[alsa-devel] [PATCH] ASoC: sigmadsp: safeload should be used from 4 bytes

[Danny Smith]

On 08/22/2018 03:05 PM, Lars-Peter Clausen wrote:
> On 08/22/2018 09:53 AM, Danny Smith wrote:
>>
>> On 08/22/2018 08:34 AM, Lars-Peter Clausen wrote:
>>> On 08/22/2018 08:11 AM, Robert Rosengren wrote:
>>>> From: Danny Smith <dannys at axis.com>
>>>>
>>>> Fixed range in safeload conditional to allow safeload to be used from
>>>> 4 bytes.
>>> Hi,
>>>
>>> Thanks for the patch. The reason why 4 bytes is excluded is that up to 4
>>> bytes can be updated atomically with a single register write. But I could
>>> see that if the firmware reads the same parameter multiple times during the
>>> same run you could get inconsistent results.
>>>
>>> Can you explain a bit more why you need this?
>>>
>>> If we want to allow safeload for 4 byte parameters the same reasoning
>>> applies for parameters with less than 4 bytes as well and so the check
>>> should be removed completely.
>>>
>>> - Lars
>> Hi,
>>
>> Thanks for the feedback. It is correct that up to 4 bytes can be updated
>> atomically but does that also guarantee that the data is safeloaded, i.e.
>> updated when the parameter is not in use? Maybe updating up to 4 byte
>> parameters without safeload does not cause audio glitches?
> The datasheet says:
>
> """
> To update parameters in real time while avoiding pop and click
> noises on the output, the ADAU1761 uses a software safeload
> mechanism. The software safeload mechanism enables the
> SigmaDSP core to load new parameters into RAM while guaranteeing
> that the parameters are not in use. This prevents an
> undesirable condition where an instruction could execute with
> a mix of old and new parameters.
> """
>
> The way I understand the last sentence is that there is no issue when
> loading only 4 bytes or less since there is no risk of mixing old and new data.
>
> I don't mind changing this, I was just curious if you had seen any issues
> with a 4 byte parameter update.
>
> - Lars
I have not seen any issues doing a 4byte parameter update without 
safeload but have not performed any extensive testing on the subject. 
The reason for the patch was that I noticed that sigmastudio uses 
safeload when doing 4byte writes but I cannot say if it is necessary or not.

Regards,
Danny