linux-kernel - Re: [PATCH] libnvdimm: Clarify nd_pfn

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Date:   Mon, 21 Jan 2019 10:04:40 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Wei Yang <richardw.yang@...ux.intel.com>
Cc:     linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] libnvdimm: Clarify nd_pfn_init() flow

On Sun, Jan 20, 2019 at 11:51 PM Wei Yang <richardw.yang@...ux.intel.com> wrote:
> On Fri, Jan 18, 2019 at 04:47:23PM -0800, Dan Williams wrote:
[..]
> Also, I have one confusion about your saying: two probes.
>
> If the two probes are:
>
>     * for dax%d.%d: 1. nd_dax_probe 2. dax_pmem_probe
>     * for pfn%d.%d: 1. nd_pfn_probe 2. nd_pmem_probe
>
> Then, if the first probe fails, the device itself would be destroyed. How the
> second probe do its job?
>
> >       rc = nd_pfn_validate(nd_pfn, sig);
> >       if (rc != -ENODEV)
> >               return rc;

Here is an example path for a device-dax instance:

    /sys/devices/platform/e820_pmem/ndbus0/region0/dax0.1/dax0.0

In this case the order of events is:

1/ region0 discovers it contains a pmem namespace and registers namespace0.0
2/ The pmem namespace driver calls nd_dax_probe() to check for the
presence of a device-dax configuration
3/ If present, nd_pfn_validate() returns 0 and nd_dax_probe()
registers the dax0.1 device (this is a libnvdimm 'personality device).
4/ When nd_pmem_probe() sees nd_dax_probe() return 0 it in turn fails
the probe of namespace0.0 with -ENXIO. All devm allocations during the
probe of namespace0.0 are released.
5/ Meanwhile the created dax0.1 device is recognized by the dax_pmem device.
6/ dax_pmem_probe() uses nd_pfn_init() to validate the configuration
and ultimately creates the dax0.0 character device

The pmem namepsace driver is effectively a router to other personality
drivers that may be layered on top of the base namespace capacity.