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

[Bruce Richardson <bruce.richardson@intel.com>]

On Wed, Jun 16, 2021 at 04:48:59PM +0000, Honnappa Nagarahalli wrote:
> <snip>
> 
> > 
> > On Wed, Jun 16, 2021 at 02:14:54PM +0200, David Marchand wrote:
> > > On Tue, Jun 15, 2021 at 3:25 PM Chengwen Feng
> > <fengchengwen@huawei.com> wrote:
> > > > +
> > > > +#define RTE_DMADEV_NAME_MAX_LEN        (64)
> > > > +/**< @internal Max length of name of DMA PMD */
> > > > +
> > > > +/** @internal
> > > > + * The data structure associated with each DMA device.
> > > > + */
> > > > +struct rte_dmadev {
> > > > +       /**< Device ID for this instance */
> > > > +       uint16_t dev_id;
> > > > +       /**< Functions exported by PMD */
> > > > +       const struct rte_dmadev_ops *dev_ops;
> > > > +       /**< Device info. supplied during device initialization */
> > > > +       struct rte_device *device;
> > > > +       /**< Driver info. supplied by probing */
> > > > +       const char *driver_name;
> > > > +
> > > > +       /**< Device name */
> > > > +       char name[RTE_DMADEV_NAME_MAX_LEN]; } __rte_cache_aligned;
> > > > +
> > >
> > > I see no queue/channel notion.
> > > How does a rte_dmadev object relate to a physical hw engine?
> > >
> > One queue, one device.
> > When looking to update the ioat driver for 20.11 release when I added the
> > idxd part, I considered adding a queue parameter to the API to look like one
> > device with multiple queues. However, since each queue acts completely
> > independently of each other, there was no benefit to doing so. It's just easier
> > to have a single id to identify a device queue.
> Does it mean, the queue is multi thread safe? Do we need queues per core to avoid locking?

The design is for each queue to be like the queue on a NIC, not
thread-safe. However, if the hardware supports thread-safe queues too, that
can be supported. But the API should be like other data-plane ones and be
lock free.

For the DMA devices that I am working on, the number of queues
is not very large, and in most cases each queue appears as a separate
entity, e.g. for ioat each queue/channel appears as a separate PCI ID, and
when using idxd kernel driver each queue is a separate dev node to mmap.
For other cases right now we just create one rawdev instance per queue in
software.

/Bruce