linux-kernel - Re: [PATCH v4 3/6] libnvdimm, acpi, nfit: Add bus level dsm mask for pass thru.

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Date:   Tue, 4 Jul 2017 13:37:43 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jerry Hoemann <Jerry.Hoemann@....com>
Cc:     "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 3/6] libnvdimm, acpi, nfit: Add bus level dsm mask for
 pass thru.

On Tue, Jul 4, 2017 at 1:08 PM, Jerry Hoemann <jerry.hoemann@....com> wrote:
> On Sat, Jul 01, 2017 at 01:46:03PM -0700, Dan Williams wrote:
>> On Sat, Jul 1, 2017 at 1:38 PM, Jerry Hoemann <jerry.hoemann@....com> wrote:
>> > On Sat, Jul 01, 2017 at 01:10:31PM -0700, Dan Williams wrote:
>> >> On Sat, Jul 1, 2017 at 1:08 PM, Dan Williams <dan.j.williams@...el.com> wrote:
>> >> > On Sat, Jul 1, 2017 at 12:58 PM, Jerry Hoemann <jerry.hoemann@....com> wrote:
>> >> >> On Fri, Jun 30, 2017 at 08:55:22PM -0700, Dan Williams wrote:
>> >> >>
>> >> >> ...
>> >> >>
>> >> >>> On Fri, Jun 30, 2017 at 9:09 AM, Jerry Hoemann <jerry.hoemann@....com> wrote:
>> >> >>> > +               if (cmd == ND_CMD_CALL)
>> >> >>> > +                       dsm_mask = nd_desc->bus_dsm_mask;
>> >> >>> >                 desc = nd_cmd_bus_desc(cmd);
>> >> >>> >                 uuid = to_nfit_uuid(NFIT_DEV_BUS);
>> >> >>> >                 handle = adev->handle;
>> >> >>> > @@ -1613,6 +1615,7 @@ static void acpi_nfit_init_dsms(struct acpi_nfit_desc *acpi_desc)
>> >> >>> >         struct nvdimm_bus_descriptor *nd_desc = &acpi_desc->nd_desc;
>> >> >>> >         const u8 *uuid = to_nfit_uuid(NFIT_DEV_BUS);
>> >> >>> >         struct acpi_device *adev;
>> >> >>> > +       unsigned long dsm_mask;
>> >> >>> >         int i;
>> >> >>> >
>> >> >>> >         nd_desc->cmd_mask = acpi_desc->bus_cmd_force_en;
>> >> >>> > @@ -1624,6 +1627,11 @@ static void acpi_nfit_init_dsms(struct acpi_nfit_desc *acpi_desc)
>> >> >>> >                 if (acpi_check_dsm(adev->handle, uuid, 1, 1ULL << i))
>> >> >>> >                         set_bit(i, &nd_desc->cmd_mask);
>> >> >>> >         set_bit(ND_CMD_CALL, &nd_desc->cmd_mask);
>> >> >>> > +
>> >> >>> > +       dsm_mask = 0x3bf;
>> >> >>>
>> >> >>> I went ahead and fixed this up to use dsm_mask defined like this:
>> >> >>>
>> >> >>> +       dsm_mask =
>> >> >>> +               (1 << ND_CMD_ARS_CAP) |
>> >> >>> +               (1 << ND_CMD_ARS_START) |
>> >> >>> +               (1 << ND_CMD_ARS_STATUS) |
>> >> >>> +               (1 << ND_CMD_CLEAR_ERROR) |
>> >> >>> +               (1 << NFIT_CMD_TRANSLATE_SPA) |
>> >> >>> +               (1 << NFIT_CMD_ARS_INJECT_SET) |
>> >> >>> +               (1 << NFIT_CMD_ARS_INJECT_CLEAR) |
>> >> >>> +               (1 << NFIT_CMD_ARS_INJECT_GET);
>> >> >>>
>> >> >>> This drops function number 0 which userspace has no need to call.
>> >> >>
>> >> >> Actually I like to call function 0.  Its an excellent test when
>> >> >> modifying the code path as its a no side effects function whose output
>> >> >> is known in advance and instantly recognizable.  I also use it when
>> >> >> testing new firmware.
>> >> >>
>> >> >> What is the downside to allowing it?  What bad things happen?
>> >> >
>> >> > It allows implementations to bypass the standardization process and
>> >> > ship new root DSMs. It's always possible to patch the kernel locally
>> >> > for development, so I see no reason to ship this capability globally.
>> >
>> > I don't understand this comment, but I think your next comment
>> > essentially says to disregard this comment?
>>
>> Yes, sorry.
>>
>> >> Actually, just the discovery portion does not lead to this leak, but
>> >> it's redundant when we have the 'dsm_mask' sysfs attribute.
>> >
>> > No.  The generation of the mask in sysfs is not done by
>> > executing the code in acpi_nfit_ctl.  One of the reasons I call
>> > function 0 to test changes I am making to the ioctl path itself.
>> > The sysfs has nothing to do with that path and cannot be used
>> > to serve this purpose.
>> >
>> > And since the content of sysfs has been edited it also can not be
>> > used as a basic test of firmware.
>> >
>> > What is the downside to allowing the calling of function 0?
>>
>> It needlessly expands the kernel ABI. I would suggest, if you want to
>
> No.  It is not needless.  It is not an ABI extension.
> Same goes for the override feature.

If the need is testing then we have a tools/testing/nvdimm for that.
Of course it's an ABI extension, it allows userspace to discover DSM
function numbers the kernel didn't know about at compile time.

> I hope that ACPI doesn't extend the specification in the future because
> we'll just have to redo these patches yet again.

Hopefully this is the last ACPI spec version where we add new DSMs to
the root device. All future methods should be named methods like what
the specification started doing for NVIDMM leaf devices with _LSI,
_LSR, and _LSW.