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Date: Tue, 27 Mar 2018 13:12:07 +0200
From: Simon Horman <horms@verge.net.au>
To: Wolfram Sang <wsa@the-dreams.de>
Cc: Ulf Hansson <ulf.hansson@linaro.org>,
        Wolfram Sang <wsa+renesas@sang-engineering.com>,
        Magnus Damm <magnus.damm@gmail.com>,
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Subject: Re: [PATCH v3 0/2] mmc: renesas_sdhi: add eMMC HS400 mode support
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On Tue, Feb 27, 2018 at 02:02:23PM +0100, Wolfram Sang wrote:
> 
> > Perhaps someone need to explain in more detail what the HW controller
> > needs to manage tuning for HS400? I don't like that we may end up
> > getting it magically to work, then it's we better understand the
> > details and if the current sequence provided by the mmc core can't
> > fulfill the need for this case, we may instead consider adding an
> > additional pre/post host ops.
> 
> Thanks. Exactly this thinking prevented me from saying "It works, let's
> ship it" ;)

Thanks, I've dug a little deeper into the history of this code and
the associated hardware documentation and it seems that the desired
sequence is as follows:

1. Reset HS400 mode (currently executed near beginning of tmio_mmc_set_ios())
2. Set clock for HS200 mode (tmio_mmc_set_ios())
3. Tune in HS200 mode
4. Set clock for HS200 mode (tmio_mmc_set_ios())
5. Set to HS400 mode (currently renesas_sdhi_prepare_hs400_tuning() which
                      is executed near the end of tmio_mmc_set_ios())

X. It is also necessary to disable the SCC after setting HS400 mode
   has occurred before setting to High Speed mode.
   This is currently executed near the beginning of tmio_mmc_set_ios().

The current implementation differs a little from the above and its
hard to tell if that is because of hardware requirements or as
an artefact of the implementation of steps 1 and 5 being hooked into
tmio_mmc_set_ios().

It seems to me that step 1 could use the MMC core prepare_hs400_tuning
hook. Its less clear to me where steps 5 and X should hook in but perhaps
tmio_mmc_set_ios() is the right place.