Re: [PATCH 2/3] mwifiex: support sysfs initiated device coredump

[Arend van Spriel]

+ Johannes (for devcoredump documentation question).

On 2/22/2018 8:35 PM, Brian Norris wrote:
> Hi Arend,
>
> On Thu, Feb 22, 2018 at 01:17:56PM +0100, Arend van Spriel wrote:
> > On 2/21/2018 11:59 PM, Brian Norris wrote:
> > > On Wed, Feb 21, 2018 at 11:50:19AM +0100, Arend van Spriel wrote:
> > > > Since commit 3c47d19ff4dc ("drivers: base: add coredump driver ops")
> > > > it is possible to initiate a device coredump from user-space. This
> > > > patch adds support for it adding the .coredump() driver callback.
> > > > As there is no longer a need to initiate it through debugfs remove
> > > > that code.
> > > >
> > > > Signed-off-by: Arend van Spriel <arend.vanspriel@xxxxxxxxxxxx>
> > > > ---
> > > >    drivers/net/wireless/marvell/mwifiex/debugfs.c | 31 +-------------------------
> > > >    drivers/net/wireless/marvell/mwifiex/pcie.c    | 19 ++++++++++++++--
> > > >    drivers/net/wireless/marvell/mwifiex/sdio.c    | 13 +++++++++++
> > > >    drivers/net/wireless/marvell/mwifiex/usb.c     | 14 ++++++++++++
> > > >    4 files changed, 45 insertions(+), 32 deletions(-)
> > >
> > > The documentation doesn't really say [1], but is the coredump supposed
> > > to happen synchronously? Because the mwifiex implementation is
> > > asynchronous, whereas it looks like the brcmfmac one is synchronous.
> >
> > Well, that depends on the eye of the beholder I guess. From user-space
> > perspective it is asynchronous regardless. A write access to the coredump
> > sysfs file eventually results in a uevent when the devcoredump entry is
> > created, ie. after driver has made a dev_coredump API call. Whether the
> > driver does that synchronously or asynchronously is irrelevant as far as
> > user-space is concerned.
>
> Is it really? The driver infrastructure seems to guarantee that the
> entirety of a driver's ->coredump() will complete before returning from
> the write. So it might be reasonable for some user to assume (based on
> implementation details, e.g., of brcmfmac) that the devcoredump will be
> ready by the time the write() syscall returns, absent documentation that
> says otherwise. But then, that's not how mwifiex works right now, so
> they might be surprised if they switch drivers.

Ok. I already agreed that the uevent behavior should be documented. The 
error handling you are bringing up below was something I realized as 
well. I am not familiar with mwifiex to determine what it can say about 
the coredump succeeding before scheduling the work.

> Anyway, *I'm* already personally used to these dumps being asynchronous,
> and writing tooling to listen for the uevent instead. But that doesn't
> mean everyone will be.
>
> Also, due to the differences in async/sync, mwifiex doesn't really
> provide you much chance for error handling, because most errors would be
> asynchronous. So brcmfmac's "coredump" has more chance for user programs
> to error-check than mwifiex's (due to the asynchronous nature) [1].
>
> BTW, I push on this mostly because this is migrating from a debugfs
> feature (that is easy to hand-wave off as not really providing a
> consistent/stable API, etc., etc.) to a documented sysfs feature. If it
> were left to rot in debugfs, I probably wouldn't be as bothered ;)

I appreciate it. The documentation is not in the stable ABI folder yet 
so I welcome any and all improvements. Might learn a thing or two from it.

> > > Brian
> > >
> > > [1] In fact, the ABI documentation really just describes kernel
> > > internals, rather than documenting any user-facing details, from what I
> > > can tell.
> >
> > You are right. Clearly I did not reach the end my learning curve here. I
> > assumed referring to the existing dev_coredump facility was sufficient, but
> > maybe it is worth a patch to be more explicit and mention the uevent
> > behavior. Also dev_coredump facility may be disabled upon which the trigger
> > will have no effect in sysfs. In the kernel the data passed by the driver is
> > simply freed by dev_coredump facility.
>
> Is there any other documentation for the coredump feature? I don't
> really see much.

Any other than the code itself you mean? I am not sure. Maybe Johannes 
knows.

> Brian
>
> [1] Oh wait, but I see that while ->coredump() has an integer return
> code...the caller ignores it:
>
> static ssize_t coredump_store(struct device *dev, struct device_attribute *attr,
> 			    const char *buf, size_t count)
> {
> 	device_lock(dev);
> 	if (dev->driver->coredump)
> 		dev->driver->coredump(dev);
> 	device_unlock(dev);
>
> 	return count;
> }
> static DEVICE_ATTR_WO(coredump);
>
> Is that a bug or a feature?

Yeah. Let's call it a bug. Just not sure what to go for. Return the 
error or change coredump callback to void return type.

Regards,
Arend