linux-kernel - Re: [PATCH 1/2] mmc: jz4740: Don't change parent clock rate for some SoCs

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Message-ID: <t9aKzNwdAtshY49d7AayzhwbD9leN37T@localhost>
Date:   Fri, 18 Nov 2022 14:18:07 +0000
From:   Aidan MacDonald <aidanmacdonald.0x0@...il.com>
To:     Paul Cercueil <paul@...pouillou.net>
Cc:     Ulf Hansson <ulf.hansson@...aro.org>,
        Siarhei Volkau <lis8215@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        linux-mips@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mmc@...r.kernel.org
Subject: Re: [PATCH 1/2] mmc: jz4740: Don't change parent clock rate for
 some SoCs


Paul Cercueil <paul@...pouillou.net> writes:

> Hi,
>
> (Ingenic SoCs maintainer here)
>
> Le ven. 18 nov. 2022 à 09:45:48 +0100, Ulf Hansson <ulf.hansson@...aro.org> a
> écrit :
>> On Tue, 8 Nov 2022 at 05:53, Siarhei Volkau <lis8215@...il.com> wrote:
>>>  Some SoCs have one clock divider for all MMC units, thus changing one
>>>  affects others as well. This leads to random hangs and memory
>>>  corruptions, observed on the JZ4755 based device with two MMC slots
>>>  used at the same time.
>> Urgh, that sounds like broken HW to me.
>> The MMC blocks could share a parent clock (that would need a fixed
>> rate for it to be applied), assuming there is a separate gate/divider
>> available per block. But there isn't'?
>
> They do share a parent clock and have separate gates, and each MMC IP block has
> an internal divider for the bus frequency derived from that shared clock.
>
>>>  List of SoCs affected includes: JZ4725b, JZ4755, JZ4760 and JZ4760b.
>>>  However, the MMC driver doesn't distinguish JZ4760 and JZ4770
>>>  which shall remain its behavior. For the JZ4755 is sufficient to
>>>  use JZ4725b's binding. JZ4750 is outside of the patch.
>>>  The MMC core has its own clock divisor, rather coarse but suitable well,
>>>  and it shall keep the role of tuning clock for the MMC host in that
>>>  case.
>> The mmc core doesn't have a clock divisor, but it does control the bus
>> clock frequency through the ->set_ios() host ops. It needs to do that,
>> to be able to conform to the (e)MMC, SD and SDIO specifications.
>> Can you please try to elaborate on the above, so I can better
>> understand your point?
>
> Yes, I don't really understand the patch, TBH.
>
> The "clk_set_rate" call will only set the shared clock to the *maximum* clock
> frequency (host->mmc->f_max) which should be the exact same across all MMC IPs.
>
> So it doesn't matter if it's set 3 times by 3 different instances of the IP, as
> long as they all request the same value.

Isn't the fact that 3 separate driver instances call clk_set_rate()
on a shared clock kind of... problematic? Nor is it documented that
all controllers need to use the same max-frequency.

Given it's a shared clock which can't realistically be controlled by
any of the consumers, I don't think it's unreasonable to assign the
clock frequency from the DT, but it needs to be backward-compatible
with old DTs.

On another note, shouldn't the MMC mux parent be assigned from the DT?
JZ4760 (and most other Ingenic SoCs) have multiple choices and I don't
see the parent being assigned anywhere.

Regards,
Aidan

>
> Besides, I know for a fact that the mainline driver works fine on the JZ4760(B)
> and JZ4725B.
>
> Finally... even if it was correct, this change would break compatibility with
> old Device Tree files.
>
> Cheers,
> -Paul
>
>>>  Signed-off-by: Siarhei Volkau <lis8215@...il.com>
>> Kind regards
>> Uffe
>>
>>>  ---
>>>   drivers/mmc/host/jz4740_mmc.c | 10 +++++++++-
>>>   1 file changed, 9 insertions(+), 1 deletion(-)
>>>  diff --git a/drivers/mmc/host/jz4740_mmc.c b/drivers/mmc/host/jz4740_mmc.c
>>>  index dc2db9c18..d390ff31d 100644
>>>  --- a/drivers/mmc/host/jz4740_mmc.c
>>>  +++ b/drivers/mmc/host/jz4740_mmc.c
>>>  @@ -114,6 +114,7 @@ enum jz4740_mmc_version {
>>>          JZ_MMC_JZ4740,
>>>          JZ_MMC_JZ4725B,
>>>          JZ_MMC_JZ4760,
>>>  +       JZ_MMC_JZ4770,
>>>          JZ_MMC_JZ4780,
>>>          JZ_MMC_X1000,
>>>   };
>>>  @@ -887,7 +888,13 @@ static int jz4740_mmc_set_clock_rate(struct
>>> jz4740_mmc_host *host, int rate)
>>>          int real_rate;
>>>          jz4740_mmc_clock_disable(host);
>>>  -       clk_set_rate(host->clk, host->mmc->f_max);
>>>  +
>>>  +       /*
>>>  +        * Changing rate on these SoCs affects other MMC units too.
>>>  +        * Make sure the rate is configured properly by the CGU driver.
>>>  +        */
>>>  +       if (host->version != JZ_MMC_JZ4725B && host->version !=
>>> JZ_MMC_JZ4760)
>>>  +               clk_set_rate(host->clk, host->mmc->f_max);
>>>          real_rate = clk_get_rate(host->clk);
>>>  @@ -992,6 +999,7 @@ static const struct of_device_id jz4740_mmc_of_match[]
>>> = {
>>>          { .compatible = "ingenic,jz4740-mmc", .data = (void *)
>>> JZ_MMC_JZ4740 },
>>>          { .compatible = "ingenic,jz4725b-mmc", .data = (void
>>> *)JZ_MMC_JZ4725B },
>>>          { .compatible = "ingenic,jz4760-mmc", .data = (void *)
>>> JZ_MMC_JZ4760 },
>>>  +       { .compatible = "ingenic,jz4770-mmc", .data = (void *)
>>> JZ_MMC_JZ4770 },
>>>          { .compatible = "ingenic,jz4775-mmc", .data = (void *)
>>> JZ_MMC_JZ4780 },
>>>          { .compatible = "ingenic,jz4780-mmc", .data = (void *)
>>> JZ_MMC_JZ4780 },
>>>          { .compatible = "ingenic,x1000-mmc", .data = (void *) JZ_MMC_X1000
>>> },
>>>  --
>>>  2.36.1
>>>