Re: [dpdk-dev] [RFC PATCH 5/6] bus/mlx5_pci: register a PCI driver

[Gaëtan Rivet <grive@u256.net>]

On 18/06/20 10:03 +0000, Parav Pandit wrote:
> 
> > From: Gaëtan Rivet <grive@u256.net>
> > Sent: Tuesday, June 16, 2020 3:17 AM
> > 
> > On 10/06/20 17:17 +0000, Parav Pandit wrote:
> > > Create a mlx5 bus driver framework for invoking drivers of multiple
> > > classes who have registered with the mlx5_pci bus driver.
> > >
> > > Validate user class arguments for supported class combinations.
> > >
> > > Signed-off-by: Parav Pandit <parav@mellanox.com>
> > > ---
> > >  drivers/bus/mlx5_pci/Makefile           |   1 +
> > >  drivers/bus/mlx5_pci/meson.build        |   2 +-
> > >  drivers/bus/mlx5_pci/mlx5_pci_bus.c     | 253
> > ++++++++++++++++++++++++
> > >  drivers/bus/mlx5_pci/rte_bus_mlx5_pci.h |   1 +
> > >  4 files changed, 256 insertions(+), 1 deletion(-)
> > >

[...]

> > > +
> > > +	while (nstr) {
> > > +		/* Extract each individual class name */
> > > +		found = strsep(&nstr, ":");
> > 
> > I have not seen the feature test macros (_DEFAULT_SOURCE) in the
> > Makefile, it seems required for strsep()?
> > 
> If its mandatory meson build should have complained?
> 

Invoking the compiler without specific standard conformance will work.
If someone adds for example -std=c11 however then _DEFAULT_SOURCE
becomes necessary.

It all depends on the range of compiler versions targeted by this code.
I don't know the full coverage, but I see -std=c11 + -D_DEFAULT_SOURCE
in most mlx5 code, which is why I'm asking for a double check here.


[...]

> > > +			continue;
> > > +
> > > +		if (class->loaded)
> > > +			class->remove(dev);
> > > +	}
> > > +	return 0;
> > > +}
> > > +
> > > +static int
> > > +mlx5_bus_pci_dma_map(struct rte_pci_device *dev, void *addr,
> > > +		     uint64_t iova, size_t len)
> > > +{
> > > +	struct rte_mlx5_pci_driver *class;
> > > +	int ret = -EINVAL;
> > > +
> > > +	TAILQ_FOREACH(class, &drv_list, next) {
> > > +		if (!class->dma_map)
> > > +			continue;
> > > +
> > > +		return class->dma_map(dev, addr, iova, len);
> > 
> > Is there a specific class that could have priority for the DMA?
> > 
> No.
> 

The code being written this way seems to point to multiple classes being
able to have DMA ops. If that's not the case, you can add a sanity check
to enforce than only the right classes have DMA ops defined.

> > > +	}
> > > +	return ret;
> > > +}
> > > +
> > > +static int
> > > +mlx5_bus_pci_dma_unmap(struct rte_pci_device *dev, void *addr,
> > > +		       uint64_t iova, size_t len)
> > > +{
> > > +	struct rte_mlx5_pci_driver *class;
> > > +	int ret = -EINVAL;
> > > +
> > > +	TAILQ_FOREACH_REVERSE(class, &drv_list, mlx5_pci_bus_drv_head,
> > next) {
> > > +		if (!class->dma_unmap)
> > > +			continue;
> > > +
> > > +		return class->dma_unmap(dev, addr, iova, len);
> > 
> > If you have two classes A -> B having dma_map() + dma_unmap(), you will
> > dma_map() with A then dma_unmap() with B, due to the _REVERSE()
> > iteration?
> > 
> There isn't plan for two drivers to do so.
> If two classes do that its source of an error.
> Will enhance the bus when that need arise.
> 

You have well-defined edge-cases, but they are not apparent reading
the code. Such error could be warned about and / or documented.

> > Why use reversed iteration at all by the way for dinit? If your ops is sound
> > any order should be ok.
> > 
> Because deinit must be always reverse of init() code regardless.
> 

This is a strong statement :)

If this is a requirement for driver inter-dependencies to be properly
implemented, this should be documented as such. Probably also explained
in the high-level design documentation in the header exposing this
driver API.

Best,
-- 
Gaëtan