Re: [PATCH 02/13] mmc: host: Add facility to support re-tuning

Ulf Hansson
2015-01-13T14:23:00Z

On 13 January 2015 at 14:23, Adrian Hunter <adrian.hunter@intel.com> wrote:
> On 13/01/15 13:25, Ulf Hansson wrote:
>> Hi Adrian,
>>
>> Thanks for working on this and apologize for my late reply!
>>
>> On 5 December 2014 at 18:41, Adrian Hunter <adrian.hunter@intel.com> wrote:
>>> Currently, there is core support for tuning during
>>> initialization. There can also be a need to re-tune
>>> periodically (e.g. sdhci) or to re-tune after the
>>> host controller is powered off (e.g. after PM
>>> runtime suspend / resume) or to re-tune in response
>>> to CRC errors.
>>>
>>> The main requirements for re-tuning are:
>>> - ability to enable /disable re-tuning
>>> - ability to flag that re-tuning is needed
>>> - ability to re-tune before any request
>>> - ability to hold off re-tuning if the card is busy
>>> - ability to hold off re-tuning if re-tuning is in
>>> progress
>>> - ability to run a re-tuning timer
>>
>> I suggest we skip the support for the re-tuning timer in this initial
>> step and thus remove the related functionality from this patchset. It
>> adds complexity, but more important it's not obvious that it actually
>> will help. I am more concerned that it randomly will cause a request
>> latency and thus decrease performance.
>>
>> The re-tuning period can't be selected "perfectly", so in this initial
>> step let's instead just rely on re-tune from the request retry path.
>>
>> If we do see a need for a doing re-tuning periodically, how about
>> using the runtime PM suspend path (of the mmc card device). In that
>> way, we should be able to minimize the impact on performance.
>
> Thank you for looking at the patches.
>
> I am not sure I know what you mean. sdhci already has a re-tuning timer, so
> this is just moving it into core, where it won't be used by other drivers
> unless they enable it.

I am kind of questioning the re-tuning timer in sdhci. What is it good for?

Can sdhci rely on that the mmc core performs a re-tune from the
request retry mechanism instead?

>
> I am not sure what you want to leave in sdhci.c and what you want in core,
> if anything.

We need all the infrastructure code in the core. Much like what your
patchset does. Except that I would like you to remove the option of
having a timer and the corresponding complexity it adds.

>
> At a minimum I need sdhci to be able to switch from hs400 to hs200, re-tune,
> and switch back.

As stated, I am only questioning the timer, nothing else.

Kind regards
Uffe