[dpdk-dev] [PATCH v9 2/9] eal/pci: get iommu class

Thomas Monjalon thomas at monjalon.net
Fri Oct 6 09:24:24 CEST 2017

06/10/2017 05:04, santosh:
> Thomas,
> You comment is annoying and infuriating both.
> Patch is their for more than 4month, had enough time for you to comment
> and understand the topic. Thorough review and testing has happened both.
> NOTE: You have already delayed this series by one release and
> I'm guessing that you intent to push by one more, if you had such
> mundane question then why not ask before? Make me think that you are
> wasting my time and effort both.

You misunderstand me.
My intent is to push this patch.
A lot of people have reviewed it during this cycle.
I was just looking for wording details in order to ease people
when they will see this abstraction in the code base.

> On Friday 06 October 2017 05:28 AM, Thomas Monjalon wrote:
> > This patch is introducing a new abstraction.
> > It is important to explain it for future readers of this code.
> If you don't know - What is iova? How to program iova?
> purpose of iova then should read and educate your know - how first.
> Yes, its is introducing new abstraction, because dpdk from
> ancient days does only one programming mode aka iova=pa.
> note:You were still using iova mode as _pa (and didn't care to ask yourself about IOVA!)
> which is one of iova mode too!.
> However, IOMMU can also generate _va address too called iova=_va mode..
> which is also correct/viable/applicable/Okiesh programming mode
> for iommu capable HW like dma for example(Note again,.. AGNOSTIC behavior of iommu).
> Now Why dpdk needs to understand IOVA programming philosophy:
> Though DPDK was _silenty_ using iova as pa mode but then there
> is a need arise to make mapping mode explicit and for that we need
> abstraction since there wasn't one existed.
> Reason:
> Because From last few years,.ONA participants like Cavium, nxp
> added ARM arch support in dpdk and included drivers for their HW.. 
> and their hw has use-case (example external mempool), such a way that
> programming those HW in iova as va mode would save cycle in fast path
> (this part, we explained so many-1000 time in series and same understood by reviewer)
> thus its vital to introduce iova infra in dpdk.
> Same applicable for intel HW blocks too. Its works for intel too!

I know all of that!
I was just thinking that you could add more explanations somewhere
in the code or the doc.

> > 20/09/2017 13:23, Santosh Shukla:
> >> +/**
> >> + * IOVA mapping mode.
> >> + */
> > Please explain what IOVA means and what is the purpose of
> > distinguish the different modes.
> >
> IOVA mapping mode is device aka iommu programming mode by which
> HW(iommu) will generate _pa or _va address accordingly.

In this doxygen block, it would be the right place to explain how the
IOVA mode will impact the rest of DPDK.

> >> +enum rte_iova_mode {
> >> +	RTE_IOVA_DC = 0,	/* Don't care mode */
> >> +	RTE_IOVA_PA = (1 << 0),
> >> +	RTE_IOVA_VA = (1 << 1)
> >> +};
> > You should explain each value of the enum.
> Aren't naming choice for each member of enum is self-explanatory?
> I don't find logic anymore in your question? are you asking about side commenting?
> if not then IFAIU, you question is basically about what is _pa and _va? if so then
> reader should have little know-how before they intent to do fast-path programming.
> Author can't write whole IOMMU spec for reader sake. Those are minute and mundate info
> incase any user want to program device in _pa or _va. I'm at loss with you question,
> I don;t see logic and it is frustrating to me. You had enough time for all this
> in case you had really cared,, we have series for external PMD and drivers waiting
> for iova infra, I see it a your move nothing bu blocking ONA series progress
> Don;t you trust Reviewer in case you have hard time understaing topic and that
> makese me to ask - Are you willing to accept this feature or not? if not then
> I'm wasting my energy on it.

Santosh, I'm sorry if you don't understand that I was just asking for
a bit more doc.
You could just add something like
	/* DMA using physical address */
	/* DMA using virtual address */

Anyway, if you don't want to add any explanation, it won't prevent
pushing this patch.

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