linux-kernel - Re: [PATCH 5/6] soundwire: qcom: update register read/write routine

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Date:   Mon, 1 Feb 2021 10:42:49 -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, sanyog.r.kale@...el.com,
        linux-kernel@...r.kernel.org, alsa-devel@...a-project.org
Subject: Re: [PATCH 5/6] soundwire: qcom: update register read/write routine



On 2/1/21 9:50 AM, Srinivas Kandagatla wrote:
> 
> 
> On 29/01/2021 19:33, Pierre-Louis Bossart wrote:
>>
>>
>> On 1/29/21 11:32 AM, Srinivas Kandagatla wrote:
>>> In the existing code every soundwire register read and register write
>>> are kinda blocked. Each of these are using a special command id that
>>
>> what does 'kinda blocked' mean?
> 
> I meant read/writes are waiting for completion interrupt!
> 
>>
>>> generates interrupt after it successfully finishes. This is really
>>> overhead, limiting and not really necessary unless we are doing
>>> something special.
>>>
>>> We can simply read/write the fifo that should also give exactly
>>> what we need! This will also allow to read/write registers in
>>> interrupt context, which was not possible with the special
>>> command approach.
>>
>> This is really unclear, sorry.
> 
> If read/writes are waiting for an interrupt, it becomes difficult to 
> read or write to any registers from same interrupt handler!

Well, yes, you need to handle the complete() at a lower level than the 
code that initiates the transactions otherwise you self-deadlock.

IIRC in the Intel initial code, the complete was in the handler and the 
register IOs in the thread.

> 
> 
>>
>>> +    if (id != SWR_BROADCAST_CMD_ID) {
>>> +        if (id < 14)
>>> +            id += 1;
>>> +        else
>>> +            id = 0;
>>
>> that is really odd. if id=13 (group2) then id becomes 14 (master 
>> address). A comment is really needed here.
> 
> This is magic value for each fifo read or write, so that we can verify 
> that them by comparing with this magic value!
> 
> This has nothing to do with device number!

You should probably add a comment here then, or use a #define instead of 
the 14 which threw me off.

> 
>>
>>> +    if (cmd_id == SWR_BROADCAST_CMD_ID) {
>>> +        /*
>>> +         * sleep for 10ms for MSM soundwire variant to allow broadcast
>>> +         * command to complete.
>>
>> that's also super-odd. There is nothing in SoundWire that makes any 
>> difference between a regular and a broadcast command. they all 
>> complete in the same time (a frame).
>>> +         */
>>> +        ret = wait_for_completion_timeout(&swrm->broadcast, (2 * 
>>> HZ/10));
>>
>> is this 10ms really or dependent on CONFIG_HZ?

comment missed?

>>
>>> +        if (!ret)
>>> +            ret = SDW_CMD_IGNORED;
>>> +        else
>>> +            ret = SDW_CMD_OK;
>>
>> no CMD_FAILED support?
> 
> Qcom controllers does not provide that information if the command is 
> ignored or failed by any means!
> 
> That was the behavior from the starting of this driver.

ah yes, now I remember this.