Re: [RFC PATCH 1/5] base: dma-mapping: Export commonly used symbols

[Tomasz Figa <tfiga@chromium.org>]

On Thu, Jul 6, 2017 at 5:26 PM, Arnd Bergmann <arnd@arndb.de> wrote:
> On Thu, Jul 6, 2017 at 3:44 AM, Tomasz Figa <tfiga@chromium.org> wrote:
>> On Thu, Jul 6, 2017 at 2:20 AM, Christoph Hellwig <hch@lst.de> wrote:
>>> On Thu, Jul 06, 2017 at 12:22:35AM +0900, Tomasz Figa wrote:
>
>>> In general I think moving dma
>>> ops and iommu implementations into modules is a bad idea
>>
>> Could you elaborate on this? I'd be interested in seeing the reasoning
>> behind this.
>>
>>> but I
>>> don't want to reject the idea before seeing the code.  Or maybe
>>> by looking at the user we can come up with an even better idea
>>> to solve the original issue you're trying to solve, so please also
>>> explain your rationale.
>
> I had pretty much the same thoughts here.
>
>> Basically we have an x86 platform with a camera subsystem that is a
>> PCI device, has its own MMU and needs cache maintenance. Moreover, the
>> V4L2 subsystem, which is the right place for camera drivers, heavily
>> relies on DMA mapping as a way to abstract memory allocation, mapping
>> and cache maintenance. So it feels natural to me to hide the hardware
>> details (additional cache maintenance, mapping into the built-in
>> IOMMU) in the DMA mapping ops for this camera subsystem and simply
>> make V4L2 just work without knowing those details.
>
> I can understand your reasoning here, but I'm also not convinced
> that this is the best approach. There may be a middle ground somewhere
> though.
>
> Generally speaking I don't want to have to deal with the horrors of
> deciding whether an IOMMU is going to be there eventually or not
> at probe() time. At some point, we had decided that IOMMUs need to
> be initialized (almost) as early as irqchips and clocksources so we can
> rely on them to be there at device discovery time. That got pushed
> back already, and now we may have to deal with -EPROBE_DEFER
> when an IOMMU has not been fully initialized at device probe time,
> but at least we can reliably see if one is there or not. Making IOMMUs
> modular will add further uncertainty here. Obviously we cannot attach
> an IOMMU to a device once we have started using DMA mapping
> calls on it.

The hardware can only work with IOMMU and so the main module is highly
tied with the IOMMU module and it initialized it directly. There is no
separate struct driver or device associated with the IOMMU, as it's a
part of the one and only one PCI device (as visible from the system
PCI bus point of view) and technically handled by one pci_driver.

>
> For your particular use case, I would instead leave the knowledge
> about the IOMMU in the driver itself, like we do for the IOMMUs
> that are integrated in desktop GPUs, and have the code use the
> DMA mapping API with the system-provided dma_map_ops to
> get dma_addr_t tokens which you then program into the device
> IOMMU.
>
> An open question however would be whether to use the IOMMU
> API without the DMA mapping API here, or whether to completely
> leave the knowledge of the IOMMU inside of the driver itself.
> I don't have a strong opinion on that part, and I guess this mostly
> depends on what the hardware looks like.

+ linux-media and some media folks

I'd say that this is something that has been consistently tried to be
avoided by V4L2 and that's why it's so tightly integrated with DMA
mapping. IMHO re-implementing the code that's already there in
videobuf2 again in the driver, only because, for no good reason
mentioned as for now, having a loadable module providing DMA ops was
disliked.

Similarly with IOMMU API. It provides a lot of help in managing the
mappings and re-implementing this would be IMHO backwards.

Best regards,
Tomasz