linux-kernel - Re: [alsa-devel] [RFC PATCH 6/6] soundwire: qcom: add support for SoundWire controller

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Message-ID: <0e836692-2297-4cb7-d681-76692db78a56@linux.intel.com>
Date:   Tue, 11 Jun 2019 07:21:20 -0500
From:   Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
To:     Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
        broonie@...nel.org, vkoul@...nel.org
Cc:     mark.rutland@....com, devicetree@...r.kernel.org,
        alsa-devel@...a-project.org, robh+dt@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [alsa-devel] [RFC PATCH 6/6] soundwire: qcom: add support for
 SoundWire controller



On 6/11/19 5:29 AM, Srinivas Kandagatla wrote:
> 
> 
> On 10/06/2019 15:12, Pierre-Louis Bossart wrote:
>>>>> +
>>>>> +    if (dev_addr == SDW_BROADCAST_DEV_NUM) {
>>>>> +        ctrl->fifo_status = 0;
>>>>> +        ret = wait_for_completion_timeout(&ctrl->sp_cmd_comp,
>>>>> +                          msecs_to_jiffies(TIMEOUT_MS));
>>>>
>>>> This is odd. The SoundWire spec does not handle writes to a single 
>>>> device or broadcast writes differently. I don't see a clear reason 
>>>> why you would only timeout for a broadcast write.
>>>>
>>>
>>> There is danger of blocking here without timeout.
>>
>> Right, and it's fine to add a timeout. The question is why add a 
>> timeout *only* for a broadcast operation? It should be added for every 
>> transaction IMO, unless you have a reason not to do so.
>>
> 
> I did try this before, the issue is when we read/write registers from 
> interrupt handler, these can be deadlocked as we will be interrupt 
> handler waiting for another completion interrupt, which will never 
> happen unless we return from the first interrupt.

I don't quite get the issue. With the Intel hardware we only deal with 
Master registers (some of which mirror the bus state) in the handler and 
will only modify Slave registers in the thread. All changes to Slave 
registers will be subject to a timeout as well as a check for no 
response or NAK. Not sure what is specific about your solution that 
requires a different handling of commands depending on which device 
number is used. It could very well be that you've uncovered a flaw in 
the bus design but I still don't see how it would be Qualcomm-specific?