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SubjectRE: [PATCH v1 3/7] platform/mellanox: mlxreg-hotplug: add extra cycle for hotplug work queue
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> -----Original Message-----
> From: Darren Hart [mailto:dvhart@infradead.org]
> Sent: Friday, April 13, 2018 7:47 PM
> To: Vadim Pasternak <vadimp@mellanox.com>
> Cc: andy.shevchenko@gmail.com; gregkh@linuxfoundation.org; linux-
> kernel@vger.kernel.org; platform-driver-x86@vger.kernel.org; jiri@resnulli.us;
> Michael Shych <michaelsh@mellanox.com>; ivecera@redhat.com
> Subject: Re: [PATCH v1 3/7] platform/mellanox: mlxreg-hotplug: add extra cycle
> for hotplug work queue
>
> On Tue, Mar 27, 2018 at 10:02:03AM +0000, Vadim Pasternak wrote:
> > It adds missed logic for signal acknowledge, by adding an extra run
> > for work queue in case a signal is received, but no specific signal
> > assertion is detected. Such case theoretically can happen for example
> > in case several units are removed or inserted at the same time. In
> > this situation acknowledge for some signal can be missed at signal top
> > aggreagation status level.
>
> Why can they be missed? What does "signal top aggregation status level"
> mean? I'm asking to confirm that we are fixing this at the right place, and not
> just applying a suboptimal bandaid by running the workqueue more.
>

Hi Darren,

Thank for review.

It could happen within the next flow:

The signal routing flow is as following (f.e. for of FANi removing):
- FAN status and event registers related bit is changed;
-- intermediate aggregation status register is changed;
--- top aggregation status register is changed;
---- interrupt routed to CPU and interrupt handler is invoked.

When interrupt handler is invoked it follows the next simple logic (f.e
FAN3 is removed):
(a1) mask top aggregation interrupt mask register;
(a2) read top aggregation interrupt status register and test to which
underling group belongs a signal (FANs in this case and is changed
from 0xff to 0xfb and 0xfb is saved as a last status value);
(b1) mask FANs interrupt mask register;
(b2) read FANs status register and test which FAN has been changed (FAN3 in
this example);
(c1) perform relevant action;
<--------------- (FAN2 is removed at this point)
(b3) clear FANs interrupt event register to acknowledge FAN3 signal;
(b4) unmask FANs interrupt mask register
(a3) unmask top aggregation interrupt mask register;

An interrupt handler is invoked, since FAN2 interrupt is not acknowledge.
It should set top aggregation interrupt status register bit 6 (0xfb).
In step (a2)
(a2) read top aggregation interrupt and comparing it with saved value doesn't
show change (same 0xfb) and after (a2) execution jumps to (a3) and
signal leaved unhandled.

> ...
>
> >
> > Fixes: 1f976f6978bf ("platform/x86: Move Mellanox platform hotplug
> > driver to platform/mellanox")
>
> You didn't mention above how this commit caused this - how did moving the
> driver create this problem?

Actually I should reference to
07b89c2b2a5e ("platform/x86: Introduce support for Mellanox hotplug driver")
which was initial driver commit, before it has been relocated.

Does this need to go to stable? I'm assuming not as
> you've called it theoretical - not something you've observed in practice?
>

It's not necessary to go to stable.

> ...
>
> > static int mlxreg_hotplug_device_create(struct
> > mlxreg_hotplug_priv_data *priv, @@ -410,6 +413,18 @@ static void
> mlxreg_hotplug_work_handler(struct work_struct *work)
> > aggr_asserted = priv->aggr_cache ^ regval;
> > priv->aggr_cache = regval;
> >
> > + /*
> > + * Handler is invoked, but no assertion is detected at top aggregation
> > + * status level. Set aggr_asserted to mask value to allow handler extra
> > + * run over all relevant signals to recover any missed signal.
> > + */
> > + if (priv->not_asserted == MLXREG_HOTPLUG_NOT_ASSERT) {
> > + priv->not_asserted = 0;
> > + aggr_asserted = pdata->mask;
> > + }
> > + if (!aggr_asserted)
>
> We seem to check aggr_asserted in several places in this routine now.
> Looks like something we could simplify. If you check it here, can you drop the
> check lower in the routine? Can you remove it from the for loop if conditional
> entirely? Please consider how to simplify.

OK, will review this code.

>
> --
> Darren Hart
> VMware Open Source Technology Center

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