Re: [PATCH 2/2] mmc: renesas_sdhi: keep SCC clock active when tuning

Wolfram Sang
2020-09-01T10:24:25Z

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Hi Shimoda-san,

> I got a report from a colleague about this issue. According to the report,
> this issue is related to retuning. When retuning happens, the mmc core
> calls mmc_hs400_to_hs200() and then mmc_hs400_to_hs200() will set the clock
> as 52MHz at first. So, it's possible to cause the issue.
>
> It's difficult to cause retuning in normal situation. But, according to
> the report, if we add a code which the sdhi driver reports an error
> at the first CMD18 once, we can cause retuning and then the issue happens.

I took the liberty of a different approach because I wanted to reproduce
the issue when doing the initial tuning and not a retune. Because my new
series adds (and checks) a flag for doing_initial_tune, so I really
wanted to excercise this code path. This is a real problem, too, because
I saw this with my boards earlier back then.

And halleluja, today I saw it again, once. I switched to my H3-ES2.0
board which I haven't used for weeks. And when booting that for the
first time, I got a failure including logs. Later boots just went fine.

And because of the logs, I could finally inject an error which will
reproducibly cause the boot to hang because of a stalled SCC. Tada, here
is the patch:

From: Wolfram Sang <wsa+renesas@sang-engineering.com>
Subject: [PATCH] GOLD: simulate stalled SCC

Geez, this took ages to find...
---
drivers/mmc/core/mmc.c | 1 +
1 file changed, 1 insertion(+)

diff --git a/drivers/mmc/core/mmc.c b/drivers/mmc/core/mmc.c
index 216bd1aed373..6b3056437b37 100644
--- a/drivers/mmc/core/mmc.c
+++ b/drivers/mmc/core/mmc.c
@@ -1218,6 +1218,7 @@ static int mmc_select_hs400(struct mmc_card *card)
host->ops->hs400_complete(host);

err = mmc_switch_status(card, true);
+ err = -EILSEQ;
if (err)
goto out_err;

Interestingly, the other mmc_switch_status() in mmc_select_hs400() was
not stalling the SCC. Anyhow, after this failute, the MMC core switches
back to 300kHz and the SCC clock is off but for some reason SCC is still
accessed. I will investigate why. The good news is that my new patch set
fixes the hang as expected. The board will continue to boot so we
probably want to have this series. However, I have the feeling that this
SCC access which hangs the board might be a bug because of an unintended
code path. I mean, this is also one reason why the bug triggers so
rarely these days. We have been fixing a lot of things and the SCC is
only accessed when it should be accessed. We will see. I also need to
test other boards, too.

So much for now, I hope I can report more later.

Happy hacking and kind regards,

Wolfram

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