[dpdk-dev] [PATCH v7 2/2] bus/pci: support MMIO in PCI ioport accessors

David Marchand david.marchand at redhat.com
Thu Feb 25 10:52:02 CET 2021


On Thu, Feb 25, 2021 at 5:00 AM 谢华伟(此时此刻) <huawei.xhw at alibaba-inc.com> wrote:
> >>> Is the 'outb_p' to 'outb' conversion intentional? And if so why?
> >>>
> >>> Same of the all 'outb_p', 'outw_p', 'outl_p'.
> >>
> >> There is no need to delay for virtio device, as we can see in virtio
> >> legacy driver.
> >>
> >> IMO, the delay is for ugly old device. The device itself should
> >> assure the previous IO completes when the subsequent IO instruction
> >> arrives.
> >>
> >
> > Can there be any virtio legacy device needing this?
>
> The pause version delays sometime by writing to 0x80 debug port. virtio
> doesn't need this. virtio legacy PMD driver doens't use this.
>
> Any device relying on this i think is buggy. How could the device rely
> on some uncertain cpu cycles to behave correct?
>
> >
> > What is the downside of using "pause until the I/O completes" versions?
>
> The downside in virtio PMD is a small performance penalty when we use it
> to notify backend. CPU executes unnecessary serializing IO instruction.
>
> I check kernel code, io wrapper for in/out doesn't use p version.

This change is a fix/optimisation.
This is a separate topic from adding MMIO support with x86 ioport.
I would split as a separate patch.


-- 
David Marchand



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