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SubjectRE: [PATCH v4 1/4] mmc: dw_mmc: Invalidate cache of current_speed after suspend/resume
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On Sat, August 10, 2013, Doug Anderson wrote:
> Seungwon and Jaehoon,
>
> On Fri, Aug 9, 2013 at 6:32 AM, Seungwon Jeon <tgih.jun@samsung.com> wrote:
> > On Wed, August 07, 2013, Doug Anderson wrote:
> >> The dw_mmc driver keeps a cache of the current slot->clock in order to
> >> avoid doing a whole lot of work every time set_ios() is called.
> >> However, after suspend/resume the register values are bogus so we need
> >> to ensure that the cached value is invalidated.
> > This mismatch comes only in case MMC_PM_KEEP_POWER, right?
>
> Actually, no. I saw problems with the SD Card slot, which doesn't
> have MMC_KEEP_POWER. Problems showed up when no card was inserted
> across suspend/resume. In other words:
>
> 1. At boot time, slot is all setup and configured to 400kHz.
>
> 2. Suspend
>
> 3. Resume; clock registers are reset (by suspend/resume) and not
> restored since dw_mmc still thinks slot is configured for 400kHz due
> to host->current_speed cache.
>
> 4. Insert card.
>
> 5. No code sees any need to change the clock for detecting the card,
> since everyone thinks it's at 400kHz. ...but it's not.

Doug, your analysis is right.
But, let me suggest another approach.
After step #1, core layer actually call mmc_power_off because slot is empthy(get_cd() is '0').
Then, set_ios is requested with 'ios->clock'.
However, because current implementation doesn't update current_speed in case ios->clock is '0'.
It causes current_speed has invalid clock rate in resume of dw-mmc.

So, if we can update slot->clock properly, it will be fixed.

-static void dw_mci_setup_bus(struct dw_mci_slot *slot, bool force_clkinit)
+static void dw_mci_setup_bus(struct dw_mci_slot *slot)
{
struct dw_mci *host = slot->host;
u32 div;
u32 clk_en_a;

- if (slot->clock != host->current_speed || force_clkinit) {
+ if (slot->clock && (slot->clock != host->current_speed)) {


@@ -807,13 +807,11 @@ static void dw_mci_set_ios(struct mmc_host *mmc, struct mmc_ios *ios)

mci_writel(slot->host, UHS_REG, regs);

- if (ios->clock) {
- /*
- * Use mirror of ios->clock to prevent race with mmc
- * core ios update when finding the minimum.
- */
- slot->clock = ios->clock;
- }
+ /*
+ * Use mirror of ios->clock to prevent race with mmc
+ * core ios update when finding the minimum.
+ */
+ slot->clock = ios->clock;

Thanks,
Seungwon Jeon

>
>
> >> In many cases we got by without this since the core mmc code fiddles
> >> with the clock a lot. If we've got a card present we're probably
> >> running it at something like 50MHz and the core will temporarily
> >> switch us to 400kHz after resume. One case that didn't work (for me)
> >> is the case of having no card in the slot. The slot is initted to
> >> 400kHz at boot time. After suspend/resume the slot thinks it's still
> >> at 400kHz (due to the cache) so doesn't adjust timing. When it tries
> >> to send the command at probe time it just times out and gets left in a
> >> bad state.
> > I understand this change although some part of commit message (boot time, probe time...) make me
> confused.
>
> Sorry to be confusing. I was trying to explain why the old code works
> fine in many cases. It's because the core MMC code tends to adjust
> the clock a lot around suspend/resume. When it does that, it works
> around this problem. ...but I found one case where suspend/resume
> would happen and the MMC core didn't adjust the clock.
>
>
> > I guess this change intends to update clock programming forcedly.
> > It looks like another version of 'dw_mci_setup_bus(slot, true)'.
> > Eventually, this change does same?
>
> Effectively, yes. As Jaehoon points out, that means we can actually
> eliminate the "force" parameter to dw_mci_setup_bus().
>
>
> I will send a new version out that eliminates the "force" parameter
> and updates the commit message to (hopefully) be clearer.
>
> -Doug
> --
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