Re: [REGRESSION] ALSA: firewire-lib: heavy digital distortion with Fireface 800

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On 25/07/24 09:38, Takashi Iwai wrote:
On Thu, 25 Jul 2024 17:11:31 +0200,
Gustavo A. R. Silva wrote:



On 25/07/24 08:08, Gustavo A. R. Silva wrote:
Hi!

On 25/07/24 07:07, Takashi Iwai wrote:
On Thu, 25 Jul 2024 00:24:29 +0200,
edmund.raile wrote:

Bisection revealed that the bitcrushing distortion with RME FireFace 800
was caused by 1d717123bb1a7555
("ALSA: firewire-lib: Avoid -Wflex-array-member-not-at-end warning").

Reverting this commit yields restoration of clear audio output.
I will send in a patch reverting this commit for now, soonTM.

#regzbot introduced: 1d717123bb1a7555

While it's OK to have a quick revert, it'd be worth to investigate
further what broke there; the change is rather trivial, so it might be
something in the macro expansion or a use of flex array stuff.


I wonder is there is any log that I can take a look at. That'd be really
helpful.

OK, I found a discrepancy in how the `header_length` field in the flexible
structure (a struct that contains a flexible-array member) below is used:

include/linux/firewire.h:
458 struct fw_iso_packet {
...
465         u32 header_length:8;    /* Length of immediate header           */
466                                 /* tx: Top of 1394 isoch. data_block    */
467         u32 header[] __counted_by(header_length);
468 };

Take a look at the following piece of code:

sound/firewire/amdtp-stream.c:
1164         if (!(s->flags & CIP_NO_HEADER))
1165                 pkt_header_length = IT_PKT_HEADER_SIZE_CIP;

In the code above `pkt_header_length` is set to `IT_PKT_HEADER_SIZE_CIP`, which based
on the following macros is 8 _bytes_:

sound/firewire/amdtp-stream.c:37:#define CIP_HEADER_QUADLETS	2
sound/firewire/amdtp-stream.c:58:#define CIP_HEADER_SIZE		(sizeof(__be32) * CIP_HEADER_QUADLETS)
sound/firewire/amdtp-stream.c:72:#define IT_PKT_HEADER_SIZE_CIP		CIP_HEADER_SIZE

Then we use the DEFINE_FLEX() macro, which internally sets `template->header_length`
to `CIP_HEADER_QUADLETS`, which based on the macros above, takes the value
of 2 _elements_. We set `header_length` because such variable is the _counter_
used during the `__counted_by()` annotation in `struct fw_iso_packet`. The
_counter_ is the variable that holds the number of _elements_ in the flex-array
member at some point at run-time[1].

So, we set the counter to `CIP_HEADER_QUADLETS` because that's the total number
of _elements_ allocated for the flexible-array member `header[]` by the DEFINE_FLEX()
macro.

1183                 DEFINE_FLEX(struct fw_iso_packet, template, header,
1184                             header_length, CIP_HEADER_QUADLETS);
1185                 bool sched_irq = false;

Then we call function `build_it_pkt_header()` and pass as arguments a pointer
to `template`, and `pkt_header_length`, which at this point might hold the
value of 8 _bytes_.

1187                 build_it_pkt_header(s, desc->cycle, template, pkt_header_length,
1188                                     desc->data_blocks, desc->data_block_counter,
1189                                     desc->syt, i, curr_cycle_time);

Then inside function `build_it_pkt_header()`, the _counter_ is updated
`params->header_length = header_length;`:

  680 static void build_it_pkt_header(struct amdtp_stream *s, unsigned int cycle,
  681                                 struct fw_iso_packet *params, unsigned int header_length,
...
  692         if (header_length > 0) {
  693                 cip_header = (__be32 *)params->header;
  694                 generate_cip_header(s, cip_header, data_block_counter, syt);
  695                 params->header_length = header_length;
  696         } else {

This causes `params->header_length == 8`; however, only enough space for 2
_elements_ was allocated for the flex array (via DEFINE_FLEX()).

So, regardless of how `pkt_header_length` is intended to be used in the rest of
the code inside `build_it_pkt_header()`, this last update to `params->header_length`
seems to be incorrect.

So, my question here is whether this `header_length` struct member was originally
intended to be used as a counter for the elements in the flex array or as size
variable to hold the total number of bytes in the array?

Based on the comment "Length of immediate header", I suppose `header_length` would
hold _elements_ not _bytes_.

Thanks, now I took a look over the whole picture, and I guess there
are two problems:

- The header_length should be in bytes, as far as I read the code in
   drivers/firwire/*.  So the assumption in the commit d3155742db89
   ("firewire: Annotate struct fw_iso_packet with __counted_by()") was
   already wrong, and it couldn't be annotated like that -- unless we
   fix up all users of header_length field.

I see. In this case, the code comment should also be changed:

s/Length/Size

to prevent any further confusion, as Size is clearly for bytes.

- By the use of DEFINE_FLEX() in amdtp-stream.c, process_rx_packets()
   sets the header_length field to CIP_HEADER_QUADLETS (= 2) as
   default.  Meanwhile, build_it_pkt_header() doesn't touch
   header_length unless non-zero pkt_header_length is passed, supposing
   it being zero.  So this may lead to a bogus header_length, which is
   processed by the firewire core code wrongly.

The actual effect we see is likely the latter.  A simple fix would be
to use DEFINE_RAW_FLEX() instead of DEFINE_FLEX() like below.

Yes, `DEFINE_RAW_FLEX()` is the way to go in this case, as long as the
following patch is also applied:

--- a/include/linux/firewire.h
+++ b/include/linux/firewire.h
@@ -462,9 +462,9 @@ struct fw_iso_packet {
                                /* rx: Sync bit, wait for matching sy   */
        u32 tag:2;              /* tx: Tag in packet header             */
        u32 sy:4;               /* tx: Sy in packet header              */
-       u32 header_length:8;    /* Length of immediate header           */
+       u32 header_length:8;    /* Size of immediate header             */
                                /* tx: Top of 1394 isoch. data_block    */
-       u32 header[] __counted_by(header_length);
+       u32 header[];
 };

Thanks
- Gustavo



thanks,

Takashi

-- 8< --
--- a/sound/firewire/amdtp-stream.c
+++ b/sound/firewire/amdtp-stream.c
@@ -1180,8 +1180,8 @@ static void process_rx_packets(struct fw_iso_context *context, u32 tstamp, size_
  		(void)fw_card_read_cycle_time(fw_parent_device(s->unit)->card, &curr_cycle_time);
for (i = 0; i < packets; ++i) {
-		DEFINE_FLEX(struct fw_iso_packet, template, header,
-			    header_length, CIP_HEADER_QUADLETS);
+		DEFINE_RAW_FLEX(struct fw_iso_packet, template, header,
+				CIP_HEADER_QUADLETS);
  		bool sched_irq = false;
build_it_pkt_header(s, desc->cycle, template, pkt_header_length,



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