netdev - Re: [net-next v4 10/12] ASoC: SOF: Introduce descriptors for SOF client

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Date:   Fri, 22 May 2020 13:48:00 -0500
From:   Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Ranjani Sridharan <ranjani.sridharan@...ux.intel.com>,
        Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
        davem@...emloft.net, gregkh@...uxfoundation.org,
        netdev@...r.kernel.org, linux-rdma@...r.kernel.org,
        nhorman@...hat.com, sassmann@...hat.com,
        Fred Oh <fred.oh@...ux.intel.com>
Subject: Re: [net-next v4 10/12] ASoC: SOF: Introduce descriptors for SOF
 client



On 5/22/20 1:40 PM, Jason Gunthorpe wrote:
> On Fri, May 22, 2020 at 01:35:54PM -0500, Pierre-Louis Bossart wrote:
>>
>>
>> On 5/22/20 12:10 PM, Jason Gunthorpe wrote:
>>> On Fri, May 22, 2020 at 10:33:20AM -0500, Pierre-Louis Bossart wrote:
>>>
>>>>> Maybe not great, but at least it is consistent with all the lifetime
>>>>> models and the operation of the driver core.
>>>>
>>>> I agree your comments are valid ones, I just don't have a solution to be
>>>> fully compliant with these models and report failures of the driver probe
>>>> for a child device due to configuration issues (bad audio topology, etc).
>>>
>>>
>>>> My understanding is that errors on probe are explicitly not handled in the
>>>> driver core, see e.g. comments such as:
>>>
>>> Yes, but that doesn't really apply here...
>>>> /*
>>>>    * Ignore errors returned by ->probe so that the next driver can try
>>>>    * its luck.
>>>>    */
>>>> https://elixir.bootlin.com/linux/latest/source/drivers/base/dd.c#L636
>>>>
>>>> If somehow we could request the error to be reported then probably we
>>>> wouldn't need this complete/wait_for_completion mechanism as a custom
>>>> notification.
>>>
>>> That is the same issue as the completion, a driver should not be
>>> making assumptions about ordering like this. For instance what if the
>>> current driver is in the initrd and the 2nd driver is in a module in
>>> the filesystem? It will not probe until the system boots more
>>> completely.
>>>
>>> This is all stuff that is supposed to work properly.
>>>
>>>> Not at the moment, no. there are no failures reported in dmesg, and
>>>> the user does not see any card created. This is a silent error.
>>>
>>> Creating a partial non-function card until all the parts are loaded
>>> seems like the right way to surface an error like this.
>>>
>>> Or don't break the driver up in this manner if all the parts are really
>>> required just for it to function - quite strange place to get into.
>>
>> This is not about having all the parts available - that's handled already
>> with deferred probe - but an error happening during card registration. In
>> that case the ALSA/ASoC core throws an error and we cannot report it back to
>> the parent.
> 
> The whole point of the virtual bus stuff was to split up a
> multi-functional PCI device into parts. If all the parts are required
> to be working to make the device work, why are you using virtual bus
> here?

It's the other way around: how does the core know that one part isn't 
functional.

There is nothing in what we said that requires that all parts are fully 
functional. All we stated is that when *one* part isn't fully functional 
we know about it.

>>> What happens if the user unplugs this sub driver once things start
>>> running?
>>
>> refcounting in the ALSA core prevents that from happening usually.
> 
> So user triggered unplug of driver that attaches here just hangs
> forever? That isn't OK either.

No, you'd get a 'module in use' error if I am not mistaken.