linux-kernel - Re: [v2 PATCH 3/3] mmc: sdhci-cadence: Update PHY delay configuration

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Date:   Thu, 9 Mar 2017 11:37:22 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     Piotr Sroka <piotrs@...ence.com>
Cc:     linux-mmc <linux-mmc@...r.kernel.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [v2 PATCH 3/3] mmc: sdhci-cadence: Update PHY delay configuration

Hi Piotr,

2017-03-07 20:00 GMT+09:00 Piotr Sroka <piotrs@...ence.com>:
> Hi Masahiro,
>
>> -----Original Message-----
>> Sent: 07 March, 2017 9:03 AM
>> To: Piotr Sroka
>> Subject: Re: [v2 PATCH 3/3] mmc: sdhci-cadence: Update PHY delay configuration
>>
>> Hi Piotr,
>>
>> 2017-03-06 22:39 GMT+09:00 Piotr Sroka <piotrs@...ence.com>:
>> > PHY settings can be different for different platforms and SoCs.
>> > Fixed PHY input delays was replaced with SoC specific compatible data.
>> > DTS properties are used for configuration new PHY DLL delays.
>>
>>
>> Probably you are familiar with this IP.
>>
>> Please teach me this.
>>
>> With this patch, we will have two groups for PHY parameters.
>>
>> (A) specified via a data array associated with a compatible string SDHCI_CDNS_PHY_DLY_SD_HS SDHCI_CDNS_PHY_DLY_SD_DEFAULT
>> SDHCI_CDNS_PHY_DLY_UHS_SDR12
>> SDHCI_CDNS_PHY_DLY_UHS_SDR25
>> SDHCI_CDNS_PHY_DLY_UHS_SDR50
>> SDHCI_CDNS_PHY_DLY_UHS_DDR50
>> SDHCI_CDNS_PHY_DLY_EMMC_LEGACY
>> SDHCI_CDNS_PHY_DLY_EMMC_SDR
>> SDHCI_CDNS_PHY_DLY_EMMC_DDR
>>
>> (B) specified with DT property
>> SDHCI_CDNS_PHY_DLY_SDCLK
>> SDHCI_CDNS_PHY_DLY_HSMMC
>> SDHCI_CDNS_PHY_DLY_STROBE
>>
>> I am confused.
>> What is the difference between (A) and (B)?
>
> The first group of delays are input delays. These delays are set in current version of sdhci-cadence driver in sdhci_cdns_phy_init function.
> Following by spec:
> They are provided to help in meeting timings relations between data window and sampling clock.
> The clock is fixed position in respect to the SDCLK. And the idea of sampling is to delay and align the data to the data window.
> If the default values of the delays are not sufficient/correct for the chip/board implementation those can be adjusted
>
> The second group are DLL delays.
> There are three delays
> SDHCI_CDNS_PHY_DLY_SDCLK  - sdclk delay line use to delay outgoing sdclk signal
> SDHCI_CDNS_PHY_DLY_HSMMC - sdclk delay line use to delay outgoing sdclk signal for for HS200, HS400 and HS400ES
> SDHCI_CDNS_PHY_DLY_STROBE - DLL strobe delay for HS400ES
> Following by spec:
> They allows to setup basic DLL parameters. In general the default values are sufficient to start working in any speed mode. The default values of delays and phase detect select can be adjusted depending on the chip/board implementation.



It was not clear what makes one group different from the other.

After all, parameters from both groups
should be adjusted depending on chip/board implementation.


> In general all PHY delays values either should be properly hardcoded in HW or they should be properly set  by FW depending on the chip/board.
> So PHY driver should do not touch PHY delays at all or should set values which are proper for specific chip/board.
>
> I am not sure where exactly they should be placed in dts file or in compatible data.

I am not quite sure, either.
(comments are appreciated.)


FWIW:
The first group (data associated with compatible) allows per-chip adjustment,
but not per-board.   Pros are, we will not break DT compatibility,
and we can avoid a list of properties difficult to understand.
Cons are, we can not make fine-grained adjustment for each board.


The second group (DT property) gives more flexibility for per-chip and
per-board adjustment.
A bad thing is we will end up with specifying a bunch of mysterious
properties from DT.





-- 
Best Regards
Masahiro Yamada