linux-kernel - Re: [PATCH v5 11/20] firmware: arm_scmi: add support for polling based SCMI transfers

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Date:   Mon, 19 Feb 2018 11:50:42 +0000
From:   Sudeep Holla <sudeep.holla@....com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Sudeep Holla <sudeep.holla@....com>,
        ALKML <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        DTML <devicetree@...r.kernel.org>,
        Roy Franz <roy.franz@...ium.com>,
        Harb Abdulhamid <harba@...eaurora.org>,
        Nishanth Menon <nm@...com>, Loc Ho <lho@....com>,
        Alexey Klimov <klimov.linux@...il.com>,
        Ryan Harkin <Ryan.Harkin@....com>,
        Jassi Brar <jassisinghbrar@...il.com>
Subject: Re: [PATCH v5 11/20] firmware: arm_scmi: add support for polling
 based SCMI transfers



On 19/02/18 11:32, Arnd Bergmann wrote:
> On Tue, Jan 2, 2018 at 3:42 PM, Sudeep Holla <sudeep.holla@....com> wrote:
> 
>> +#define SCMI_MAX_POLLING_TIMEOUT_NS    (100 * NSEC_PER_USEC)
>>  /**
>>   * scmi_do_xfer() - Do one transfer
>>   *
>> @@ -389,14 +406,30 @@ int scmi_do_xfer(const struct scmi_handle *handle, struct scmi_xfer *xfer)
> 
>> +       if (xfer->hdr.poll_completion) {
>> +               ktime_t stop, cur;
>> +
>> +               stop = ktime_add_ns(ktime_get(), SCMI_MAX_POLLING_TIMEOUT_NS);
>> +               do {
>> +                       udelay(5);
>> +                       cur = ktime_get();
>> +               } while (!scmi_xfer_poll_done(info, xfer) &&
>> +                        ktime_before(cur, stop));
> 
> The 5 microsecond back-off isn't that much smaller than the 100 microsecond
> timeout, given that udelay() often waits much longer than the specified time.
> 
> How did you come up with those two numbers? Are you sure this is better
> than just using a cpu_relax() instead of the udelay()?
> 

Somehow I assumed that cpu_relax will schedule out and since this is
called in the fast switching path, I can't do that. But now I see that
it's just an hint and so I can use it. Sorry for missing it earlier, you
did point this out in previous version and I retained it based on my
wrong assumption. Thanks.

-- 
Regards,
Sudeep