Re: [PATCH 1/5] i2c: smbus: add unlocked __i2c_smbus_xfer variant

From: Peter Rosin
Date: Sun Jul 01 2018 - 13:14:14 EST

On July 1, 2018 2:13:20 PM GMT+02:00, Wolfram Sang <wsa@xxxxxxxxxxxxx> wrote:
>On Wed, Jun 27, 2018 at 09:08:18AM +0200, Wolfram Sang wrote:
>> > Because, thinking more about it, the problem with those allocs are
>> > related to the locking details; adding another trylock to the mix
>> > makes it so much more obvious. I mean, first we would specifically
>> > handle atomic/irq context with a trylock "documenting" that
>> > users are welcome to at least try xfers, and then we blattantly
>> > the rulez with a GFP_KERNEL alloc...
>> Yes, thinking more about it, I came to the conclusion that we should
>> add trylock to SMBus and keep the requirement to allow sleeping.
>> True, SMBus is not consistent with I2C then, but actually, I'd prefer
>> the consistency the other way around: I wish we had a clear statement
>> that i2c_transfer may sleep. And have a dedicated irqless,
>> callback for handling the atomic case instead.
>> I really don't like the commit which introduced the trylock
>> into i2c_transfer[1]. Its commit message even says: "It is the
>> reponsability of the caller to ensure that the underlying i2c bus
>> will not sleep either." Which seems broken to me because I can't see
>> the caller should do that? And most bus drivers will sleep. But that
>> commit is upstream for 10 years now, so there are probably users.
>> also are very hard to spot, I am afraid. I wouldn't see a way to
>> them off the top of my head.
>> [1] cea443a81c9c ("i2c: Support i2c_transfer in atomic contexts")
>> > Currently, I assume they are only broken when the alloc happens to
>> > need to do more than is allowed from the given context. Something
>> > which might or might not be common?
>> The only regression now would be using smbus_emulated from atomic
>> context. Which is a bug on the caller side because it cannot know if
>> smbus_emulated will be used or not. For the non-emulated case, it
>> not be atomic anyhow.
>> So, unless I overlooked something, if we decide to not add trylock to
>> smbus_xfer, we are all fine?
>> And I think we should really keep this clean rule of smbus functions
>> being non-atomic.
>> D'accord?
>So, if no other arguments drop in, I'll apply this series as is next

Right, I had the below response sitting in my drafts folder. I thought I had sent it, but apparently I didn't...

Well, IF there are SMBus users that in fact do rely on the emulation allowing calls from atomic/irq context then it will be a regression even if those users are "buggy". But if someone complains, I think the correct response is to open-code the trylock dance and call the new unlocked __i2c_smbus_xfer at the affected location. So, I think we have a contingency plan.

Other than that, we are in violent agreement, and I think you also agree with the above.

I guess that also means the series is fine as-is (modulo the fixes recently made in the tail of the first hunk of patch 1/5 that causes a trivial but annoying conflict when applying it, i.e. below the i2c_transfer -> __i2c_transfer update in the emulation function).

As far as I'm concerned, you can take this whole series directly even if most patches are i2c-mux patches. I don't have anything in my tree yet so I'll simply base any other stuff on this once I can fetch it from you...