Date:	Tue, 27 May 2014 11:32:00 +0200
From:	Ulf Hansson <ulf.hansson@...aro.org>
To:	Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
Cc:	Russell King <linux@....linux.org.uk>,
	linux-mmc <linux-mmc@...r.kernel.org>,
	Chris Ball <chris@...ntf.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-arm-msm@...r.kernel.org,
	Linus Walleij <linus.walleij@...aro.org>
Subject: Re: [PATCH v3 12/13] mmc: mmci: add explicit clk control

On 27 May 2014 00:39, Srinivas Kandagatla
<srinivas.kandagatla@...aro.org> wrote:
> Hi Ulf,
> Thankyou for the comments.
>
>
> On 26/05/14 15:21, Ulf Hansson wrote:
>>
>> On 23 May 2014 14:52,  <srinivas.kandagatla@...aro.org> wrote:
>>>
>>> From: Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
>>>
>>> On Controllers like Qcom SD card controller where cclk is mclk and mclk
>>> should
>>> be directly controlled by the driver.
>>>
>>> This patch adds support to control mclk directly in the driver, and also
>>> adds explicit_mclk_control and cclk_is_mclk flags in variant structure
>>> giving
>>> more flexibility to the driver.
>>>
>>> Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
>>> Reviewed-by: Linus Walleij <linus.walleij@...aro.org>
>>> ---
>>>   drivers/mmc/host/mmci.c | 30 +++++++++++++++++++++++++-----
>>>   1 file changed, 25 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/drivers/mmc/host/mmci.c b/drivers/mmc/host/mmci.c
>>> index 5cbf644..f6dfd24 100644
>>> --- a/drivers/mmc/host/mmci.c
>>> +++ b/drivers/mmc/host/mmci.c
>>> @@ -73,6 +73,8 @@ static unsigned int fmax = 515633;
>>>    * @pwrreg_nopower: bits in MMCIPOWER don't controls ext. power supply
>>>    * @mclk_delayed_writes: enable delayed writes to ensure, subsequent
>>> updates
>>>    *                      are not ignored.
>>> + * @explicit_mclk_control: enable explicit mclk control in driver.
>>> + * @cclk_is_mclk: enable iff card clock is multimedia card adapter
>>> clock.
>>>    */
>>>   struct variant_data {
>>>          unsigned int            clkreg;
>>> @@ -94,6 +96,8 @@ struct variant_data {
>>>          bool                    busy_detect;
>>>          bool                    pwrreg_nopower;
>>>          bool                    mclk_delayed_writes;
>>> +       bool                    explicit_mclk_control;
>>> +       bool                    cclk_is_mclk;
>>
>>
>> I can't see why you need to have both these new configurations. Aren't
>> "cclk_is_mclk" just a fact when you use "explicit_mclk_control".
>>
>
>
>> I also believe I would prefer something like "qcom_clkdiv" instead.
>
>
> There is a subtle difference between both the flags.  Am happy to change it
> to qcom_clkdiv.
>
>
>>
>>>   };
>>>
>>>   static struct variant_data variant_arm = {
>>> @@ -202,6 +206,8 @@ static struct variant_data variant_qcom = {
>>>           * for 3 clk cycles.
>>>           */
>>>          .mclk_delayed_writes    = true,
>>> +       .explicit_mclk_control  = true,
>>> +       .cclk_is_mclk   = true,
>>>   };
>>>
>>>   static inline u32 mmci_readl(struct mmci_host *host, u32 off)
>>> @@ -317,7 +323,9 @@ static void mmci_set_clkreg(struct mmci_host *host,
>>> unsigned int desired)
>>>          host->cclk = 0;
>>>
>>>          if (desired) {
>>> -               if (desired >= host->mclk) {
>>> +               if (variant->cclk_is_mclk) {
>>> +                       host->cclk = host->mclk;
>>> +               } else if (desired >= host->mclk) {
>>>                          clk = MCI_CLK_BYPASS;
>>>                          if (variant->st_clkdiv)
>>>                                  clk |= MCI_ST_UX500_NEG_EDGE;
>>> @@ -1354,6 +1362,16 @@ static void mmci_set_ios(struct mmc_host *mmc,
>>> struct mmc_ios *ios)
>>>          if (!ios->clock && variant->pwrreg_clkgate)
>>>                  pwr &= ~MCI_PWR_ON;
>>>
>>> +       if (ios->clock != host->mclk &&
>>> host->variant->explicit_mclk_control) {
>>
>>
>> I suggest you should clarify the statement by adding a pair of extra
>> parentheses. Additionally it seems like a good idea to reverse the
>> order of the statements, to clarify this is for qcom clock handling
>> only.
>
> Yes, sure Will fix this in next version.
>
>
>>
>> More important, what I think you really want to do is to compare
>> "ios->clock" with it's previous value it had when ->set_ios were
>> invoked. Then let a changed value act as the trigger to set a new clk
>> rate. Obvoiusly you need to cache the clock rate in the struct mmci
>> host to handle this.
>
>
> host->mclk already has this cached value.

