Date:   Tue, 15 Mar 2022 13:27:10 +0100
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Brian Norris <briannorris@...omium.org>,
        Heiner Kallweit <hkallweit1@...il.com>
Cc:     Adrian Hunter <adrian.hunter@...el.com>,
        Linux Kernel <linux-kernel@...r.kernel.org>,
        Douglas Anderson <dianders@...omium.org>,
        Shawn Lin <shawn.lin@...k-chips.com>, linux-mmc@...r.kernel.org
Subject: Re: [PATCH] mmc: core: Set HS clock speed before sending HS CMD13

+ Heiner

On Tue, 15 Mar 2022 at 00:11, Brian Norris <briannorris@...omium.org> wrote:
>
> On Mon, Mar 14, 2022 at 6:13 AM Adrian Hunter <adrian.hunter@...el.com> wrote:
> >
> > On 10.3.2022 22.56, Brian Norris wrote:
> > > Way back in commit 4f25580fb84d ("mmc: core: changes frequency to
> > > hs_max_dtr when selecting hs400es"), Rockchip engineers noticed that
> > > some eMMC don't respond to SEND_STATUS commands very reliably if they're
> > > still running at a low initial frequency. As mentioned in that commit,
> > > JESD84-B51 P49 suggests a sequence in which the host:
> > > 1. sets HS_TIMING
> > > 2. bumps the clock ("<= 52 MHz")
> > > 3. sends further commands
> > >
> > > It doesn't exactly require that we don't use a lower-than-52MHz
> > > frequency, but in practice, these eMMC don't like it.
> > >
> > > Anyway, the aforementioned commit got that right for HS400ES, but the
> > > refactoring in 53e60650f74e ("mmc: core: Allow CMD13 polling when
> > > switching to HS mode for mmc") messed that back up again, by reordering
> > > step 2 after step 3.
> >
> > That description might not be accurate.
>
> I've been struggling to track where things were working, where things
> were broken, and what/why Shawn's original fix was, precisely. So you
> may be correct in many ways :) Thanks for looking.
>
> > It looks like 4f25580fb84d did not have the intended effect because
> > CMD13 was already being sent by mmc_select_hs(), still before increasing
> > the frequency.  53e60650f74e just kept that behaviour.
>
> You may be partially right, or fully right. But anyway, I think I have
> some additional explanation, now that you've pointed that out: that
> behavior changed a bit in this commit:
>
> 08573eaf1a70 mmc: mmc: do not use CMD13 to get status after speed mode switch
>
> While that patch was merged in July 2016 and Shawn submitted his v1
> fix in September, there's a very good chance that a lot of his work
> was actually done via backports, and even if not, he may not have been
> testing precisely the latest -next kernel when submitting. So his fix
> may have worked out for _some_ near-upstream kernel he was testing in
> 2016, you may be correct that it didn't really work in the state it
> was committed to git history.
>
> This may also further explain why my attempts at bisection were rather
> fruitless (notwithstanding the difficulties in getting RK3399 running
> on that old of a kernel).
>
> Anyway, I'll see if I can improve the messaging if/when a v2 comes around.
>
> > > --- a/drivers/mmc/core/mmc.c
> > > +++ b/drivers/mmc/core/mmc.c
> ...
> > > @@ -1487,6 +1492,12 @@ static int mmc_select_hs200(struct mmc_card *card)
> > >               old_timing = host->ios.timing;
> > >               mmc_set_timing(host, MMC_TIMING_MMC_HS200);
> > >
> > > +             /*
> > > +              * Bump to HS frequency. Some cards don't handle SEND_STATUS
> > > +              * reliably at the initial frequency.
> > > +              */
> > > +             mmc_set_clock(host, card->ext_csd.hs_max_dtr);
> >
> > Is card->ext_csd.hs_max_dtr better than card->ext_csd.hs200_max_dtr here?
>
> I believe either worked in practice. I ended up choosing hs_max_dtr
> because it's lower and presumably safer. But frankly, I don't know
> what the Right thing to do is here, since the spec just talks about
> "<=", and yet f_init (which is also "<=") does not work. I think it
> might be like Ulf was guessing way back in the first place [1], and
> the key is that there is *some* increase (i.e., not using f_init).
>
> So assuming either works, would you prefer hs200_max_dtr here, since
> that does seem like the appropriate final rate?

I think that makes most sense, as we are switching to that rate anyway
just a few cycles later in mmc_select_timing(), when it calls
mmc_set_bus_speed().

That said, I have recently queued a patch that improves the
speed-mode-selection-fallback, when switching to HS200 mode fails [2].
We need to make sure this part still works as expected. I have looped
in Heiner who has been in the loop around this change, hopefully he
can help with further testing or so. Maybe $subject patch (or a new
version of it) can even make HS200 to work on Heiner's platform!?

>
> Brian
>
> [1] https://lore.kernel.org/all/CAPDyKFrNp=Y3BhVE_kxtggv7Qc6m=2kef2U8Dn2Bb3ANHPYV-Q@mail.gmail.com/
> Re: [PATCH 3/5] mmc: core: changes frequency to hs_max_dtr when
> selecting hs400es

Kind regards
Uffe

[2]
https://patchwork.kernel.org/project/linux-mmc/patch/20220303164522.129583-1-ulf.hansson@linaro.org/

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