Date:   Fri, 22 Nov 2019 14:42:39 -0700
From:   Dave Jiang <dave.jiang@...el.com>
To:     Dan Williams <dan.j.williams@...el.com>,
        Logan Gunthorpe <logang@...tatee.com>
Cc:     Vinod Koul <vkoul@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>
Subject: Re: [PATCH 1/5] dmaengine: Store module owner in dma_device struct



On 11/22/19 2:01 PM, Dan Williams wrote:
> On Fri, Nov 22, 2019 at 12:56 PM Logan Gunthorpe <logang@...tatee.com> wrote:
>>
>>
>>
>> On 2019-11-22 1:50 p.m., Dan Williams wrote:
>>> On Fri, Nov 22, 2019 at 8:53 AM Dave Jiang <dave.jiang@...el.com> wrote:
>>>>
>>>>
>>>>
>>>> On 11/21/19 10:20 PM, Vinod Koul wrote:
>>>>> On 14-11-19, 10:03, Logan Gunthorpe wrote:
>>>>>>
>>>>>>
>>>>>> On 2019-11-13 9:55 p.m., Vinod Koul wrote:
>>>>>>>> But that's the problem. We can't expect our users to be "nice" and not
>>>>>>>> unbind when the driver is in use. Killing the kernel if the user
>>>>>>>> unexpectedly unbinds is not acceptable.
>>>>>>>
>>>>>>> And that is why we review the code and ensure this does not happen and
>>>>>>> behaviour is as expected
>>>>>>
>>>>>> Yes, but the current code can kill the kernel when the driver is unbound.
>>>>>>
>>>>>>>>>> I suspect this is less of an issue for most devices as they wouldn't
>>>>>>>>>> normally be unbound while in use (for example there's really no reason
>>>>>>>>>> to ever unbind IOAT seeing it's built into the system). Though, the fact
>>>>>>>>>> is, the user could unbind these devices at anytime and we don't want to
>>>>>>>>>> panic if they do.
>>>>>>>>>
>>>>>>>>> There are many drivers which do modules so yes I am expecting unbind and
>>>>>>>>> even a bind following that to work
>>>>>>>>
>>>>>>>> Except they will panic if they unbind while in use, so that's a
>>>>>>>> questionable definition of "work".
>>>>>>>
>>>>>>> dmaengine core has module reference so while they are being used they
>>>>>>> won't be removed (unless I complete misread the driver core behaviour)
>>>>>>
>>>>>> Yes, as I mentioned in my other email, holding a module reference does
>>>>>> not prevent the driver from being unbound. Any driver can be unbound by
>>>>>> the user at any time without the module being removed.
>>>>>
>>>>> That sounds okay then.
>>>>
>>>> I'm actually glad Logan is putting some work in addressing this. I also
>>>> ran into the same issue as well dealing with unbinds on my new driver.
>>>
>>> This was an original mistake of the dmaengine implementation that
>>> Vinod inherited. Module pinning is distinct from preventing device
>>> unbind which ultimately can't be prevented. Longer term I think we
>>> need to audit dmaengine consumers to make sure they are prepared for
>>> the driver to be removed similar to how other request based drivers
>>> can gracefully return an error status when the device goes away rather
>>> than crashing.
>>
>> Yes, but that will be a big project because there are a lot of drivers.
> 
> Oh yes, in fact I think it's something that can only reasonably be
> considered for new consumers.
> 
>> But I think the dmaengine common code needs to support removal properly,
>> which essentially means changing how all the drivers allocate and free
>> their structures, among other things.
>>
>> The one saving grace is that most of the drivers are for SOCs which
>> can't be physically removed and there's really no use-case for the user
>> to call unbind.
> 
> Yes, the SOC case is not so much my concern as the generic offload use
> cases, especially if those offloads are in a similar hotplug domain as
> a cpu.
> 

It becomes a bigger issue when "channels" can be reconfigured and can 
come and go in a hot plug fashion.

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