There are no guarantees clk_set_rate() will manage to set the exact
requested rate as ios->clock. At least if you follow the clk API
documentation.

>
>
>>
>>> +               int rc = clk_set_rate(host->clk, ios->clock);
>>> +               if (rc < 0) {
>>> +                       dev_err(mmc_dev(host->mmc),
>>> +                               "Error setting clock rate (%d)\n", rc);
>>> +               } else {
>>> +                       host->mclk = clk_get_rate(host->clk);
>>
>>
>> So here you actually find out the new clk rate, but shouldn't you
>> update "host->mclk" within the spin_lock? Or it might not matter?
>>
> I think it does not matter in this case, as this is the only place mclk gets
> modified.

OK.

>
>>> +               }
>>> +       }
>>> +
>>>          spin_lock_irqsave(&host->lock, flags);
>>>
>>>          mmci_set_clkreg(host, ios->clock);
>>> @@ -1540,10 +1558,12 @@ static int mmci_probe(struct amba_device *dev,
>>>           * is not specified. Either value must not exceed the clock rate
>>> into
>>>           * the block, of course.
>>>           */
>>> -       if (mmc->f_max)
>>> -               mmc->f_max = min(host->mclk, mmc->f_max);
>>> -       else
>>> -               mmc->f_max = min(host->mclk, fmax);
>>> +       if (!host->variant->explicit_mclk_control) {
>>> +               if (mmc->f_max)
>>> +                       mmc->f_max = min(host->mclk, mmc->f_max);
>>> +               else
>>> +                       mmc->f_max = min(host->mclk, fmax);
>>> +       }
>>
>>
>> This means your mmc->f_max value will either be zero or the one you
>> provided through DT. And since zero won't work, that means you
>> _require_ to get the value from DT. According to the documentation of
>> this DT binding, f_max is optional.
>>
>> So unless you fine another way of dynamically at runtime figure out
>> the value of f_max (using the clk API), you need to update the DT
>> documentation for mmci.
>>
>
> You are right there is a possibility of f_max to be zero.
>
> This logic could fix it.
>
> if (host->variant->explicit_mclk_control) {
>         if (mmc->f_max)
>                 mmc->f_max = max(host->mclk, mmc->f_max);
>         else
>                 mmc->f_max = max(host->mclk, fmax);
> } else {
>         if (mmc->f_max)
>
>                 mmc->f_max = min(host->mclk, mmc->f_max);
>         else
>
>                 mmc->f_max = min(host->mclk, fmax);
> }
>

Hmm. Looking a bit deeper into this, we have some additional related
code to fixup. :-)

In ->probe(), we do clk_set_rate(100MHz), if the mclk > 100MHz. That's
due to the current variants don't support higher frequency than this.
It seems like the Qcom variant may support up to 208MHz? Now, if
that's the case, we need to add "f_max" to the struct variant_data to
store this information, so we can respect different values while doing
clk_set_rate() at ->probe().

While updating mmc->f_max for host->variant->explicit_mclk_control, we
shouldn't care about using the host->mclk as a limiter, instead, use
min(mmc->f_max, host->variant->f_max) and fallback to fmax.

Not sure how that will affect the logic. :-)

>
>
>> Additionally, this makes me wonder about f_min. I haven't seen
>> anywhere in this patch were that value is being set to proper value,
>> right?
>>
>
> f_min should be 400000 for qcom, I think with the default mclk frequency and
> a divider of 512 used for calculating the f_min is bringing down the f_min
> to lessthan 400Kz. Which is why its working fine.
> I think the possibility of mclk default frequency being greater than 208Mhz
> is very less. so I could either leave it as it is Or force this to 400000
> all the time for qcom chips.

No, this seems like a wrong approach.

I think you would like to do use the clk_round_rate() find out the
lowest possible rate. Or just use a fixed fallback value somehow.

Kind regards
Ulf Hansson
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