On Mon, 2 Mar 2020 11:43:52 +0200
Tzvetomir Stoyanov <tz.stoyanov@xxxxxxxxx> wrote:
> On Sat, Feb 29, 2020 at 2:28 AM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> >
> >
> > Now that I'm playing with patch 14, I took more interest in this code.
> >
> > On Thu, 27 Feb 2020 16:19:59 +0200
> > "Tzvetomir Stoyanov (VMware)" <tz.stoyanov@xxxxxxxxx> wrote:
> >
> > > +
> > > +#define PROTO_MASK_SIZE (sizeof(char))
> >
> > Hmm, this is the size in bytes of the mask, not bits. You may need both.
> >
> > #define PROTO_MASK_BITS (PROTO_MASK_SIZE * 8)
> >
> > Because we can have up to 8 protocols per mask size (8 bits in a byte).
> >
> > > +/**
> > > + * tracecmd_tsync_proto_select - Select time sync protocol, to be used for
> > > + * timestamp synchronization with a peer
> > > + *
> > > + * @proto_mask: bitmask array of time sync protocols, supported by the peer
> > > + * @length: size of the @protos array
> > > + *
> > > + * Retuns Id of a time sync protocol, that can be used with the peer, or 0
> > > + * in case there is no match with supported protocols
> > > + */
> > > +unsigned int tracecmd_tsync_proto_select(char *proto_mask, int length)
> > > +{
> > > + struct tsync_proto *selected = NULL;
> > > + struct tsync_proto *proto;
> > > + int word;
> > > + int id;
> > > +
> > > + for (word = 0; word < length; word++) {
> > > + for (proto = tsync_proto_list; proto; proto = proto->next) {
> > > + if (proto->proto_id < word * PROTO_MASK_SIZE)
> > > + continue;
> >
> > The above should be: proto->proto_id < word * PROTO_MASK_BITS
> >
> > Because what you have currently is:
> >
> > proto->proto_id < word * 1
> >
> >
> > > +
> > > + id = proto->proto_id - word * PROTO_MASK_SIZE;
> >
> > And here you want PROTO_MASK_BITS, otherwise if we have a proto_id of 2
> > (which would fit as a bit in a char), this would become:
> >
> > id = 2 - 0 * 1 = 1
> >
> > > + if (id >= PROTO_MASK_SIZE)
> >
> > Then this is: 2 >= 1 which would skip it.
> >
> > Hmm, maybe you don't even need PROTO_MASK_SIZE and only need
> > PROTO_MASK_BITS.
> >
> The PROTO_MASK_SIZE is the size of 1 word of the mask array, in bytes.
> In the first implementation the array was of integers, and the define
> was 4. It was very easy to switch to char, that's why I prefer to keep
> the implementation as is - even though that "proto->proto_id < word *
> 1".
> The confusion is the name of the variable "id", which is actually the
> index of the bit inside the given bitmask's word. In the example of
> proto_id 2:
> - the word is 0, this proto_id is in the first word from the bitmask array.
> - the index of the bit in this word is 2 ( id = 2 - 0 * 1 = 2 )
Yes, 2 (that's what I meant), but then my point still is valid.
> - then we can check if the bit, representing the proto_id 2 is up:
> if ((1 << id) & proto_mask[word]) ...
I'm talking about this:
+ id = proto->proto_id - word * PROTO_MASK_SIZE;
+ if (id >= PROTO_MASK_SIZE)
+ continue;
Where id = 2, then if (2 >= 1) continue, means we skip it.
You need: if (id >= PROTO_MASK_SIZE_BITS)
-- Steve
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