[alsa-devel] firewire-lib: an issue to generate packet with 'no data' in blocking mode

[Takashi Sakamoto]

Hi Clemens,

 > I guess I just didn't notice that empty packets don't need to change
 > their FDF field.  This is a bug.

I also paied no attension to empty packets. Then BeBoB chipset teach me.

 > Empty packets should be fine for all devices.  (NO-DATA packets would
 > waste DMA bandwidth.)

I completely agree with you. Would you review an attached patch?


Thanks

Takashi Sakamoto

(Nov 22 2013 20:47), Clemens Ladisch wrote:
> Takashi Sakamoto wrote:
>> I have a question about generate packet with 'no data' in blocking mode.
>> I think there is out of specification in current firewire-lib.
>>
>> In my understanding of IEC 61883-6, there are two ways:
>>
>> 1. generate 'empty packet' defined in IEC 61883-1
>>     - size of packet is 2 quadlets
>>     - FDF = sfc
>>     - packet includes just CIP headers
>>
>> 2. generate 'special non-empty packet' defined in IEC 61883-6
>>     - size of packet is following to blocking mode
>>     - FDF = 0xff ('NO-DATA' code)
>>     - packet includes dummy data
>>
>> But current implementation is a strange combination of them.
>>   - size of packet is 2 (way 1)
>>   - FDF = 0xff (way 2)
>
> It's an empty NO-DATA packet.  ;-)
>
> I guess I just didn't notice that empty packets don't need to change
> their FDF field.  This is a bug.
>
>> If this is a qurk for some devices, I'll prepare patches to switch
>> generating mode because BeBoB cannot sound with current firewire-lib. If
>> this is a bug, then I want to discuss which is better for firewire-lib.
>
> Empty packets should be fine for all devices.  (NO-DATA packets would
> waste DMA bandwidth.)
>
>
> Regards,
> Clemens
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