at arm.com; joro at 8bytes.org;
> >> bhelgaas at google.com; Gabriele Paoloni <gabriele.paoloni at huawei.com>;
> >> John Garry <john.garry at huawei.com>; iommu at lists.linux-foundation.org;
> >> linux-arm-kernel at lists.infradead.org; linux-acpi at vger.kernel.org; linux-
> >> pci at vger.kernel.org; devel at acpica.org; Linuxarm
> <linuxarm at huawei.com>;
> >> Wangzhou (B) <wangzhou1 at hisilicon.com>; Guohanjun (Hanjun Guo)
> >> <guohanjun at huawei.com>
> >> Subject: Re: [PATCH v9 2/4] iommu/dma: Add a helper function to reserve
> HW
> >> MSI address regions for IOMMU drivers
> >>
> >> On Fri, Oct 06, 2017 at 03:04:48PM +0100, Shameer Kolothum wrote:
> >>> IOMMU drivers can use this to implement their .get_resv_regions callback
> >>> for HW MSI specific reservations(e.g. ARM GICv3 ITS MSI region).
> >>>
> >>> Signed-off-by: Shameer Kolothum
> <shameerali.kolothum.thodi at huawei.com>
> >>> ---
> >>>  drivers/iommu/dma-iommu.c | 20 ++++++++++++++++++++
> >>>  include/linux/dma-iommu.h |  7 +++++++
> >>>  2 files changed, 27 insertions(+)
> >>
> >> I'd like to see Robin's Ack on this, because this is his code and he had
> >> ideas on ways to solve this problem properly.
> >
> > Please let us know if it is ok to go ahead with ACPI support for now.
> > It will help our customers to start using pass-through for PCIe.
> >
> > Thanks,
> > Shameer
> >
> >>
> >> Will
> >>
> >>> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> >>> index 9d1cebe..bae677e 100644
> >>> --- a/drivers/iommu/dma-iommu.c
> >>> +++ b/drivers/iommu/dma-iommu.c
> >>> @@ -19,6 +19,7 @@
> >>>   * along with this program.  If not, see <http://www.gnu.org/licenses/>.
> >>>   */
> >>>
> >>> +#include <linux/acpi_iort.h>
> >>>  #include <linux/device.h>
> >>>  #include <linux/dma-iommu.h>
> >>>  #include <linux/gfp.h>
> >>> @@ -27,6 +28,7 @@
> >>>  #include <linux/iova.h>
> >>>  #include <linux/irq.h>
> >>>  #include <linux/mm.h>
> >>> +#include <linux/of_iommu.h>
> >>>  #include <linux/pci.h>
> >>>  #include <linux/scatterlist.h>
> >>>  #include <linux/vmalloc.h>
> >>> @@ -198,6 +200,24 @@ void iommu_dma_get_resv_regions(struct device
> >> *dev, struct list_head *list)
> >>>  }
> >>>  EXPORT_SYMBOL(iommu_dma_get_resv_regions);
> >>>
> >>> +/**
> >>> + * iommu_dma_get_msi_resv_regions - Reserved region driver helper
> >>> + * @dev: Device from iommu_get_resv_regions()
> >>> + * @list: Reserved region list from iommu_get_resv_regions()
> >>> + *
> >>> + * IOMMU drivers can use this to implement their .get_resv_regions
> >>> + * callback for HW MSI specific reservations. For now, this only
> 
> This doesn't make an awful lot of sense - there's only one reserved
> region callback, so iommu-dma shouldn't be offering two separate and
> non-overlapping implementations.
> 
> >>> + * covers ITS MSI region reservation using ACPI IORT helper function.
> >>> + */
> >>> +int iommu_dma_get_msi_resv_regions(struct device *dev, struct
> list_head
> >> *list)
> >>> +{
> >>> +	if (!is_of_node(dev->iommu_fwspec->iommu_fwnode))
> >>> +		return iort_iommu_msi_get_resv_regions(dev, list);
> 
> Either this call knows how to do the right thing for any platform and
> should be made from iommu_dma_get_reserved_regions() directly, or it's
> tightly coupled to the HiSilicon quirk in the SMMUv3 driver and
> iommu-dma doesn't need to know - the middle ground presented here is
> surely the worst of both worlds.

Right. I think we have discussed this earlier[1] and had a v4 based on invoking
the iort_iommu_its_get_resv_regions() within the iommu_dma_get_resv_regions().
But later as you rightly pointed out, we were not checking for platforms which
requires this quirk inside the iort code and that will break the platforms which are 
happy with MSI translations. Hence moved to the current implementation in v6
after this discussion here[2]
 
And earlier I think in the v3 version we had the function called from smmu driver
directly and the feedback was that it should be abstracted from the driver.

May be it is still possible to move the function call inside the 
iommu_dma_get_resv_regions() and do the smmu model check inside
 the iort helper function and selectively apply the HW MSI reservations.

But I think it is much neater if we can invoke the iort_get_msi_regions() directly
from SMMUv3 based on the model.

Thoughts?

Thanks,
Shameer
1. https://patches.linaro.org/patch/106268/
2. https://www.spinics.net/lists/arm-kernel/msg599162.html