Re: [PATCH v3 09/10] drivers: qcom: rpmh: add support for batch RPMH request

[Lina Iyer]

On Fri, Mar 16 2018 at 11:00 -0600, Stephen Boyd wrote:
> Quoting Lina Iyer (2018-03-08 14:55:40)
> > On Thu, Mar 08 2018 at 14:59 -0700, Stephen Boyd wrote:
> > >Quoting Lina Iyer (2018-03-02 08:43:16)
> > >> @@ -343,6 +346,146 @@ int rpmh_write(struct rpmh_client *rc, enum rpmh_state state,
> > >>  }
> > >>  EXPORT_SYMBOL(rpmh_write);
> > >>
> > >> +static int cache_batch(struct rpmh_client *rc,
> > >> +                     struct rpmh_request **rpm_msg, int count)
> > >> +{
> > >> +       struct rpmh_ctrlr *rpm = rc->ctrlr;
> > >> +       unsigned long flags;
> > >> +       int ret = 0;
> > >> +       int index = 0;
> > >> +       int i;
> > >> +
> > >> +       spin_lock_irqsave(&rpm->lock, flags);
> > >> +       while (rpm->batch_cache[index])
> > >
> > >If batch_cache is full.
> > >And if adjacent memory has bits set....
> > >
> > >This loop can go forever?
> > >
> > >Please add bounds.
> > >
> > How so? The if() below will ensure that it will not exceed bounds.
>
> Right, the if below will make sure we don't run off the end, but unless
> rpm->batch_cache has a sentinel at the end we can't guarantee we won't
> run off the end of the array and into some other portion of memory that
> also has a bit set in a word. And then we may read into some unallocated
> space. Or maybe I missed something.
The rpmh_write_batch checks to make sure the number of requests do not
exceed the max and would not exceed the value. This is ensured by the
write_batch request.
A write_batch follows an invalidate and therefore would ensure that that
the batch_cache does not overflow, but I can add a simple check, though
its unnecessary with the general use of this API.

> > >> +               index++;
> > >> +       if (index + count >=  2 * RPMH_MAX_REQ_IN_BATCH) {
> > >> +               ret = -ENOMEM;
> > >> +               goto fail;
> > >> +       }
> > >> +
> > >> +       for (i = 0; i < count; i++)
> > >> +               rpm->batch_cache[index + i] = rpm_msg[i];
> > >> +fail:
> > >> +       spin_unlock_irqrestore(&rpm->lock, flags);
> > >> +
> > >> +       return ret;
> > >> +}
> > >> +
> >
> > >> + * @state: Active/sleep set
> > >> + * @cmd: The payload data
> > >> + * @n: The array of count of elements in each batch, 0 terminated.
> > >> + *
> > >> + * Write a request to the mailbox controller without caching. If the request
> > >> + * state is ACTIVE, then the requests are treated as completion request
> > >> + * and sent to the controller immediately. The function waits until all the
> > >> + * commands are complete. If the request was to SLEEP or WAKE_ONLY, then the
> > >> + * request is sent as fire-n-forget and no ack is expected.
> > >> + *
> > >> + * May sleep. Do not call from atomic contexts for ACTIVE_ONLY requests.
> > >> + */
> > >> +int rpmh_write_batch(struct rpmh_client *rc, enum rpmh_state state,
> > >> +                   struct tcs_cmd *cmd, int *n)
> > >
> > >I'm lost why n is a pointer, and cmd is not a double pointer if n stays
> > >as a pointer. Are there clients calling this API with a contiguous chunk
> > >of commands but then they want to break that chunk up into many
> > >requests?
> > >
> > That is correct. Clients want to provide a big buffer that this API will
> > break it up into requests specified in *n.
>
> Is that for bus scaling?
Yes.

> > >> +               /* For those unsent requests, spoof tx_done */
> > >
> > >Why? Comments shouldn't say what the code is doing, but explain why
> > >things don't make sense.
> > >
> > Will remove..
>
> Oh, I was hoping for more details, not less.
Hmm.. I thought it was fairly obvious that we spoof tx_done so we could
complete when the wait_count reaches 0. I will add that to the comments.

Thanks,
Lina