linux-kernel - RE: [PATCH v2] drivers:soc:fsl:qbman:qman.c: Sleep instead of stuck hacking jiffies.

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Date:   Fri,  5 May 2017 08:01:54 +0200
From:   Karim Eshapa <karim.eshapa@...il.com>
To:     oss@...error.net
Cc:     claudiu.manoil@....com, roy.pledge@....com,
        colin.king@...onical.com, linuxppc-dev@...ts.ozlabs.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Karim Eshapa <karim.eshapa@...il.com>
Subject: RE: [PATCH v2] drivers:soc:fsl:qbman:qman.c: Sleep instead of stuck hacking jiffies.

>On 5/4/2017 5:07 PM, Scott Wood wrote:
>> On Thu, 2017-05-04 at 06:58 +0200, Karim Eshapa wrote:
>>> +    stop = jiffies + 10000;
>>> +    /*
>>> +     * if MR was full and h/w had other FQRNI entries to produce, we
>>> +     * need to allow it time to produce those entries once the
>>> +     * existing entries are consumed. A worst-case situation
>>> +     * (fully-loaded system) means h/w sequencers may have to do 3-4
>>> +     * other things before servicing the portal's MR pump, each of
>>> +     * which (if slow) may take ~50 qman cycles (which is ~200
>>> +     * processor cycles). So rounding up and then multiplying this
>>> +     * worst-case estimate by a factor of 10, just to be
>>> +     * ultra-paranoid, goes as high as 10,000 cycles. NB, we consume
>>> +     * one entry at a time, so h/w has an opportunity to produce new
>>> +     * entries well before the ring has been fully consumed, so
>>> +     * we're being *really* paranoid here.
>>> +     */
>> OK, upon reading this more closely it seems the intent was to delay for 10,000
>> *processor cycles* and somehow that got turned into 10,000 jiffies (which is
>> 40 seconds at the default Hz!).  We could just replace this whole thing with
>> msleep(1) and still be far more paranoid than was originally intended.
>>
>> Claudiu and Roy, any comments?
>Yes the timing here is certainly off, the code changed a few times since
>the comment was originally written.
>An msleep(1) seems reasonable here to me.

If the previous patch with msleep(1) is OK.
can I send a patch to slightly change the comments.

Thanks,
Karim