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From:   Evan Green <evgreen@chromium.org>
Date:   Thu, 22 Feb 2018 15:41:47 -0800
Message-ID: <CAE=gft6rBXxST3ha0VaO=A9tmOYfzeETPz8FzGsiZ1z80yqk3g@mail.gmail.com>
Subject: Re: [PATCH v2 07/10] drivers: qcom: rpmh: cache sleep/wake state requests
To:     Lina Iyer <ilina@codeaurora.org>
Cc:     Andy Gross <andy.gross@linaro.org>,
        David Brown <david.brown@linaro.org>,
        linux-arm-msm@vger.kernel.org, linux-soc@vger.kernel.org,
        Rajendra Nayak <rnayak@codeaurora.org>,
        Bjorn Andersson <bjorn.andersson@linaro.org>,
        linux-kernel@vger.kernel.org
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Sender: linux-kernel-owner@vger.kernel.org
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Hi Lina,

On Thu, Feb 22, 2018 at 8:58 AM, Lina Iyer <ilina@codeaurora.org> wrote:
> On Wed, Feb 21 2018 at 22:07 +0000, Evan Green wrote:
>>
>> Hi Lina,
>>
>> On Thu, Feb 15, 2018 at 9:35 AM, Lina Iyer <ilina@codeaurora.org> wrote:
>
> [...]
>
>>> +static struct cache_req *cache_rpm_request(struct rpmh_client *rc,
>>> +                                         enum rpmh_state state,
>>> +                                         struct tcs_cmd *cmd)
>>> +{
>>> +       struct cache_req *req;
>>> +       struct rpmh_ctrlr *rpm = rc->ctrlr;
>>> +       unsigned long flags;
>>> +
>>> +       spin_lock_irqsave(&rpm->lock, flags);
>>> +       req = __find_req(rc, cmd->addr);
>>> +       if (req)
>>> +               goto existing;
>>> +
>>> +       req = kzalloc(sizeof(*req), GFP_ATOMIC);
>>> +       if (!req) {
>>> +               req = ERR_PTR(-ENOMEM);
>>> +               goto unlock;
>>> +       }
>>> +
>>> +       req->addr = cmd->addr;
>>> +       req->sleep_val = req->wake_val = UINT_MAX;
>>
>>
>> So UINT_MAX is really never a valid value to write? Maybe it would be
>> good to at least print some sort of complaint if somebody sends down a
>> request with this value. Otherwise the request is silently ignored and
>> would be quite challenging to track down.
>>
> Yes, UINT_MAX is a invalid value. I have not encountered anything in the
> spec that points to hold UINT_MAX as a valid value.
> It would also be quite inefficient to validate each input. Drivers
> generally know what they are doing. These are data sent to the hardware
> and there is a general understanding that incorrect data may fail
> silently.
>
>

I see. I added this suggestion as it seemed worth calling out a
software-based assumption about an unknown hardware-specific value. In
the off-chance that UINT_MAX were to be a valid value in the future,
this would certainly surprise a driver writer who essentially did
everything right and wasn't expecting their request to be dropped
before reaching the hardware. But if you're confident this won't ever
happen, I'm willing to let this go.

>>> +/**
>>> + * rpmh_flush: Flushes the buffered active and sleep sets to TCS
>>> + *
>>> + * @rc: The RPMh handle got from rpmh_get_dev_channel
>>> + *
>>> + * This function is generally called from the sleep code from the last
>>> CPU
>>> + * that is powering down the entire system.
>>> + *
>>> + * Returns -EBUSY if the controller is busy, probably waiting on a
>>> response
>>> + * to a RPMH request sent earlier.
>>> + */
>>> +int rpmh_flush(struct rpmh_client *rc)
>>> +{
>>> +       struct cache_req *p;
>>> +       struct rpmh_ctrlr *rpm = rc->ctrlr;
>>> +       int ret;
>>> +       unsigned long flags;
>>> +
>>> +       if (IS_ERR_OR_NULL(rc))
>>> +               return -EINVAL;
>>> +
>>> +       spin_lock_irqsave(&rpm->lock, flags);
>>> +       if (!rpm->dirty) {
>>> +               pr_debug("Skipping flush, TCS has latest data.\n");
>>> +               spin_unlock_irqrestore(&rpm->lock, flags);
>>> +               return 0;
>>> +       }
>>> +       spin_unlock_irqrestore(&rpm->lock, flags);
>>> +
>>> +       /*
>>> +        * Nobody else should be calling this function other than system
>>> PM,,
>>> +        * hence we can run without locks.
>>> +        */
>>> +       list_for_each_entry(p, &rc->ctrlr->cache, list) {
>>> +               if (!is_req_valid(p)) {
>>> +                       pr_debug("%s: skipping RPMH req: a:0x%x s:0x%x
>>> w:0x%x",
>>> +                               __func__, p->addr, p->sleep_val,
>>> p->wake_val);
>>> +                       continue;
>>> +               }
>>> +               ret = send_single(rc, RPMH_SLEEP_STATE, p->addr,
>>> p->sleep_val);
>>> +               if (ret)
>>> +                       return ret;
>>> +               ret = send_single(rc, RPMH_WAKE_ONLY_STATE, p->addr,
>>> +                                               p->wake_val);
>>> +               if (ret)
>>> +                       return ret;
>>> +       }
>>> +
>>> +       spin_lock_irqsave(&rpm->lock, flags);
>>> +       rpm->dirty = false;
>>> +       spin_unlock_irqrestore(&rpm->lock, flags);
>>> +
>>
>>
>> I've got some questions on the locking in this function.
>>
>> I understand that the lock protects the list, and I'm surmising that
>> you don't want to hold the lock across send_single (even though
>> there's another lock in there that's held for most of that time, so I
>> think you could). I'm still a newbie to Linux in general, so I'll pose
>> this as a question: is it generally okay in Linux to traverse across a
>> list that may have items concurrently added to it? You're never
>> removing items from this list, so I think there are no actual bugs,
>> but it does seem like it relies on the implementation details of the
>> list. And if you ever did remove items from the list, this would bite
>> you.
>>
> It is generally not advisable to traverse a list if the list is being
> modified externally. The reason why it is okay here, is the context of
> this function call. The only caller of this function will be a system PM
> driver. As noted in the comments, this is called when the last CPU is
> powering down and in such a context, there are no other active users of
> this library.
>

