Message-ID: <20180514150008.GA25503@codeaurora.org>
Date:   Mon, 14 May 2018 09:00:08 -0600
From:   Lina Iyer <ilina@...eaurora.org>
To:     Doug Anderson <dianders@...omium.org>
Cc:     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 v7 04/10] drivers: qcom: rpmh: add RPMH helper functions

On Fri, May 11 2018 at 14:14 -0600, Doug Anderson wrote:
>Hi,
>
>On Fri, May 11, 2018 at 8:06 AM, Lina Iyer <ilina@...eaurora.org> wrote:
>>> 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 tried to avoid such pointer references and keep it object oriented
>> with this approach. I agree that we run through a list of 2 (at the max)
>> RSC to get the drv* from the rpmh_ctrlr. It is not going to be
>> expensive.
>
>Even if you wanted to keep things "object oriented" then IMHO your
>code still should change.  Sure, it's not computationally expensive to
>iterate through this list, but it adds an extra level of complexity
>that doesn't seem justified.
>
>If you _really_ needed an abstraction barrier then at least add a
>"void *client_data" to "struct rsc_drv.c".  At least you'd get rid of
>the ugly global list and store your pointer directly in the correct
>structure rather than creating an external entity.  Now it becomes
>100% obvious that there is exactly 1 "struct rpmh_ctrlr" for each
>controller.  ...but IMO there's enough intertwining between "rpmh.c"
>and "rpmh-rsc.c" that it would just be a waste because now you'd need
>to do extra memory allocation and freeing.  ...and if you just
>allocated the pointer in get_rpmh_ctrlr() it would also seem non-ideal
>because this one-time allocation (that affects _all_ RPMh clients)
>happens whenever one client happens to do the first access.  This is
>one-time init so it makes sense to do it at init time.
>
>I say that there's intertwining between "rpmh.c" and "rpmh-rsc.c"
>because both C files call directly into one another and have intimate
>knowledge of how the other works.  They aren't really separate things.
>Specifically I see that "rpmh-rsc" directly calls into "rpmh.c" when
>it calls rpmh_tx_done(), and of coruse "rpmh-rsc.c" directly calls
>into "rpmh.c".
>
>
>OK, so I've been browsing through the source code more so I can be a
>little more informed here.  As far as I can tell "rpmh.c"'s goal is:
>
>1. Put a simpler API atop "rpmh-rsc.c".  ...but why not just put that
>API directly into "rpmh-rsc.c" anyway?  If there was someone that
>needed the more complex API then having a "simpler" wrapper makes
>sense, but that's not the case, is it?
>
>2. Add caching atop "rpmh-rsc"
>
>
>I'll respond to some of your other patches too, but I think that the
>amount of code for caching is not very much.  I don't see the benefit
>of trying to split the code into two files.  Put them into one and
>then delete all the extra code you needed just the try to maintain
>some abstraction.
>
>
>> Another things this helps with is that, if the driver is not a child of
>> the RSC nodes in DT, then the drvdata of the parent would not a RSC node
>> and accessing that would result in a crash. This offers a cleaner exit
>> path for the error case.
>
>Why would the driver not be a child of the RSC nodes?  That's kinda
>like saying "if you try to instantiate an i2c device as a platform
>device then its probe will crash".  Yeah, it will.  Doctor, it hurts
>if I poke myself in my eye with this sharp stick, do you have any
>medicine that can help fix that?
>
>
>>>>> 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.
>>>
>> I saw your approach. I am okay with it for your tree,
>
>I'm not okay with it for the Chrome OS tree.  We need to match
>upstream, not fork for style reasons.  I'd rather take a driver that I
>think it overly complex but matches upstream than a private driver.
>
>
>> my approach comes
>> out of experiences in qcom platforms and how things tend to shape up in
>> the future. I would want you to consider my reasoning as well, before we
>> go forward.
>
>I suppose we can get advice from others who have worked in qcom
>platforms and see what they think.  My opinions come out of years of
>experience working with Linux drivers, so I guess we both have some
>experience under our belts that we're trying to leverage.
>
Doug, I am sorry it came out the wrong way. I meant to say, knowing
qualcomm platforms, as it has been in the past, we need this
flexibility. Things change with hardware variants just a wee bit that it
doesn't warrant a new interface, just a new driver or part of it to be
re-written. Keeping code separate out like this helps us maintain the
driver better.

Thanks for your reviews. I will try to address your comments before my
vacation, but I doubt I will get to all of it.

Thanks,
Lina

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