[PATCH 1/3] ALSA USB MIDI: Fix port starvation

[Clemens Ladisch]

Andreas Steinmetz wrote:
> On Mon, 2020-03-16 at 13:03 +0100, Clemens Ladisch wrote:
>> Andreas Steinmetz wrote:
>>> the snd_usbmidi_transmit_byte helper had to be converted to
>>> return output notification to allow for the 'repeat' shortcut.
>>
>> Why not simply handle one MIDI byte per port in each iteration?  It
>> could be argued that single-byte MIDI commands are likely to be real-
>> time messages and deserve to go first.
>
> Actually the patch does exactly this. As soon as the helper signals
> that a message is complete the next port is processed. The "repeat"
> loop is just necessary to get a complete class compliant message for
> transfer as the helper processes one byte at a time.

Why is it necessary to immedately get a complete class-compliant message?

> The range optimization is there to prevent O(2) performance cost if
> possible.

Do you mean O(n^2)?  For what n?

>> My original idea for that FIXME was to use round robin until the packet
>> is filled (or all ports are empty), and to store the next port index
>> (where to start for the next packet) in the endpoint.  This would be
>> able to distribute the balancing over multiple packets.
>
> The problem with "round robin until the packet is filled" is that in
> case of a large wMaxPacketSize there's then a huge latency.

This mechanism does not require using the original packet size.
The point is that we should not restart with the first port in each
packet.


Regards,
Clemens