Re: [PATCH 1/2] arm64: dts: mediatek: mt8192: Make sure MSDCPLL's rate is 400MHz

[AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>]

Il 15/06/23 13:16, Matthias Brugger ha scritto:
> 
> 
> On 15/06/2023 11:51, TingHan Shen (沈廷翰) wrote:
>> Hi AngeloGioacchino,
>>
>> On Mon, 2023-05-22 at 11:30 +0200, AngeloGioacchino Del Regno wrote:
>>> External email : Please do not click links or open attachments until you have 
>>> verified the sender or the content.
>>>
>>>
>>> Some bootloaders will set MSDCPLL's rate lower than 400MHz: what I have
>>> seen is this clock being set at around 384MHz.
>>> This is a performance concern (and possibly a stability one, for picky
>>> eMMC/SD cards) as the MSDC controller's internal divier will choose a
>>> frequency that is lower than expected, in the end causing a difference
>>> in the expected mmc/sd device's timings.
>>>
>>> Make sure that the MSDCPLL frequency is always set to 400MHz to both
>>> improve performance and reliability of the sd/mmc storage.
>>>
>>> Fixes: 5d2b897bc6f5 ("arm64: dts: mediatek: Add mt8192 clock controllers")
>>> Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
>>> ---
>>>   arch/arm64/boot/dts/mediatek/mt8192.dtsi | 2 ++
>>>   1 file changed, 2 insertions(+)
>>>
>>> diff --git a/arch/arm64/boot/dts/mediatek/mt8192.dtsi 
>>> b/arch/arm64/boot/dts/mediatek/mt8192.dtsi
>>> index 5c30caf74026..6fc14004f6fd 100644
>>> --- a/arch/arm64/boot/dts/mediatek/mt8192.dtsi
>>> +++ b/arch/arm64/boot/dts/mediatek/mt8192.dtsi
>>> @@ -677,6 +677,8 @@ apmixedsys: syscon@1000c000 {
>>>                          compatible = "mediatek,mt8192-apmixedsys", "syscon";
>>>                          reg = <0 0x1000c000 0 0x1000>;
>>>                          #clock-cells = <1>;
>>> +                       assigned-clocks = <&apmixedsys CLK_APMIXED_MSDCPLL>;
>>> +                       assigned-clock-rates = <400000000>;
>>>                  };
>>>
>>>                  systimer: timer@10017000 {
>>> -- 
>>> 2.40.1
>>>
>>
>> Comment from mtk emmc owner,
>>
>> "As we all know, the clock has some jitter, when we set MSDCPLL to 400M,
>> but the actual measurement is not exactly 200M.
>> For eMMC, the spec stipulates that clock cannot exceed 200M.
>> If MSDCPLL is set to 400M, the actual measurement may exceed the spec.
>> So we set MSDCPLL to 384M in the bootloader stage to avoid exceeding the spec."
>>

Thanks for the comment,
I haven't seen any issue with this commit, if not a slight performance improvement,
on MT8192 and MT8195, but if there's risk to overclock the card, then it's not ok.

In any case, what is the expected jitter percentage?
eMMC/SD cards do expect jitter by spec anyway.

Thanks,
Angelo


> 
> Thanks for the feedback. Given that I'm not aware of any regressions that got fixed 
> by this commits I will drop this series for now. We can keep on the discussion and 
> if needed add them in a later stage.
> 
> Regards,
> Matthias
>