Re: [PATCH v4 21/21] cxl/core: Split decoder setup into alloc + add

From: Dan Williams <dan.j.williams@intel.com>
To: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: linux-cxl@vger.kernel.org, kernel test robot <lkp@intel.com>,
	 Nathan Chancellor <nathan@kernel.org>,
	Dan Carpenter <dan.carpenter@oracle.com>,
	 Vishal L Verma <vishal.l.verma@intel.com>,
	Linux NVDIMM <nvdimm@lists.linux.dev>,
	 Ben Widawsky <ben.widawsky@intel.com>,
	"Schofield, Alison" <alison.schofield@intel.com>,
	 "Weiny, Ira" <ira.weiny@intel.com>
Subject: Re: [PATCH v4 21/21] cxl/core: Split decoder setup into alloc + add
Date: Fri, 10 Sep 2021 11:36:05 -0700	[thread overview]
Message-ID: <CAPcyv4jFYw6O2OrrDHb5qSCrNQ3jDH7Jmn4cCBCYsC9bW2yCQQ@mail.gmail.com> (raw)
In-Reply-To: <20210910113348.00007360@Huawei.com>

On Fri, Sep 10, 2021 at 3:34 AM Jonathan Cameron
<Jonathan.Cameron@huawei.com> wrote:
>
> On Wed, 8 Sep 2021 22:13:26 -0700
> Dan Williams <dan.j.williams@intel.com> wrote:
>
> > The kbuild robot reports:
> >
> >     drivers/cxl/core/bus.c:516:1: warning: stack frame size (1032) exceeds
> >     limit (1024) in function 'devm_cxl_add_decoder'
> >
> > It is also the case the devm_cxl_add_decoder() is unwieldy to use for
> > all the different decoder types. Fix the stack usage by splitting the
> > creation into alloc and add steps. This also allows for context
> > specific construction before adding.
> >
> > With the split the caller is responsible for registering a devm callback
> > to trigger device_unregister() for the decoder rather than it being
> > implicit in the decoder registration. I.e. the routine that calls alloc
> > is responsible for calling put_device() if the "add" operation fails.
> >
> > Reported-by: kernel test robot <lkp@intel.com>
> > Reported-by: Nathan Chancellor <nathan@kernel.org>
> > Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
> > Signed-off-by: Dan Williams <dan.j.williams@intel.com>
>
> A few minor things inline. This one was definitely a case where diff
> wasn't being helpful in how it chose to format things!
>
> I haven't taken the time to figure out if the device_lock() changes
> make complete sense as I don't understand the intent.
> I think they should be called out in the patch description as they
> seem a little non obvious.
>
> Jonathan
>
> > ---
> >  drivers/cxl/acpi.c      |   84 +++++++++++++++++++++++++----------
> >  drivers/cxl/core/bus.c  |  114 ++++++++++++++---------------------------------
> >  drivers/cxl/core/core.h |    5 --
> >  drivers/cxl/core/pmem.c |    7 ++-
> >  drivers/cxl/cxl.h       |   16 +++----
> >  5 files changed, 106 insertions(+), 120 deletions(-)
> >
> > diff --git a/drivers/cxl/acpi.c b/drivers/cxl/acpi.c
> > index 9d881eacdae5..654a80547526 100644
> > --- a/drivers/cxl/acpi.c
> > +++ b/drivers/cxl/acpi.c
> > @@ -82,7 +82,6 @@ static void cxl_add_cfmws_decoders(struct device *dev,
> >       struct cxl_decoder *cxld;
> >       acpi_size len, cur = 0;
> >       void *cedt_subtable;
> > -     unsigned long flags;
> >       int rc;
> >
> >       len = acpi_cedt->length - sizeof(*acpi_cedt);
> > @@ -119,24 +118,36 @@ static void cxl_add_cfmws_decoders(struct device *dev,
> >               for (i = 0; i < CFMWS_INTERLEAVE_WAYS(cfmws); i++)
> >                       target_map[i] = cfmws->interleave_targets[i];
> >
> > -             flags = cfmws_to_decoder_flags(cfmws->restrictions);
> > -             cxld = devm_cxl_add_decoder(dev, root_port,
> > -                                         CFMWS_INTERLEAVE_WAYS(cfmws),
> > -                                         cfmws->base_hpa, cfmws->window_size,
> > -                                         CFMWS_INTERLEAVE_WAYS(cfmws),
> > -                                         CFMWS_INTERLEAVE_GRANULARITY(cfmws),
> > -                                         CXL_DECODER_EXPANDER,
> > -                                         flags, target_map);
> > -
> > -             if (IS_ERR(cxld)) {
> > +             cxld = cxl_decoder_alloc(root_port,
> > +                                      CFMWS_INTERLEAVE_WAYS(cfmws));
> > +             if (IS_ERR(cxld))
> > +                     goto next;
> > +
> > +             cxld->flags = cfmws_to_decoder_flags(cfmws->restrictions);
> > +             cxld->target_type = CXL_DECODER_EXPANDER;
> > +             cxld->range = (struct range) {
> > +                     .start = cfmws->base_hpa,
> > +                     .end = cfmws->base_hpa + cfmws->window_size - 1,
> > +             };
> > +             cxld->interleave_ways = CFMWS_INTERLEAVE_WAYS(cfmws);
