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From:   Patrick Venture <venture@google.com>
Date:   Thu, 20 Sep 2018 11:08:31 -0700
Message-ID: <CAO=notw5bjRbiPY6xfkNpQ+dkziNKxKfxWhT6=VVRcfUfAQ1tw@mail.gmail.com>
Subject: Re: [PATCH v2] ipmi: looped device detection
To:     Corey Minyard <minyard@acm.org>
Cc:     Arnd Bergmann <arnd@arndb.de>,
        Greg KH <gregkh@linuxfoundation.org>,
        openipmi-developer@lists.sourceforge.net,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        OpenBMC Maillist <openbmc@lists.ozlabs.org>
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On Wed, Sep 19, 2018 at 1:20 PM Corey Minyard <minyard@acm.org> wrote:
>
> On 09/19/2018 02:56 PM, Patrick Venture wrote:
> > On Tue, Sep 18, 2018 at 2:37 PM Corey Minyard <tcminyard@gmail.com> wrote:
> >> On 09/18/2018 01:42 PM, Patrick Venture wrote:
> >>> On Wed, Sep 12, 2018 at 3:54 PM Patrick Venture <venture@google.com> wrote:
> >>>> On Wed, Sep 12, 2018 at 3:10 PM Corey Minyard <minyard@acm.org> wrote:
> >>>>> On 09/11/2018 05:56 PM, Patrick Venture wrote:
> >>>>>> Try to get the device ID repeatedly during initialization before giving up.
> >>>>>> The BMC isn't always responsive, and this allows it to be slightly flaky
> >>>>>> during early boot.
> >>>>>>
> >>>>>> Tested: Installed on a system with the BMC software disabled
> >>>>>> such that it was non-responsive.  The driver correctly detected this
> >>>>>> and gave up as expected.  Then I re-enabled the BMC software unloaded
> >>>>>> and reloaded the driver and it was detected properly.
> >>>>> The patch looks fine, but I wonder if this is something that is really
> >>>>> valuable.
> >>>>> I have wondered about this before.
> >>>>>
> >>>>> The question is: If the BMC is unavailable, what are the chances of it
> >>>>> becoming
> >>>>> available by the time you do 5 attempts?  I would guess that is a pretty
> >>>>> small
> >>>>> chance, which is why I haven't done this already.
> >>> Friendly ping.  I'd like to get a sense of whether you're likely to
> >>> accept this.  If not, it's fine, will close out patch in current
> >>> downstream rebase.
> >> I'm ok with doing this, but I lied about the patch being fine, there are
> >> some issue.
> >> Well, I didn't lie, but I didn't look closely enough.
> >>
> >> Can you use dev_xxx() instead of pr_xxx().  I know the driver isn't
> >> currently
> >> consistent, but there are a number of patches I have pending to make it
> >> better and it's a longer-term goal.
> > Ack.
> >
> >> Can you make GET_DEVICE_ID_ATTEMPTS more specific, add IPMI_SI_ to
> >> the beginning or something.
> > Ack.
> >
> >> I am not sure that I'm ok with waiting up to 1.25 seconds in the init
> >> function.
> >> As I mentioned before, a large number of systems have broken ACPI/SMBIOS
> >> information, and for those it will add 1.25 seconds to the boot time of
> >> every
> >> one of those systems.  That won't make me a popular guy :-).
> > Yeah, that's problematic for the systems that'll never get a valid
> > response.  I don't think it makes sense to gate the feature with a
> > configuration option, do you?
>
> Yeah, a config option wouldn't really help.  You could do a module parm,
> but that's
> really not very easy to use.
>
> Pushing the detection off to a thread would solve that problem, but it
> creates its
> own problems.  The driver may be in the process of detecting something, but
> it may not be ready when the module load or kernel boot finishes.
>
> I don't think this has been an issue in the past because generally the
> BMC is
> integral to the boot of most systems.  So it's always operational by the
> time
> the OS comes up.
>
> So I'm not sure what to do.

Let's drop the patch.  I'll maintain it downstream.  Thanks for your
thoughts and time on this.

>
> -corey
>
> >> This is a harder problem to figure out what to do.  To solve it properly
> >> would
> >> mean having a timer or thread drive this, and unload the module later if
> >> the process fails.
