Re: [PATCH 4/4] ARM: dts: Fix-up EMMC2 controller's frequency

[Alan Cooper <alcooperx@gmail.com>]

Nicolas,

I got a better description of the failure and it looks like the bus
clock needs to be limited to 300KHz for a 500MHz core clock.
What's happening is that an internal reset sequence is needed after a
command timeout and the reset signal needs to be asserted for at least
2 ticks of the bus clock. This is done using a 12 bit counter clocked
by the core clock. That means a 500MHz core clock produces a 122KHz
reset signal which is too fast for 2 ticks of the 200KHz bus clock
(100KHz) but is okay for the 300KHz (150Khz) bus clock.

Al

On Mon, Apr 5, 2021 at 4:45 AM Nicolas Saenz Julienne
<nsaenzjulienne@suse.de> wrote:
>
> Hi Alan,
>
> On Thu, 2021-04-01 at 11:23 -0400, Alan Cooper wrote:
> > Nicolas,
> >
> > Sorry, I just noticed this thread.
> > This is a known bug in some newer Arasan cores.
> > The problem happens when the difference between the core clock and the bus
> > clock is too great.
> > Limiting the clock to 200KHz minimum should be a good fix.
>
> Great, that's what I was hoping to hear :). Out of curiosity, can you share
> more details on how the failure occurs?
>
> > In my experience, it's only eMMC that needs the clock to be retried
>
> > below 400KHz and not SD or SDIO. That's because the CMD signal for
> > eMMC starts out as open-drain during identification and the size of
> > the pull-up on the CMD signal can require the <400KHz clock. Once eMMC
> > is out of identification mode the CMD signal is switched to push-pull
> > and can run at much higher clock rates.
>
> Fair enough, I need to do some tests, some of the compute modules use an eMMC.
>
> > I don't think that SD and SDIO have any open-drain signals, so they
> > shouldn't need to retry at slower clock speeds.
>
> Noted.
>
> > I'm trying to get more detail on the bug, like the exact ratio of core
> > clock to bus clock that causes the problem. When I first found this
> > bug I was told that the failure would not happen at 200KHz, but we
> > were using a 405MHz core clock.
>
> That would be nice to have.
>
> > One other question. Why are you using polling for the SD card, this
> > newer controller supports the interrupt driven "Card Inserted" signal
> > and avoids wasting time polling?
>
> I believe the line isn't routed on RPi4.
>
> > Al
>