> > +             cxld->interleave_granularity =
> > +                     CFMWS_INTERLEAVE_GRANULARITY(cfmws);
> > +
> > +             rc = cxl_decoder_add(dev, cxld, target_map);
> > +             if (rc)
> > +                     put_device(&cxld->dev);
> > +             else
> > +                     rc = cxl_decoder_autoremove(dev, cxld);
> > +             if (rc) {
> >                       dev_err(dev, "Failed to add decoder for %#llx-%#llx\n",
> >                               cfmws->base_hpa, cfmws->base_hpa +
> >                               cfmws->window_size - 1);
> > -             } else {
> > -                     dev_dbg(dev, "add: %s range %#llx-%#llx\n",
> > -                             dev_name(&cxld->dev), cfmws->base_hpa,
> > -                              cfmws->base_hpa + cfmws->window_size - 1);
> > +                     goto next;
> >               }
> > +             dev_dbg(dev, "add: %s range %#llx-%#llx\n",
> > +                     dev_name(&cxld->dev), cfmws->base_hpa,
> > +                     cfmws->base_hpa + cfmws->window_size - 1);
> > +next:
> >               cur += c->length;
> >       }
> >  }
> > @@ -268,6 +279,7 @@ static int add_host_bridge_uport(struct device *match, void *arg)
> >       struct acpi_device *bridge = to_cxl_host_bridge(host, match);
> >       struct acpi_pci_root *pci_root;
> >       struct cxl_walk_context ctx;
> > +     int single_port_map[1], rc;
> >       struct cxl_decoder *cxld;
> >       struct cxl_dport *dport;
> >       struct cxl_port *port;
> > @@ -303,22 +315,46 @@ static int add_host_bridge_uport(struct device *match, void *arg)
> >               return -ENODEV;
> >       if (ctx.error)
> >               return ctx.error;
> > +     if (ctx.count > 1)
> > +             return 0;
> >
> >       /* TODO: Scan CHBCR for HDM Decoder resources */
> >
> >       /*
> > -      * In the single-port host-bridge case there are no HDM decoders
> > -      * in the CHBCR and a 1:1 passthrough decode is implied.
> > +      * Per the CXL specification (8.2.5.12 CXL HDM Decoder Capability
> > +      * Structure) single ported host-bridges need not publish a decoder
> > +      * capability when a passthrough decode can be assumed, i.e. all
> > +      * transactions that the uport sees are claimed and passed to the single
> > +      * dport. Default the range a 0-base 0-length until the first CXL region
> > +      * is activated.
> >        */
> > -     if (ctx.count == 1) {
> > -             cxld = devm_cxl_add_passthrough_decoder(host, port);
> > -             if (IS_ERR(cxld))
> > -                     return PTR_ERR(cxld);
> > +     cxld = cxl_decoder_alloc(port, 1);
> > +     if (IS_ERR(cxld))
> > +             return PTR_ERR(cxld);
> > +
> > +     cxld->interleave_ways = 1;
> > +     cxld->interleave_granularity = PAGE_SIZE;
> > +     cxld->target_type = CXL_DECODER_EXPANDER;
> > +     cxld->range = (struct range) {
> > +             .start = 0,
> > +             .end = -1,
> > +     };
> >
> > -             dev_dbg(host, "add: %s\n", dev_name(&cxld->dev));
> > -     }
>
> This was previously done without taking the device lock... (see below)
>
> > +     device_lock(&port->dev);
> > +     dport = list_first_entry(&port->dports, typeof(*dport), list);
> > +     device_unlock(&port->dev);
> >
> > -     return 0;
> > +     single_port_map[0] = dport->port_id;
> > +
> > +     rc = cxl_decoder_add(host, cxld, single_port_map);
> > +     if (rc)
> > +             put_device(&cxld->dev);
> > +     else
> > +             rc = cxl_decoder_autoremove(host, cxld);
> > +
> > +     if (rc == 0)
> > +             dev_dbg(host, "add: %s\n", dev_name(&cxld->dev));
> > +     return rc;
> >  }
> >
> >  static int add_host_bridge_dport(struct device *match, void *arg)
> > diff --git a/drivers/cxl/core/bus.c b/drivers/cxl/core/bus.c
> > index 176bede30c55..be787685b13e 100644
> > --- a/drivers/cxl/core/bus.c
> > +++ b/drivers/cxl/core/bus.c
> > @@ -455,16 +455,18 @@ EXPORT_SYMBOL_GPL(cxl_add_dport);
> >
> >  static int decoder_populate_targets(struct device *host,
> >                                   struct cxl_decoder *cxld,
> > -                                 struct cxl_port *port, int *target_map,
> > -                                 int nr_targets)
> > +                                 struct cxl_port *port, int *target_map)
> >  {
> >       int rc = 0, i;
> >
> > +     if (list_empty(&port->dports))
> > +             return -EINVAL;
> > +
>
> The code before this patch did this under the device_lock() Perhaps call
> out the fact there is no need to do that if we don't need to?

