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

[Robin Murphy <robin.murphy@arm.com>]

On 2022-09-07 19:24, Saravana Kannan wrote:
> 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?

Er, it doesn't? The only non-optional bus here is platform, since the 
others *can* be configured out and *are* #ifdefed accordingly. 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.

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