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

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Date:   Wed, 7 Sep 2022 17:14:20 -0700
From:   Saravana Kannan <saravanak@...gle.com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     "Gupta, Nipun" <Nipun.Gupta@....com>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "krzysztof.kozlowski+dt@...aro.org" 
        <krzysztof.kozlowski+dt@...aro.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "rafael@...nel.org" <rafael@...nel.org>,
        "eric.auger@...hat.com" <eric.auger@...hat.com>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "cohuck@...hat.com" <cohuck@...hat.com>,
        "Gupta, Puneet (DCG-ENG)" <puneet.gupta@....com>,
        "song.bao.hua@...ilicon.com" <song.bao.hua@...ilicon.com>,
        "mchehab+huawei@...nel.org" <mchehab+huawei@...nel.org>,
        "maz@...nel.org" <maz@...nel.org>,
        "f.fainelli@...il.com" <f.fainelli@...il.com>,
        "jeffrey.l.hugo@...il.com" <jeffrey.l.hugo@...il.com>,
        "Michael.Srba@...nam.cz" <Michael.Srba@...nam.cz>,
        "mani@...nel.org" <mani@...nel.org>,
        "yishaih@...dia.com" <yishaih@...dia.com>,
        "jgg@...pe.ca" <jgg@...pe.ca>, "jgg@...dia.com" <jgg@...dia.com>,
        "will@...nel.org" <will@...nel.org>,
        "joro@...tes.org" <joro@...tes.org>,
        "masahiroy@...nel.org" <masahiroy@...nel.org>,
        "ndesaulniers@...gle.com" <ndesaulniers@...gle.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kbuild@...r.kernel.org" <linux-kbuild@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "okaya@...nel.org" <okaya@...nel.org>,
        "Anand, Harpreet" <harpreet.anand@....com>,
        "Agarwal, Nikhil" <nikhil.agarwal@....com>,
        "Simek, Michal" <michal.simek@....com>,
        "Radovanovic, Aleksandar" <aleksandar.radovanovic@....com>,
        "git (AMD-Xilinx)" <git@....com>
Subject: Re: [RFC PATCH v3 3/7] iommu/arm-smmu-v3: support ops registration
 for CDX bus

On Wed, Sep 7, 2022 at 1:40 PM Robin Murphy <robin.murphy@....com> wrote:
>
> On 2022-09-07 19:24, Saravana Kannan wrote:
> > On Wed, Sep 7, 2022 at 1:27 AM Robin Murphy <robin.murphy@....com> wrote:
> >>
> >> On 2022-09-07 04:17, Gupta, Nipun wrote:
> >>> [AMD Official Use Only - General]
> >>>
> >>>
> >>>
> >>>> -----Original Message-----
> >>>> From: Saravana Kannan <saravanak@...gle.com>
> >>>> Sent: Wednesday, September 7, 2022 5:41 AM
> >>>> To: Gupta, Nipun <Nipun.Gupta@....com>
> >>>> Cc: robh+dt@...nel.org; krzysztof.kozlowski+dt@...aro.org;
> >>>> gregkh@...uxfoundation.org; rafael@...nel.org; eric.auger@...hat.com;
> >>>> alex.williamson@...hat.com; cohuck@...hat.com; Gupta, Puneet (DCG-ENG)
> >>>> <puneet.gupta@....com>; song.bao.hua@...ilicon.com;
> >>>> mchehab+huawei@...nel.org; maz@...nel.org; f.fainelli@...il.com;
> >>>> jeffrey.l.hugo@...il.com; Michael.Srba@...nam.cz; mani@...nel.org;
> >>>> yishaih@...dia.com; jgg@...pe.ca; jgg@...dia.com; robin.murphy@....com;
> >>>> will@...nel.org; joro@...tes.org; masahiroy@...nel.org;
> >>>> ndesaulniers@...gle.com; linux-arm-kernel@...ts.infradead.org; linux-
> >>>> kbuild@...r.kernel.org; linux-kernel@...r.kernel.org;
> >>>> devicetree@...r.kernel.org; kvm@...r.kernel.org; okaya@...nel.org; Anand,
> >>>> Harpreet <harpreet.anand@....com>; Agarwal, Nikhil
> >>>> <nikhil.agarwal@....com>; Simek, Michal <michal.simek@....com>;
> >>>> Radovanovic, Aleksandar <aleksandar.radovanovic@....com>; git (AMD-Xilinx)
> >>>> <git@....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@....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@....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?
>
> 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.