> >>
> >> -corey
> >>
> >>> Thanks
> >>>
> >>>> This patch was actually critical for us to provide a reliable IPMI
> >>>> interface.  The version of OpenBMC or the state of the BMC at the
> >>>> point the kernel was loading was flaky, so following the example in
> >>>> the BIOS source, we just re-try a few times.  We also can hold boot X
> >>>> seconds until it's responding, but, this avoided some issues inherent
> >>>> with that.
> >>>>
> >>>>> You could have something that re-tested periodically, but there are so many
> >>>>> systems with IPMI specified in ACPI or SMBIOS that is wrong, and it would
> >>>>> try forever.  Also not really a good thing.
> >>>> If we did a periodic check, it could check X times, but I felt going
> >>>> for a simple solution was ideal -- and this idea was proved out on a
> >>>> few platforms.  We have other drivers that are loaded by the kernel
> >>>> (not at run-time) and they depend on IPMI, and without this patch they
> >>>> would then have a non-trivial probability of failure.
> >>>>
> >>>>> So I've left it to reload the driver or use the hotmod interface.
> >>>>>
> >>>>> -corey
> >>>>>
> >>>>>> Signed-off-by: Patrick Venture <venture@google.com>
> >>>>>> ---
> >>>>>> v2:
> >>>>>>     - removed extra variable that was set but not used.
> >>>>>> ---
> >>>>>>     drivers/char/ipmi/ipmi_si_intf.c | 23 ++++++++++++++++++++++-
> >>>>>>     1 file changed, 22 insertions(+), 1 deletion(-)
> >>>>>>
> >>>>>> diff --git a/drivers/char/ipmi/ipmi_si_intf.c b/drivers/char/ipmi/ipmi_si_intf.c
> >>>>>> index 90ec010bffbd..5fed96897fe8 100644
> >>>>>> --- a/drivers/char/ipmi/ipmi_si_intf.c
> >>>>>> +++ b/drivers/char/ipmi/ipmi_si_intf.c
> >>>>>> @@ -1918,11 +1918,13 @@ int ipmi_si_add_smi(struct si_sm_io *io)
> >>>>>>      * held, primarily to keep smi_num consistent, we only one to do these
> >>>>>>      * one at a time.
> >>>>>>      */
> >>>>>> +#define GET_DEVICE_ID_ATTEMPTS       5
> >>>>>>     static int try_smi_init(struct smi_info *new_smi)
> >>>>>>     {
> >>>>>>         int rv = 0;
> >>>>>>         int i;
> >>>>>>         char *init_name = NULL;
> >>>>>> +     unsigned long sleep_rm;
> >>>>>>
> >>>>>>         pr_info(PFX "Trying %s-specified %s state machine at %s address 0x%lx, slave address 0x%x, irq %d\n",
> >>>>>>                 ipmi_addr_src_to_str(new_smi->io.addr_source),
> >>>>>> @@ -2003,7 +2005,26 @@ static int try_smi_init(struct smi_info *new_smi)
> >>>>>>          * Attempt a get device id command.  If it fails, we probably
> >>>>>>          * don't have a BMC here.
> >>>>>>          */
> >>>>>> -     rv = try_get_dev_id(new_smi);
> >>>>>> +     for (i = 0; i < GET_DEVICE_ID_ATTEMPTS; i++) {
> >>>>>> +             pr_info(PFX "Attempting to read BMC device ID\n");
> >>>>>> +             rv = try_get_dev_id(new_smi);
> >>>>>> +             /* If it succeeded, stop trying */
> >>>>>> +             if (!rv)
> >>>>>> +                     break;
> >>>>>> +
> >>>>>> +             /* Sleep for ~0.25s before trying again instead of hammering
> >>>>>> +              * the BMC.
> >>>>>> +              */
> >>>>>> +             sleep_rm = msleep_interruptible(250);
> >>>>>> +             if (sleep_rm != 0) {
> >>>>>> +                     pr_info(PFX "Find BMC interrupted\n");
> >>>>>> +                     rv = -EINTR;
> >>>>>> +                     goto out_err;
> >>>>>> +             }
> >>>>>> +     }
> >>>>>> +
> >>>>>> +     /* If we exited the loop above and rv is non-zero we ran out of tries.
> >>>>>> +      */
> >>>>>>         if (rv) {
> >>>>>>                 if (new_smi->io.addr_source)
> >>>>>>                         dev_err(new_smi->io.dev,
>
>