Re: [RFC PATCH v3 3/7] iommu/arm-smmu-v3: support ops registration for CDX bus

[Saravana Kannan <saravanak@google.com>]

On Wed, Sep 7, 2022 at 1:27 AM Robin Murphy <robin.murphy@arm.com> wrote:
>
> On 2022-09-07 04:17, Gupta, Nipun wrote:
> > [AMD Official Use Only - General]
> >
> >
> >
> >> -----Original Message-----
> >> From: Saravana Kannan <saravanak@google.com>
> >> Sent: Wednesday, September 7, 2022 5:41 AM
> >> To: Gupta, Nipun <Nipun.Gupta@amd.com>
> >> Cc: robh+dt@kernel.org; krzysztof.kozlowski+dt@linaro.org;
> >> gregkh@linuxfoundation.org; rafael@kernel.org; eric.auger@redhat.com;
> >> alex.williamson@redhat.com; cohuck@redhat.com; Gupta, Puneet (DCG-ENG)
> >> <puneet.gupta@amd.com>; song.bao.hua@hisilicon.com;
> >> mchehab+huawei@kernel.org; maz@kernel.org; f.fainelli@gmail.com;
> >> jeffrey.l.hugo@gmail.com; Michael.Srba@seznam.cz; mani@kernel.org;
> >> yishaih@nvidia.com; jgg@ziepe.ca; jgg@nvidia.com; robin.murphy@arm.com;
> >> will@kernel.org; joro@8bytes.org; masahiroy@kernel.org;
> >> ndesaulniers@google.com; linux-arm-kernel@lists.infradead.org; linux-
> >> kbuild@vger.kernel.org; linux-kernel@vger.kernel.org;
> >> devicetree@vger.kernel.org; kvm@vger.kernel.org; okaya@kernel.org; Anand,
> >> Harpreet <harpreet.anand@amd.com>; Agarwal, Nikhil
> >> <nikhil.agarwal@amd.com>; Simek, Michal <michal.simek@amd.com>;
> >> Radovanovic, Aleksandar <aleksandar.radovanovic@amd.com>; git (AMD-Xilinx)
> >> <git@amd.com>
> >> Subject: Re: [RFC PATCH v3 3/7] iommu/arm-smmu-v3: support ops registration
> >> for CDX bus
> >>
> >> [CAUTION: External Email]
> >>
> >> On Tue, Sep 6, 2022 at 6:48 AM Nipun Gupta <nipun.gupta@amd.com> wrote:
> >>>
> >>> With new CDX bus supported for AMD FPGA devices on ARM
> >>> platform, the bus requires registration for the SMMU v3
> >>> driver.
> >>>
> >>> Signed-off-by: Nipun Gupta <nipun.gupta@amd.com>
> >>> ---
> >>>   drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 16 ++++++++++++++--
> >>>   1 file changed, 14 insertions(+), 2 deletions(-)
> >>>
> >>> diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> >> b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> >>> index d32b02336411..8ec9f2baf12d 100644
> >>> --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> >>> +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> >>> @@ -29,6 +29,7 @@
> >>>   #include <linux/platform_device.h>
> >>>
> >>>   #include <linux/amba/bus.h>
> >>> +#include <linux/cdx/cdx_bus.h>
> >>>
> >>>   #include "arm-smmu-v3.h"
> >>>   #include "../../iommu-sva-lib.h"
> >>> @@ -3690,16 +3691,27 @@ static int arm_smmu_set_bus_ops(struct
> >> iommu_ops *ops)
> >>>                  if (err)
> >>>                          goto err_reset_pci_ops;
> >>>          }
> >>> +#endif
> >>> +#ifdef CONFIG_CDX_BUS
> >>> +       if (cdx_bus_type.iommu_ops != ops) {
> >>> +               err = bus_set_iommu(&cdx_bus_type, ops);
> >>> +               if (err)
> >>> +                       goto err_reset_amba_ops;
> >>> +       }
> >>
> >> I'm not an expert on IOMMUs, so apologies if the question is stupid.
> >>
> >> Why does the CDX bus need special treatment here (like PCI) when there
> >> are so many other busses (eg: I2C, SPI, etc) that don't need any
> >> changes here?
> >
> > AFAIU, the devices on I2C/SPI does not use SMMU. Apart from PCI/AMBA,
> > FSL-MC is another similar bus (on SMMUv2) which uses SMMU ops.
> >
> > The devices here are behind SMMU. Robin can kindly correct or add
> > more here from SMMU perspective.
>
> Indeed, there is no need to describe and handle how DMA may or may not
> be translated for I2C/SPI/USB/etc. because they are not DMA-capable
> buses (in those cases the relevant bus *controller* often does DMA, but
> it does that for itself as the platform/PCI/etc. device it is).

Ok this is what I was guessing was the reason, but didn't want to make
that assumption.

So if there are other cases like AMBA, FSL-MC where the devices can do
direct DMA, why do those buses not need a #ifdef section in this
function like CDX? Or put another way, why does CDX need special treatment?

> Note that I have a series pending[1] that will make this patch a whole
> lot simpler.

Thanks for the pointer. I'll make some comments in that series about
bus notifiers.


-Saravana

>
> Thanks,
> Robin.
>
> [1]
> https://lore.kernel.org/linux-iommu/cover.1660572783.git.robin.murphy@arm.com/T/#t
>
> >
> > Thanks,
> > Nipun
> >
> >>
> >> -Saravana
> >>
> >>>   #endif
> >>>          if (platform_bus_type.iommu_ops != ops) {
> >>>                  err = bus_set_iommu(&platform_bus_type, ops);
> >>>                  if (err)
> >>> -                       goto err_reset_amba_ops;
> >>> +                       goto err_reset_cdx_ops;
> >>>          }
> >>>
> >>>          return 0;
> >>>
> >>> -err_reset_amba_ops:
> >>> +err_reset_cdx_ops:
> >>> +#ifdef CONFIG_CDX_BUS
> >>> +       bus_set_iommu(&cdx_bus_type, NULL);
> >>> +#endif
> >>> +err_reset_amba_ops: __maybe_unused;
> >>>   #ifdef CONFIG_ARM_AMBA
> >>>          bus_set_iommu(&amba_bustype, NULL);
> >>>   #endif
> >>> --
> >>> 2.25.1
> >>>