Re: [PATCH v6 05/10] drivers: qcom: rpmh-rsc: write sleep/wake requests to TCS

[Lina Iyer]

On Fri, Apr 27 2018 at 17:24 -0600, Doug Anderson wrote:
> Hi,
>
> On Fri, Apr 27, 2018 at 2:54 PM, Matthias Kaehlcke <mka@xxxxxxxxxxxx> wrote:
> > > > Am I getting something wrong here?
> > >
> > > The for_each_set_bit() should increment the 'i' and we would attempt to
> > > compare the first address in the request with the next command in the
> > > TCS cache. If they don't match we repeat the process again. If it does,
> > > then we loop through 'j' to find if the sequence matches.
> > >
> > > Did I miss something?
> >
> > One of us is clearly in need of more caffeine or ready for the
> > weekend, it might be me :) Maybe another pair of eyeballs could help
I need them both. Sorry about the back and forth. I understand what the
problem is. The code doesnt look right. I seem to have messed it up.
Thanks Matthias for being patient and going through this.

> > to resolve this deadlock ...
> >
> > My single stepping above assumes that tcs->cmd_cache[i] matches
> > cmd[0].addr, i.e. we either found the start of the sequence we are
> > looking for or another sequence that starts with the same address. My
> > claim is that the code returns i in either case, whether the
> > subsequent addresses match or not.
>
> I haven't reviewed this patch in detail, but I attempted to be another
> pair of eyes here.  Something is definitely wrong with the "for (j =
> 0; j < len; j++)" loop.  I believe the code that's written right now
> is equivalent to this much shorter function:
>
> +static int find_match(const struct tcs_group *tcs, const struct tcs_cmd *cmd,
> +                     int len)
> +{
> +       int i, j;
> +
> +       /* Check for already cached commands */
> +       for_each_set_bit(i, tcs->slots, MAX_TCS_SLOTS) {
> +               if (tcs->cmd_cache[i] == cmd[0].addr)
> +                       return i;
> +       }
> +
> +       return -ENODATA;
> +}
>
> Specifically the test "if (tcs->cmd_cache[i] != cmd[0].addr)" does not
> take "j" into account.  Thus if it was false when "j == 0" it will
> continue to be false for "j == 1", "j == 2", etc.  Eventually you'll
> hit the "else if (j == len - 1)" and return.
>
> I believe that's what Matthias has been saying.  I personally haven't
> looked at the rest of the patch to see how things out to be fixed, but
> I'm quite convinced that the function either has a bug or should be
> written as the shorter version I've written above.
Yes, this is incorrect in its current form. This is what it should be -

static int find_match(const struct tcs_group *tcs, const struct tcs_cmd *cmd,
                     int len)
{
       int i, j;

       /* Check for already cached commands */
       for_each_set_bit(i, tcs->slots, MAX_TCS_SLOTS) {
               if (tcs->cmd_cache[i] != cmd[0].addr)
                       continue;
               for (j = 0; j < len; j++) {
                       WARN(tcs->cmd_cache[i + j] != cmd[j].addr,
                            "Message does not match previous sequence.\n");
                       return -EINVAL;
               }
               if (j == len - 1)
                       return i;
       }

       return -ENODATA;
}


Thanks,
Lina