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

[Arend van Spriel <arend@broadcom.com>]

On 01/13/15 16:41, Ulf Hansson wrote:
> On 13 January 2015 at 16:11, Arend van Spriel<arend@broadcom.com>  wrote:
>> On 01/13/15 15:56, Ulf Hansson wrote:
>>>
>>> [...]
>>>
>>>>>> 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?
>>>>
>>>>
>>>> It is part of the SD Host Controller Standard Specification. The timer
>>>> ensures that re-tuning is done before temperature changes could affect
>>>> the
>>>> "sampling point". It is needed for re-tuning mode 1 for UHS-I modes like
>>>> SDR104.
>>>
>>>
>>> Does the spec say what value the timer should have?
>>
>>
>> It is read from the Capabilities register in the SD host controller, ie. in
>> field "Timer Count for Re-Tuning" (see below).
>>
>> Regards,
>> Arend
>>
>> Timer Count for Re-Tuning
>> This field indicates an initial value of the Re-Tuning Timer for Re-Tuning
>> Mode 1 to 3. Setting to 0 disables Re-Tuning Timer.
>> 0h      Re-Tuning Timer disabled
>> 1h      1 seconds
>> 2h      2 seconds
>> 3h      4 seconds
>> 4h      8 seconds
>> .....   ......................
>> n       2(n-1) seconds
>> .....   ......................
>> Bh      1024 seconds
>> Eh - Ch Reserved
>> Fh      Get information from other source
>
> Thanks for sharing this information, but unfortunate I don't
> understand much from it.
>
> Is the host driver intended to read/poll this register to find a good value?

You can download the spec (and others) here [1]. sdhci currently 
implements retuning mode 1, which is decribed in the spec:

Re-Tuning Timer Control Example for Re-Tuning Mode 1
The initial value of re-tuning timer is provided by Timer Count for 
Re-Tuning field in this register. The timer starts counting by loading 
the initial value. When the timer expires, the Host Driver marks an 
expiration flag. On receiving a command request, the Host driver checks 
the expiration flag. If the expiration flag is set, then the Host Driver 
should perform the re-tuning procedure before issuing a command. If the 
expiration flag is not set, then the Host Driver issues a command 
without performing the re-tuning procedure. Every time the re-tuning 
procedure is performed, the timer loads the new initial value and the 
expiration flag is cleared.

So the host controller could indeed update this register for subsequent 
retuning.

Regards,
Arend

[1] https://www.sdcard.org/downloads/pls/

> Isn't heat one of the most important factor that could effect the need
> for a re-tune? Does then the controller internally dynamically update
> this register (since it keep track of heat or similar)?

>
> Kind regards
> Uffe