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

[Lina Iyer <ilina@codeaurora.org>]

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@chromium.org> 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