Re: [PATCH v1] mmc: mediatek: Fixed bug where clock frequency could be set wrong

From: Yong Mao
Date: Sat Mar 04 2017 - 01:57:56 EST

On Thu, 2017-03-02 at 20:20 +0800, Daniel Kurtz wrote:
> On Wed, Mar 1, 2017 at 5:56 PM, Yong Mao <yong.mao@xxxxxxxxxxxx> wrote:
> > On Tue, 2017-02-28 at 14:56 +0800, Daniel Kurtz wrote:
> >> On Fri, Feb 24, 2017 at 5:38 PM, Yong Mao <yong.mao@xxxxxxxxxxxx> wrote:
> >> > From: Yong Mao <yong.mao@xxxxxxxxxxxx>
> >> > To: Daniel Kurtz <djkurtz@xxxxxxxxxxxx>
> >> > Subject: Re: [PATCH v1] mmc: mediatek: Fixed bug where clock frequency
> >> > could be set wrong
> >> > Date: Fri, 24 Feb 2017 17:33:37 +0800
> >> >
> >> >
> >> > On Fri, 2017-02-24 at 17:52 +0900, Daniel Kurtz wrote:
> >> >> On Fri, Feb 24, 2017 at 5:22 PM, Yong Mao <yong.mao@xxxxxxxxxxxx>
> >> > wrote:
> >> >> >
> >> >> > From: yong mao <yong.mao@xxxxxxxxxxxx>
> >> >> >
> >> >> > This patch can fix two issues:
> >> >> >
> >> >> > Issue 1:
> >> >> > The maximum value of clock divider is 0xff.
> >> >> > Because the type of div is u32, div may be larger than max_div.
> >> >> > In this case, we should use max_div to set the clock frequency.
> >> >> >
> >> >> > Issue 2:
> >> >> > In previous code, we can not set the correct clock frequency when
> >> >> > div equals 0xff.
> >> >> >
> >> >> > Signed-off-by: Yong Mao <yong.mao@xxxxxxxxxxxx>
> >> >> > Signed-off-by: Chaotian Jing <chaotian.jing@xxxxxxxxxxxx>
> >> >> > ---
> >> >> > drivers/mmc/host/mtk-sd.c | 13 ++++++++++++-
> >> >> > 1 file changed, 12 insertions(+), 1 deletion(-)
> >> >> >
> >> >> > diff --git a/drivers/mmc/host/mtk-sd.c b/drivers/mmc/host/mtk-sd.c
> >> >> > index 07f3236..3174445 100644
> >> >> > --- a/drivers/mmc/host/mtk-sd.c
> >> >> > +++ b/drivers/mmc/host/mtk-sd.c
> >> >> > @@ -540,6 +540,7 @@ static void msdc_set_mclk(struct msdc_host
> >> > *host, unsigned char timing, u32 hz)
> >> >> > u32 mode;
> >> >> > u32 flags;
> >> >> > u32 div;
> >> >> > + u32 max_div;
> >> >>
> >> >> There's really no need for this variable. Just use 0xff below.
> >> > For all of our IC, max_div is not a constant.
> >> > We will upstream another patch which max_div will get the different
> >> > value depending on the IC.
> >> > Therefore, we keep the max_div as a variable here.
> >>
> >> Please add the variable in the patch that uses it as a variable.
> >>
> >> >
> >> >>
> >> >> > u32 sclk;
> >> >> >
> >> >> > if (!hz) {
> >> >> > @@ -590,8 +591,18 @@ static void msdc_set_mclk(struct msdc_host
> >> > *host, unsigned char timing, u32 hz)
> >> >> > sclk = (host->src_clk_freq >> 2) / div;
> >> >> > }
> >> >> > }
> >> >> > +
> >> >> > + /**
> >> >> > + * The maximum value of div is 0xff.
> >> >> > + * Check if the div is larger than max_div.
> >> >> > + */
> >> >> > + max_div = 0xff;
> >> >> > + if (div > max_div) {
> >> >> > + div = max_div;
> >> >> > + sclk = (host->src_clk_freq >> 2) / div;
> >> >> > + }
> >> >> > sdr_set_field(host->base + MSDC_CFG, MSDC_CFG_CKMOD |
> >> > MSDC_CFG_CKDIV,
> >> >> > - (mode << 8) | (div % 0xff));
> >> >> > + (mode << 8) | div);
> >> >>
> >> >> Hmm, I don't know much about this sub-system, but should we even be
> >> >> allowing requests to set a frequency that we can't actually achieve
> >> >> with the divider?
> >> >>
> >> >
> >> > No. We can not get a frequency that we can't actually achieve with the
> >> > divider. This patch is to solve this kind of issue.
> >>
> >> Sorry, I am trying to understand why we need this patch.
> >>
> >> AFAICT, it looks like sometimes msdc_set_mclk() is being called with
> >> hz that cannot be generated by your hardware. In particular,
> >> sometimes the original code computes "div > 255".
> >> To work around this problem, this patch just caps the divider value to
> >> 255, which is the maximum divider provided by the hardware. However,
> >> presumably this means that in this case we won't actually be
> >> generating the requested hz value.
> >>
> >> So, can you please explain in what exact scenario this patch is
> >> required, and justify why it is ok to generate a clock other than the
> >> requested in this case?
> >>
> >> -Dan
> >>
> >
> > This issue is hidden deeply. It is a boundary related issue.
> > Let me take the real value to explain how this issue occurs.
> >
> > 1. mmc->f_min = host->src_clk_freq / (4 * 255);
> > mmc->f_min = 400000000 / (4 * 255) = 392156;
> > 2. mmc core tries to initialize emmc by using 400000hz.
> > If the first try failed, it will retry by using max(300000hz,
> > mmc->f_min)= 392156hz
> > 3. msdc_set_mclk will be invoked by mmc core to set the clock.
> > and then following code will be executed.
> > div = (host->src_clk_freq + ((hz << 2) - 1)) / (hz << 2);
> > (div = (400000000 + (392156 << 2) - 1)) / (392156 << 2) = 256)
> >
> > Why do we use (host->src_clk_freq + ((hz << 2) - 1))?
> > ==> Because if we can not get a proper clock frequency, we should set a
> > clock frequency which is less than proper clock frequency.
> >
> > 4. In this IC, it only use 8 bits to indicate the value of clock
> > divider. Therefore, 256 is overflow, it is equal 0 here.
> > And then we will get a wrong 100000000Hz clock frequency.
> >
> > Can you understand how this issue occurs now?
>
> Thanks for the excellent explanation. Things are a lot clearer now.
>
> I think this can be solved even easier by computing f_min the same way:
>
> mmc->f_min = DIV_ROUND_UP(400000000, 4 * 255); /* = 392157 */
>
That is really a easier way to solve this issue.
I will submit a new version as soon as possible.
Thanks.

>
> >
> >> >
> >> >
> >> >> > sdr_set_bits(host->base + MSDC_CFG, MSDC_CFG_CKPDN);
> >> >> > while (!(readl(host->base + MSDC_CFG) & MSDC_CFG_CKSTB))
> >> >> > cpu_relax();
> >> >> > --
> >> >> > 1.7.9.5
> >> >> >
> >> >> >
> >> >> > _______________________________________________
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> >> >
> >> >
> >> >
> >
> >