linux-kernel - Re: [PATCH v3 06/20] iommu/mtk: Remove detach

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Date:   Tue, 29 Nov 2022 11:45:45 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Baolu Lu <baolu.lu@...ux.intel.com>,
        Jason Gunthorpe <jgg@...dia.com>
Cc:     Joerg Roedel <joro@...tes.org>,
        Christoph Hellwig <hch@...radead.org>,
        Kevin Tian <kevin.tian@...el.com>,
        Will Deacon <will@...nel.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        Hector Martin <marcan@...can.st>,
        Sven Peter <sven@...npeter.dev>,
        Rob Clark <robdclark@...il.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Yong Wu <yong.wu@...iatek.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Heiko Stuebner <heiko@...ech.de>,
        Matthew Rosato <mjrosato@...ux.ibm.com>,
        Orson Zhai <orsonzhai@...il.com>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Chunyan Zhang <zhang.lyra@...il.com>,
        Chen-Yu Tsai <wens@...e.org>,
        Thierry Reding <thierry.reding@...il.com>,
        iommu@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 06/20] iommu/mtk: Remove detach_dev callback

On 2022-11-29 02:07, Baolu Lu wrote:
> On 11/28/22 9:59 PM, Robin Murphy wrote:
>> On 2022-11-28 13:49, Jason Gunthorpe wrote:
>>> On Mon, Nov 28, 2022 at 02:46:34PM +0800, Lu Baolu wrote:
>>>> The IOMMU driver supports default domain, so the detach_dev op will 
>>>> never
>>>> be called. Remove it to avoid dead code.
>>>>
>>>> Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>
>>>> ---
>>>>   drivers/iommu/mtk_iommu.c | 9 ---------
>>>>   1 file changed, 9 deletions(-)
>>>
>>> I listed this driver as not supporting default domains:
>>>
>>> https://lore.kernel.org/linux-iommu/20220516135741.GV1343366@nvidia.com/
>>>
>>> ?
>>>
>>> Has something changed? Did I get it wrong?
>>
>> static struct iommu_domain *mtk_iommu_domain_alloc(unsigned type)
>> {
>>      struct mtk_iommu_domain *dom;
>>
>>      if (type != IOMMU_DOMAIN_DMA && type != IOMMU_DOMAIN_UNMANAGED)
>>          return NULL;
>> ...
>>
>>
>> This one runs on arm64, so has always supported default domains for 
>> iommu-dma to work.
> 
> This, together with several other ones, only support IOMMU_DOMAIN_DMA
> type of default domain. The iommu core handle this by falling back to
> IOMMU_DOMAIN_DMA if other types fail.
> 
>          dom = __iommu_domain_alloc(bus, type);
>          if (!dom && type != IOMMU_DOMAIN_DMA) {
>                  dom = __iommu_domain_alloc(bus, IOMMU_DOMAIN_DMA);
>                  if (dom)
>                          pr_warn("Failed to allocate default IOMMU 
> domain of type %u for group %s - Falling back to IOMMU_DOMAIN_DMA",
>                                  type, group->name);
>          }
> 
> I have another cleanup series:
> 
> https://github.com/LuBaolu/intel-iommu/commits/iommu-use-def_default_type-wip
> 
> which adds IOMMU_DOMAIN_DMA default domain type requirement in the
> def_domain_type callback. I planed to bring that to discussion after
> this one.

Per the discussion over on the s390 thread, I think that would be a step 
in the wrong direction. I'd prefer to keep .def_domain_type for 
device-specific requirements and express general driver domain support a 
different way. If the IOMMU_DOMAIN_DMA fallback is worth removing then 
the one for IOMMU_DOMAIN_BLOCKED is as well - no point doing half the job ;)

Thanks,
Robin.