Nope, bug, good catch.

>
> >       if (!target_map)
> >               return 0;
> >
> >       device_lock(&port->dev);
> > -     for (i = 0; i < nr_targets; i++) {
> > +     for (i = 0; i < cxld->nr_targets; i++) {
> >               struct cxl_dport *dport = find_dport(port, target_map[i]);
> >
> >               if (!dport) {
> > @@ -479,27 +481,15 @@ static int decoder_populate_targets(struct device *host,
> >       return rc;
> >  }
> >
> > -static struct cxl_decoder *
> > -cxl_decoder_alloc(struct device *host, struct cxl_port *port, int nr_targets,
> > -               resource_size_t base, resource_size_t len,
> > -               int interleave_ways, int interleave_granularity,
> > -               enum cxl_decoder_type type, unsigned long flags,
> > -               int *target_map)
> > +struct cxl_decoder *cxl_decoder_alloc(struct cxl_port *port, int nr_targets)
> >  {
> >       struct cxl_decoder *cxld;
> >       struct device *dev;
> >       int rc = 0;
> >
> > -     if (interleave_ways < 1)
> > +     if (nr_targets > CXL_DECODER_MAX_INTERLEAVE || nr_targets < 1)
> >               return ERR_PTR(-EINVAL);
> >
> > -     device_lock(&port->dev);
> > -     if (list_empty(&port->dports))
> > -             rc = -EINVAL;
> > -     device_unlock(&port->dev);
> > -     if (rc)
> > -             return ERR_PTR(rc);
> > -
> >       cxld = kzalloc(struct_size(cxld, target, nr_targets), GFP_KERNEL);
> >       if (!cxld)
> >               return ERR_PTR(-ENOMEM);
> > @@ -508,22 +498,8 @@ cxl_decoder_alloc(struct device *host, struct cxl_port *port, int nr_targets,
> >       if (rc < 0)
> >               goto err;
> >
> > -     *cxld = (struct cxl_decoder) {
> > -             .id = rc,
> > -             .range = {
> > -                     .start = base,
> > -                     .end = base + len - 1,
> > -             },
> > -             .flags = flags,
> > -             .interleave_ways = interleave_ways,
> > -             .interleave_granularity = interleave_granularity,
> > -             .target_type = type,
> > -     };
> > -
> > -     rc = decoder_populate_targets(host, cxld, port, target_map, nr_targets);
> > -     if (rc)
> > -             goto err;
> > -
> > +     cxld->id = rc;
> > +     cxld->nr_targets = nr_targets;
> >       dev = &cxld->dev;
> >       device_initialize(dev);
> >       device_set_pm_not_required(dev);
> > @@ -541,72 +517,48 @@ cxl_decoder_alloc(struct device *host, struct cxl_port *port, int nr_targets,
> >       kfree(cxld);
> >       return ERR_PTR(rc);
> >  }
> > +EXPORT_SYMBOL_GPL(cxl_decoder_alloc);
> >
> > -struct cxl_decoder *
> > -devm_cxl_add_decoder(struct device *host, struct cxl_port *port, int nr_targets,
> > -                  resource_size_t base, resource_size_t len,
> > -                  int interleave_ways, int interleave_granularity,
> > -                  enum cxl_decoder_type type, unsigned long flags,
> > -                  int *target_map)
> > +int cxl_decoder_add(struct device *host, struct cxl_decoder *cxld,
> > +                 int *target_map)
> >  {
> > -     struct cxl_decoder *cxld;
> > +     struct cxl_port *port;
> >       struct device *dev;
> >       int rc;
> >
> > -     if (nr_targets > CXL_DECODER_MAX_INTERLEAVE)
> > -             return ERR_PTR(-EINVAL);
> > +     if (!cxld)
> > +             return -EINVAL;
> >
> > -     cxld = cxl_decoder_alloc(host, port, nr_targets, base, len,
> > -                              interleave_ways, interleave_granularity, type,
> > -                              flags, target_map);
> >       if (IS_ERR(cxld))
> > -             return cxld;
> > +             return PTR_ERR(cxld);
>
> It feels like the checks here are overly paranoid.
> Obviously harmless, but if we get as far as cxl_decoder_add() with either
> cxld == NULL or IS_ERR() then something horrible is going on.

True in the current users...

>
> I think you could reasonably drop them both.

They could be dropped, but it actually already paid dividends in
indicating to static analysis checkers that the input could be NULL.
If these were internal static functions I would agree with dropping
the extra error checking, but since this is exported and the call
sites will grow I'm inclined to keep them.