Re: [PATCH 04/13] cxl: Introduce endpoint decoders

[Ben Widawsky <ben.widawsky@intel.com>]

On 21-09-13 15:07:44, Dan Williams wrote:
> On Mon, Sep 13, 2021 at 9:11 AM Ben Widawsky <ben.widawsky@intel.com> wrote:
> >
> > On 21-09-10 12:19:24, Dan Williams wrote:
> > > On Thu, Sep 2, 2021 at 12:50 PM Ben Widawsky <ben.widawsky@intel.com> wrote:
> > > >
> > > > Endpoints have decoders too. It is useful to share the same
> > > > infrastructure from cxl_core. Endpoints do not have dports (downstream
> > > > targets), only the underlying physical medium. As a result, some special
> > > > casing is needed.
> > > >
> > > > There is no functional change introduced yet as endpoints don't actually
> > > > enumerate decoders yet.
> > > >
> > > > Signed-off-by: Ben Widawsky <ben.widawsky@intel.com>
> > > > ---
> > > >  drivers/cxl/core/bus.c | 29 +++++++++++++++++++++++++----
> > > >  1 file changed, 25 insertions(+), 4 deletions(-)
> > > >
> > > > diff --git a/drivers/cxl/core/bus.c b/drivers/cxl/core/bus.c
> > > > index 8d5061b0794d..6202ce5a5ac2 100644
> > > > --- a/drivers/cxl/core/bus.c
> > > > +++ b/drivers/cxl/core/bus.c
> > > > @@ -175,6 +175,12 @@ static const struct attribute_group *cxl_decoder_switch_attribute_groups[] = {
> > > >         NULL,
> > > >  };
> > > >
> > > > +static const struct attribute_group *cxl_decoder_endpoint_attribute_groups[] = {
> > > > +       &cxl_decoder_base_attribute_group,
> > > > +       &cxl_base_attribute_group,
> > > > +       NULL,
> > > > +};
> > > > +
> > > >  static void cxl_decoder_release(struct device *dev)
> > > >  {
> > > >         struct cxl_decoder *cxld = to_cxl_decoder(dev);
> > > > @@ -184,6 +190,12 @@ static void cxl_decoder_release(struct device *dev)
> > > >         kfree(cxld);
> > > >  }
> > > >
> > > > +static const struct device_type cxl_decoder_endpoint_type = {
> > > > +       .name = "cxl_decoder_endpoint",
> > > > +       .release = cxl_decoder_release,
> > > > +       .groups = cxl_decoder_endpoint_attribute_groups,
> > > > +};
> > > > +
> > > >  static const struct device_type cxl_decoder_switch_type = {
> > > >         .name = "cxl_decoder_switch",
> > > >         .release = cxl_decoder_release,
> > > > @@ -196,6 +208,11 @@ static const struct device_type cxl_decoder_root_type = {
> > > >         .groups = cxl_decoder_root_attribute_groups,
> > > >  };
> > > >
> > > > +static bool is_endpoint_decoder(struct device *dev)
> > > > +{
> > > > +       return dev->type == &cxl_decoder_endpoint_type;
> > > > +}
> > > > +
> > > >  bool is_root_decoder(struct device *dev)
> > > >  {
> > > >         return dev->type == &cxl_decoder_root_type;
> > > > @@ -472,7 +489,7 @@ struct cxl_decoder *cxl_decoder_alloc(struct cxl_port *port, int nr_targets)
> > > >         struct device *dev;
> > > >         int rc = 0;
> > > >
> > > > -       if (nr_targets > CXL_DECODER_MAX_INTERLEAVE || nr_targets < 1)
> > > > +       if (nr_targets > CXL_DECODER_MAX_INTERLEAVE)
> > > >                 return ERR_PTR(-EINVAL);
> > > >
> > > >         cxld = kzalloc(struct_size(cxld, target, nr_targets), GFP_KERNEL);
> > > > @@ -491,8 +508,11 @@ struct cxl_decoder *cxl_decoder_alloc(struct cxl_port *port, int nr_targets)
> > > >         dev->parent = &port->dev;
> > > >         dev->bus = &cxl_bus_type;
> > > >
> > > > +       /* Endpoints don't have a target list */
> > > > +       if (nr_targets == 0)
> > > > +               dev->type = &cxl_decoder_endpoint_type;
> > >
> > > Do you also plan to introduce the concept of endpoint ports, and if
> > > yes should that come before this patch? That would seem to be more
> > > robust than, for example, allowing a switch port to carry an endpoint
> > > decoder object as this allows.
> >
> > I didn't see a need as of yet to differentiate between endpoint ports and other
> > ports. I don't entirely understand what you mean by "allowing a switch port to
> > carry an endpoint decoder" means. Can you please elaborate?
> 
> If endpoint ports were an explicit type then this check could make
> sure that someone did not pass nr_targets set to 0 where the @port
> argument is referring to a switch where the target_list must be
> specified.
> 
> Either that, or a comment in kernel-doc for this routine about the
> special meaning of nr_targets == 0 and expected usage.

Well, since Jonathan also brought up a concern here perhaps I should entertain
other ideas. I suppose future versions of the spec could break things, but as it
stands today the only CXL component that implements decoders that can have a 0
value for this are endpoint devices (T2 or T3, or LD). I think it's fine to wait
until we have a second reason to make an endpoint port type and update kdocs for
now, but maybe it will also be a natural fit with a proper port driver.