Response below.

>> Also, why do you need to acquire the lock just to set dirty to false?
>> Right now it looks like there's a race where someone could add an
>> element to this list just after you've terminated this loop (but
>> before you have the lock), but then the dirty = false here clobbers
>> their dirty = true, and the item is never sent during future flushes.
>>
>> I think it would be safer and faster to set dirty = false before
>> iterating through the list (either within the lock or outside of it
>> given that this is the only place that reads or clears dirty). That
>> way if new elements sneak in you know that they will either be flushed
>> already or dirty will be true for them on a subsequent flush.
>>
> ->dirty will be accessed as a u32 and so does not require a lock per-se.
> The lock here guarantees a certain system state when the flag is
> changed. While in this function it is not needed since, there will be no
> active threads changing the system state. I could possibly remove the
> lock in this function.
>

Yes, please remove all the locking in this function, as it doesn't
successfully synchronize anything. I suppose you still need locking in
cache_rpm_request to prevent multiple threads calling that function
concurrently, but you can remove it everywhere else. Adding broken
locking only invites someone in the future to assume that it works.

>
>>> +       return 0;
>>> +}
>>> +EXPORT_SYMBOL(rpmh_flush);
>>> +
>>> +/**
>>> + * rpmh_invalidate: Invalidate all sleep and active sets
>>> + * sets.
>>> + *
>>> + * @rc: The RPMh handle got from rpmh_get_dev_channel
>>> + *
>>> + * Invalidate the sleep and active values in the TCS blocks.
>>> + */
>>> +int rpmh_invalidate(struct rpmh_client *rc)
>>> +{
>>> +       struct rpmh_ctrlr *rpm = rc->ctrlr;
>>> +       int ret;
>>> +       unsigned long flags;
>>> +
>>> +       if (IS_ERR_OR_NULL(rc))
>>> +               return -EINVAL;
>>> +
>>> +       spin_lock_irqsave(&rpm->lock, flags);
>>> +       rpm->dirty = true;
>>> +       spin_unlock_irqrestore(&rpm->lock, flags);
>>
>>
>> I don't think the lock acquire/release provides anything here, can't
>> you just set dirty = true?
>>
> It helps synchronize to the state of the controller.

No, I don't think it does. Please remove the locking here. If you need
a wmb after setting dirty, then add it (but I don't think you do).

>
>> So rpmh_invalidate clears any pending requests in the hardware, but
>> all the cached address/data pairs are all still in the cache, right?
>> As soon as someone else adds a new request and sets dirty to true, all
>> of these old ones get resent as well at flush, right? Is that the
>> desired behavior? Does anyone ever need to remove an address/data pair
>> from the RPMh's to-do list?
>>
> Drivers overwrite their requests and I have not had a requirement to
> remove their previous votes. Removing from the controller's registers is
> good enough to not send the data. If a driver calls invalidate, it is
> generally followed by a new sleep/wake request.
>

OK. There's an implicit assumption there that drivers need to update
_all_ of the address/data pairs they've ever submitted, or accept
stale ones being applied even after rpmh_invalidate. This is not true
for your invalidate_cache patch later, which does remove all the saved
entries. I wonder if that should at least be called out somewhere? Up
to you.

> Thanks,
> Lina
>
>>> +
>>> +       do {
>>> +               ret = rpmh_rsc_invalidate(rc->ctrlr->drv);
>>> +       } while (ret == -EAGAIN);
>>> +
>>> +       return ret;
>>> +}
>>> +EXPORT_SYMBOL(rpmh_invalidate);
>>> +