Re: [Stratos-dev] [PATCH V3 1/3] gpio: Add virtio-gpio driver

[Viresh Kumar]

On 11-06-21, 10:34, Arnd Bergmann wrote:
> Extraneous __packed annotations do cause real problems:
> 
> - On architectures without hardware unaligned accesses, the compiler is
>   forced to emit byte load/store instructions, which is slower and breaks
>   atomic updates to shared variables
> 
> - If a function takes a pointer of a packed struct member, and passes that
>   pointer to a function that expects a regular aligned pointer, you
> get undefined
>   behavior. Newer compilers produce a warning if you do that (we currently
>   shut up that warning because there are many false positives in the kernel),
>   but you can also run into CPU exceptions or broken code even on CPUs
>   that do support unaligned accesses when the variable ends up being
>   actually unaligned (as you just told the compiler that it is allowed to do).

I understand that these problems will happen if the structure isn't
aligned, but in this case the structure is aligned properly, just that
we are explicitly telling the compiler to not add any padding (it
won't have added any in here). Is it still harmful ?

-- 
viresh
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Re: [Stratos-dev] [PATCH V3 1/3] gpio: Add virtio-gpio driver

[Jean-Philippe Brucker]

On Thu, Jun 10, 2021 at 04:00:39PM +, Enrico Weigelt, metux IT consult via 
Stratos-dev wrote:
> On 10.06.21 15:22, Arnd Bergmann wrote:
> 
> > Can you give an example of how this would be hooked up to other drivers
> > using those gpios. Can you give an example of how using the "gpio-keys" or
> > "gpio-leds" drivers in combination with virtio-gpio looks like in the DT?
> 
> Connecting between self-probing bus'es and DT is generally tricky. IMHO
> we don't have any generic mechanism for that.

DT does have a generic description of PCI endpoints, which virtio-iommu
relies on to express the relation between IOMMU and endpoint nodes [1].
I think the problem here is similar: the client node needs a phandle to
the GPIO controller which may use virtio-pci transport?

Note that it mostly works if the device is on the root PCI bus. Behind a
bridge the OS may change the device's bus number as needed, so the BDF
reference in DT is only valid if the software providing the DT description
(VMM or firmware) initializes bus numbers accordingly (and I don't
remember if Linux supports this case well).

Thanks,
Jean

[1] Documentation/devicetree/bindings/virtio/iommu.txt

> 
> I've made a few attempts, but nothing practically useful, which would be
> accepted by the corresponding maintainers, yet. We'd either need some
> very special logic in DT probing or pseudo-bus'es for the mapping.
> (DT wants to do those connections via phandle's, which in turn need the
> referenced nodes to be present in the DT).
> 
> >  From what I can tell, both the mmio and pci variants of virtio can have 
> > their
> > dev->of_node populated, but I don't see the logic in 
> > register_virtio_device()
> > that looks up the of_node of the virtio_device that the of_gpio code then
> > tries to refer to.
> 
> Have you ever successfully bound a virtio device via DT ?
> 
> 
> --mtx
> 
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> Enrico Weigelt, metux IT consult
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