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

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Date:   Fri, 27 Apr 2018 16:24:10 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Matthias Kaehlcke <mka@...omium.org>
Cc:     Lina Iyer <ilina@...eaurora.org>,
        Andy Gross <andy.gross@...aro.org>,
        David Brown <david.brown@...aro.org>,
        linux-arm-msm@...r.kernel.org,
        "open list:ARM/QUALCOMM SUPPORT" <linux-soc@...r.kernel.org>,
        Rajendra Nayak <rnayak@...eaurora.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Stephen Boyd <sboyd@...nel.org>,
        Evan Green <evgreen@...omium.org>
Subject: Re: [PATCH v6 05/10] drivers: qcom: rpmh-rsc: write sleep/wake
 requests to TCS

Hi,

On Fri, Apr 27, 2018 at 2:54 PM, Matthias Kaehlcke <mka@...omium.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
> 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.

-Doug