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From:   Doug Anderson <dianders@chromium.org>
Date:   Thu, 10 May 2018 15:37:54 -0700
Message-ID: <CAD=FV=X2k-g_LLuVZEbyJvc=_e68gcXREPRCOvvD=Fz566_Bgg@mail.gmail.com>
Subject: Re: [PATCH v7 04/10] drivers: qcom: rpmh: add RPMH helper functions
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,
        "open list:ARM/QUALCOMM SUPPORT" <linux-soc@vger.kernel.org>,
        Rajendra Nayak <rnayak@codeaurora.org>,
        Bjorn Andersson <bjorn.andersson@linaro.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Stephen Boyd <sboyd@kernel.org>,
        Evan Green <evgreen@chromium.org>
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Hi,

On Tue, May 8, 2018 at 9:05 AM,  <ilina@codeaurora.org> wrote:
> On 2018-05-03 14:26, Doug Anderson wrote:
> Hi Doug,
>
>
>> Hi,
>>
>> On Wed, May 2, 2018 at 12:37 PM, Lina Iyer <ilina@codeaurora.org> wrote:
>>>
>>> +static struct rpmh_ctrlr rpmh_rsc[RPMH_MAX_CTRLR];
>>> +static DEFINE_SPINLOCK(rpmh_rsc_lock);
>>> +
>>> +static struct rpmh_ctrlr *get_rpmh_ctrlr(const struct device *dev)
>>> +{
>>> +       int i;
>>> +       struct rsc_drv *p, *drv = dev_get_drvdata(dev->parent);
>>> +       struct rpmh_ctrlr *ctrlr = ERR_PTR(-EINVAL);
>>> +       unsigned long flags;
>>> +
>>> +       if (!drv)
>>> +               return ctrlr;
>>> +
>>> +       for (i = 0; i < RPMH_MAX_CTRLR; i++) {
>>> +               if (rpmh_rsc[i].drv == drv) {
>>> +                       ctrlr = &rpmh_rsc[i];
>>> +                       return ctrlr;
>>> +               }
>>> +       }
>>> +
>>> +       spin_lock_irqsave(&rpmh_rsc_lock, flags);
>>> +       list_for_each_entry(p, &rsc_drv_list, list) {
>>> +               if (drv == p) {
>>> +                       for (i = 0; i < RPMH_MAX_CTRLR; i++) {
>>> +                               if (!rpmh_rsc[i].drv)
>>> +                                       break;
>>> +                       }
>>> +                       if (i == RPMH_MAX_CTRLR) {
>>> +                               ctrlr = ERR_PTR(-ENOMEM);
>>> +                               break;
>>> +                       }
>>> +                       rpmh_rsc[i].drv = drv;
>>> +                       ctrlr = &rpmh_rsc[i];
>>> +                       break;
>>> +               }
>>> +       }
>>> +       spin_unlock_irqrestore(&rpmh_rsc_lock, flags);
>>
>>
>> I may have missed something, but to me it appears that this whole
>> "rsc_drv_list" is pretty pointless.  I wrote up a patch atop your
>> series to remove it at
>>
>> <https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/1042883/>
>> and it simplifies the code a whole bunch.  From that patch, my
>> justification was:
>>
>>> The global rsc_drv_list was (as far as I can tell) racy and not useful
>>> for anything.
>>>
>>> I say it is racy because in general you need some sort of mutual
>>> exclusion for lists.  If someone is adding to a list while someone
>>> else is iterating over it then you get badness.
>>>
>>> I say it is not useful because the only user of it was
>>> get_rpmh_ctrlr() and the only thing it did was to verify that the
>>> "struct rsc_drv *" that it alrady had was in the list.  How could it
>>> not be?
>>
>>
>> Note that in v7 of your series you added a spinlock around your access
>> of "rsc_drv_list", but this doesn't actually remove the race.
>> Specifically I'm pretty sure that the list primitives don't support
>> calling list_add() while someone might be iterating over the list and
>> your spinlock isn't grabbed in rpmh_rsc_probe().
>>
>> Note that I also say in my patch:
>>
>>> NOTE: After this patch get_rpmh_ctrlr() still seems a bit fishy.  I'm
>>> not sure why every caller would need its own private global cache of
>>> stuff.  ...but I left that part alone.
>>
>>
> I am not sure I understand this.

As I've said I haven't reviewed RPMh in any amount of detail and so
perhaps I don't understand something.

OK, I dug a little more and coded up something for you.  Basically
you're doing a whole bunch of iteration / extra work here to try to
come up with a way to associate an extra bit of data with each "struct
rsc_drv".  Rather than that, just add an extra field into "struct
rsc_drv".  Problem solved.

See http://crosreview.com/1054646 for what I mean.


>> I'll try to dig into this more so I could just be confused, but in
>> general it seems really odd to have a spinlock and something called a
>> "cache" at this level.  If we need some sort of mutual exclusion or
>> caching it seems like it should be stored in memory directly
>> associated with the RPMh device, not some external global.
>>
> The idea behind the locking is not to avoid the race between rpmh.c and
> rpmh-rsc.c. From the DT, the devices that are dependent on the RSCs are
> probed following the probe of the controller. And init is not that we are
> worried about.
> The condition here is to prevent the rpmh_rsc[] from being modified
> concurrently by drivers.

OK, I see the point of the locking now, but not the list still.
Sounds like Matthias agrees with me that the list isn't useful.  Seems
like you should squash my patch at http://crosreview.com/1042883 into
yours.

-Doug