Return-Path: Subject: Re: [PATCH 2/3] mwifiex: support sysfs initiated device coredump To: Brian Norris References: <1519210220-22437-1-git-send-email-arend.vanspriel@broadcom.com> <1519210220-22437-3-git-send-email-arend.vanspriel@broadcom.com> <20180221225903.GA42395@rodete-desktop-imager.corp.google.com> <5A8EB4F4.2030103@broadcom.com> <20180222193547.GA78462@rodete-desktop-imager.corp.google.com> Cc: Kalle Valo , Marcel Holtmann , linux-wireless@vger.kernel.org, linux-bluetooth@vger.kernel.org, linux-kernel@vger.kernel.org, Greg Kroah-Hartman , Johannes Berg From: Arend van Spriel Message-ID: <5A8FEF68.5080900@broadcom.com> Date: Fri, 23 Feb 2018 11:39:36 +0100 MIME-Version: 1.0 In-Reply-To: <20180222193547.GA78462@rodete-desktop-imager.corp.google.com> Content-Type: text/plain; charset=windows-1252; format=flowed List-ID: + 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 >>>> --- >>>> 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