On Sat, Jul 3, 2021 at 5:54 PM Morten Brørup [off-list ref] wrote:

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From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Jerin Jacob
Sent: Saturday, 3 July 2021 11.09

On Sat, Jul 3, 2021 at 2:23 PM Morten Brørup [off-list ref]
wrote:

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From: fengchengwen [mailto:fengchengwen@huawei.com]
Sent: Saturday, 3 July 2021 02.32

On 2021/7/2 22:57, Morten Brørup wrote:

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In the DPDK framework, many data-plane API names contain queues.

e.g.

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eventdev/crypto..
The concept of virt queues has continuity.

I was also wondering about the name "virtual queue".

Usually, something "virtual" would be an abstraction of something

physical, e.g. a software layer on top of something physical.

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Back in the days, a "DMA channel" used to mean a DMA engine on a

CPU.

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If a CPU had 2 DMA channels, they could both be set up

simultaneously.

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The current design has the "dmadev" representing a CPU or other

chip,

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which has one or more "HW-queues" representing DMA channels (of the
same type), and then "virt-queue" as a software abstraction on top,

for

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using a DMA channel in different ways through individually

configured

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contexts (virt-queues).

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It makes sense to me, although I would consider renaming "HW-

queue"

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to "channel" and perhaps "virt-queue" to "queue".

The 'DMA channel' is more used than 'DMA queue', at least google

show

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that there are at least 20+ times more.

It's a good idea build the abstraction layer: queue <> channel <>

dma-

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controller.
In this way, the meaning of each layer is relatively easy to
distinguish literally.

will fix in V2

After re-reading all the mails in this thread, I have found one more

important high level detail still not decided:

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Bruce had suggested flattening the DMA channels, so each dmadev

represents a DMA channel. And DMA controllers with multiple DMA
channels will have to instantiate multiple dmadevs, one for each DMA
channel.

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Just like a four port NIC instantiates four ethdevs.

Then, like ethdevs, there would only be two abstraction layers:

dmadev <> queue, where a dmadev is a DMA channel on a DMA controller.

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However, this assumes that the fast path functions on the individual

DMA channels of a DMA controller can be accessed completely
independently and simultaneously by multiple threads. (Otherwise, the
driver would need to implement critical regions or locking around
accessing the common registers in the DMA controller shared by the DMA
channels.)

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Unless any of the DMA controller vendors claim that this assumption

about independence of the DMA channels is wrong, I strongly support
Bruce's flattening suggestion.

It is wrong from alteast octeontx2_dma PoV.

# The PCI device is DMA controller where the driver/device is
mapped.(As device driver is based on PCI bus, We dont want to have
vdev for this)
# The PCI device has HW queue(s)
# Each HW queue has different channels.

In the current configuration, we have only one queue per device and it
has 4 channels. 4 channels are not threaded safe as it is based on
single queue.

Please clarify "current configuration": Is that a configuration modifiable by changing some software/driver, or is it the chip that was built that way in the RTL code?

We have 8 queues per SoC, Based on some of HW versions it can be
configured as (a) or (b) using FW settings.
a) One PCI devices with 8 Queues
b) 8 PCI devices with each one has one queue.

Everyone is using mode (b) as it helps 8 different applications to use
DMA as if one application binds the PCI device other applications can
not use the same PCI device.
If one application needs 8 queues, it is possible that 8 dmadevice can
be bound to a single application with mode (b).


I think, in above way we can flatten to <device> <> <channel/queue>

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I think, if we need to flatten it, I think, it makes sense to have
dmadev <> channel (and each channel can have thread-safe capability
based on how it mapped on HW queues based on the device driver
capability).

The key question is how many threads can independently call data-plane dmadev functions (rte_dma_copy() etc.) simultaneously. If I understand your explanation correctly, only one - because you only have one DMA device, and all access to it goes through a single hardware queue.

I just realized that although you only have one DMA Controller with only one HW queue, your four DMA channels allows four sequentially initiated transactions to be running simultaneously. Does the application have any benefit by knowing that the dmadev can have multiple ongoing transactions, or can the fast-path dmadev API hide that ability?

In my view it is better to hide and I have similar proposal at
http://mails.dpdk.org/archives/dev/2021-July/213141.html
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  7) Because data-plane APIs are not thread-safe, and user could determine
     virt-queue to HW-queue's map (at the queue-setup stage), so it is user's
     duty to ensure thread-safe.

+1. But I am not sure how easy for the fast-path application to have this logic,
Instead, I think, it is better to tell the capa for queue by driver
and in channel configuration,
the application can request for requirement (Is multiple producers enq
to the same HW queue or not).
Based on the request, the implementation can pick the correct function
pointer for enq.(lock vs lockless version if HW does not support
lockless)

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