linux-kernel - Re: [PATCH 3/5] soundwire: qcom: wait for fifo to be empty before suspend

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Date:   Thu, 9 Feb 2023 12:24:41 -0600
From:   Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
To:     Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
        vkoul@...nel.org
Cc:     yung-chuan.liao@...ux.intel.com, alsa-devel@...a-project.org,
        linux-kernel@...r.kernel.org, steev@...i.org,
        johan+linaro@...nel.org, quic_bjorande@...cinc.com
Subject: Re: [PATCH 3/5] soundwire: qcom: wait for fifo to be empty before
 suspend




>>>>> Wait for Fifo to be empty before going to suspend or before bank
>>>>> switch happens. Just to make sure that all the reads/writes are done.
>>>>
>>>> For the suspend case that seems like a valid approach, but for bank
>>>> switch don't we already have a bus->msg_lock mutex that will prevent
>>>> the
>>>> bank switch command from being sent before the other commands are
>>>> handled?
>>>
>>> All read/writes are fifo based, so writes could be still pending.
>>
>> I am not following. The bank switch happens with this function, where a
>> mutex is taken.
>>
>> int sdw_transfer(struct sdw_bus *bus, struct sdw_msg *msg)
>> {
>>     int ret;
>>
>>     mutex_lock(&bus->msg_lock);
>>
>>     ret = sdw_transfer_unlocked(bus, msg);
> 
> Qualcomm controller uses fifo to read/write, so return after writing to
> fifo might not always indicate that write is completed.
> 
> Qcom Soundwire controller do not have any synchronous interrupt
> mechanism to indicate write complete.

Ack, I forgot that one. Might be worth a comment or reworded commit message?