Re: [dpdk-dev] [RFC PATCH] dmadev: introduce DMA device library

["Morten Brørup" <mb@smartsharesystems.com>]

> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Bruce Richardson
> Sent: Tuesday, 15 June 2021 18.39
> 
> On Tue, Jun 15, 2021 at 09:22:07PM +0800, Chengwen Feng wrote:
> > This patch introduces 'dmadevice' which is a generic type of DMA
> > device.
> >
> > The APIs of dmadev library exposes some generic operations which can
> > enable configuration and I/O with the DMA devices.
> >
> > Signed-off-by: Chengwen Feng <fengchengwen@huawei.com>
> > ---
> Thanks for sending this.
> 
> Of most interest to me right now are the key data-plane APIs. While we
> are
> still in the prototyping phase, below is a draft of what we are
> thinking
> for the key enqueue/perform_ops/completed_ops APIs.
> 
> Some key differences I note in below vs your original RFC:
> * Use of void pointers rather than iova addresses. While using iova's
> makes
>   sense in the general case when using hardware, in that it can work
> with
>   both physical addresses and virtual addresses, if we change the APIs
> to use
>   void pointers instead it will still work for DPDK in VA mode, while
> at the
>   same time allow use of software fallbacks in error cases, and also a
> stub
>   driver than uses memcpy in the background. Finally, using iova's
> makes the
>   APIs a lot more awkward to use with anything but mbufs or similar
> buffers
>   where we already have a pre-computed physical address.
> * Use of id values rather than user-provided handles. Allowing the
> user/app
>   to manage the amount of data stored per operation is a better
> solution, I
>   feel than proscribing a certain about of in-driver tracking. Some
> apps may
>   not care about anything other than a job being completed, while other
> apps
>   may have significant metadata to be tracked. Taking the user-context
>   handles out of the API also makes the driver code simpler.
> * I've kept a single combined API for completions, which differs from
> the
>   separate error handling completion API you propose. I need to give
> the
>   two function approach a bit of thought, but likely both could work.
> If we
>   (likely) never expect failed ops, then the specifics of error
> handling
>   should not matter that much.
> 
> For the rest, the control / setup APIs are likely to be rather
> uncontroversial, I suspect. However, I think that rather than xstats
> APIs,
> the library should first provide a set of standardized stats like
> ethdev
> does. If driver-specific stats are needed, we can add xstats later to
> the
> API.
> 
> Appreciate your further thoughts on this, thanks.
> 
> Regards,
> /Bruce

I generally agree with Bruce's points above.

I would like to share a couple of ideas for further discussion:

1. API for bulk operations.
The ability to prepare a vector of DMA operations, and then post it to the DMA driver.

2. Prepare the API for more complex DMA operations than just copy/fill.
E.g. blitter operations like "copy A bytes from the source starting at address X, to the destination starting at address Y, masked with the bytes starting at address Z, then skip B bytes at the source and C bytes at the destination, rewind the mask to the beginning of Z, and repeat D times". This is just an example.
I'm suggesting to use a "DMA operation" union structure as parameter to the command enqueue function, rather than having individual functions for each possible DMA operation.
I know I'm not the only one old enough on the mailing list to have worked with the Commodore Amiga's blitter. :-)
DPDK has lots of code using CPU vector instructions to shuffle bytes around. I can easily imagine a DMA engine doing similar jobs, possibly implemented in an FPGA or some other coprocessor.

-Morten