[alsa-devel] [PATCH 42/44] bebob/firewire-lib: Add a quirk of wrong dbc in empty packet for M-Audio special Firewire series

[Takashi Sakamoto]

I realize this patch includes a bug to cause wrong detection of packet 
discontinuity.

> @@ -745,6 +745,8 @@ static void handle_in_packet(struct amdtp_stream *s,
>
>   	/* Check data block counter continuity */
>   	data_block_counter = cip_header[0] & AMDTP_DBC_MASK;
> +	if (data_blocks == 0 && (s->flags & CIP_EMPTY_HAS_WRONG_DBC))
> +		data_block_counter = s->data_block_counter;
>   	if (!(s->flags & CIP_DBC_IS_END_EVENT)) {
>   		lost = data_block_counter != s->data_block_counter;
>   	} else {

These lines should be:

 > +	if (data_blocks == 0 && (s->flags & CIP_EMPTY_HAS_WRONG_DBC) &&
 > +         s->data_block_counter != UINT_MAX)
 > +		data_block_counter = s->data_block_counter;


When CIP_SKIP_INIT_DBC_CHECK is enabled, initial value of 
s->data_block_counter is set to UINT_MAX. An intention of this is to 
skip detecting discontinuity for a first packet [31/44].

This initial value has a bad effect when CIP_EMPTY_HAS_WRONG_DBC is enabled.

A first packet is an empty packet. When this packet is handled, 
data_block_counter is set to initial value of s->data_block_counter 
(UINT_MAX). Later, s->data_block_counter is set to 0xff with bitmask in 
below lines.

         if (s->flags & CIP_DBC_IS_END_EVENT)
                 s->data_block_counter = data_block_counter;
         else
                 s->data_block_counter =
                                 (data_block_counter + data_blocks) & 0xff;

When a packet with data blocks is handled later, s->data_block_counter 
is 0xff, not UINT_MAX. This causes discontinuity detection because this 
packet has 0x00 in itx dbc in below lines.

         if (lost && s->data_block_counter != UINT_MAX) {
                 dev_info(&s->unit->device,
                          "Detect discontinuity of CIP: %02X %02X\n",
                          s->data_block_counter, data_block_counter);

See this log:
[ 2933.102892] 01020000 9002FFFF 02
[ 2933.102894] 01020000 9002FFFF 02
[ 2933.102896] 01020000 9002FFFF 02
[ 2933.102899] 01020000 9002FFFF 02
[ 2933.102901] 01020000 9002FFFF 02
[ 2933.102903] 01020000 9002FFFF 02
[ 2933.104871] 01110000 900258B5 8A
[ 2933.104877] snd-bebob fw1.0: Detect discontinuity of CIP: FF 00

I'm sorry to post a patch including a bug.


Regards

Takashi Sakamoto
o-takashi at sakamocchi.jp