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

[Saravana Kannan]

On Wed, Sep 7, 2022 at 1:40 PM Robin Murphy <robin.murphy@xxxxxxx> wrote:
>
> On 2022-09-07 19:24, Saravana Kannan wrote:
> > On Wed, Sep 7, 2022 at 1:27 AM Robin Murphy <robin.murphy@xxxxxxx> wrote:
> >>
> >> On 2022-09-07 04:17, Gupta, Nipun wrote:
> >>> [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>
> >>>> Cc: robh+dt@xxxxxxxxxx; krzysztof.kozlowski+dt@xxxxxxxxxx;
> >>>> gregkh@xxxxxxxxxxxxxxxxxxx; rafael@xxxxxxxxxx; eric.auger@xxxxxxxxxx;
> >>>> alex.williamson@xxxxxxxxxx; cohuck@xxxxxxxxxx; Gupta, Puneet (DCG-ENG)
> >>>> <puneet.gupta@xxxxxxx>; song.bao.hua@xxxxxxxxxxxxx;
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> >>>> jeffrey.l.hugo@xxxxxxxxx; Michael.Srba@xxxxxxxxx; mani@xxxxxxxxxx;
> >>>> yishaih@xxxxxxxxxx; jgg@xxxxxxxx; jgg@xxxxxxxxxx; robin.murphy@xxxxxxx;
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> >>>> Harpreet <harpreet.anand@xxxxxxx>; Agarwal, Nikhil
> >>>> <nikhil.agarwal@xxxxxxx>; Simek, Michal <michal.simek@xxxxxxx>;
> >>>> Radovanovic, Aleksandar <aleksandar.radovanovic@xxxxxxx>; git (AMD-Xilinx)
> >>>> <git@xxxxxxx>
> >>>> 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.
> >>
> >> 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?
>
> Er, it doesn't? The only non-optional bus here is platform, since the
> others *can* be configured out and *are* #ifdefed accordingly.

Ah ok. Also I somehow missed the #ifdef AMBA there and thought there
was only #ifdef PCI and the rest of the buses somehow got it working
without having to muck around arm-smmu-v3.c.

Thanks for the explanation. I'm done here :)

-Saravana

> This
> patch is fine for the kernel it was based on, it'll just want rewriting
> now that I've cleaned all this horrible driver boilerplate up. And
> according to the thread on patch #4 there might need to be additional
> changes for CDX to express a reserved MSI region for SMMU support to
> actually work properly.
>
> Robin.