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

[Gupta, Nipun]

[AMD Official Use Only - General]



> -----Original Message-----
> From: Saravana Kannan <saravanak@xxxxxxxxxx>
> Sent: Wednesday, September 7, 2022 5:41 AM
> To: Gupta, Nipun <Nipun.Gupta@xxxxxxx>
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> 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@xxxxxxx> 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@xxxxxxx>
> > ---
> >  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.

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
> >