linux-kernel - Re: [RFC 1/2] mmc: sdhci: dt: Add device tree properties sdhci-caps and sdhci-caps-mask

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Message-id: <c254c113-8f21-ca71-da5b-02fb95fed412@samsung.com>
Date:   Tue, 01 Nov 2016 07:13:29 +0900
From:   Jaehoon Chung <jh80.chung@...sung.com>
To:     Adrian Hunter <adrian.hunter@...el.com>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Zach Brown <zach.brown@...com>
Cc:     Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        linux-mmc <linux-mmc@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC 1/2] mmc: sdhci: dt: Add device tree properties sdhci-caps
 and sdhci-caps-mask

On 10/31/2016 09:34 PM, Adrian Hunter wrote:
> On 31/10/16 13:59, Jaehoon Chung wrote:
>> On 10/28/2016 05:12 PM, Ulf Hansson wrote:
>>> On 25 October 2016 at 21:58, Zach Brown <zach.brown@...com> wrote:
>>>> On some systems the sdhci capabilty registers are incorrect for one
>>>> reason or another.
>>>>
>>>> The sdhci-caps-mask property specifies which bits in the registers
>>>> are incorrect and should be turned off before using sdhci-caps to turn
>>>> on bits.
>>>>
>>>> The sdhci-caps property specifies which bits should be turned on.
>>>>
>>>> Signed-off-by: Zach Brown <zach.brown@...com>
>>>> ---
>>>>  Documentation/devicetree/bindings/mmc/mmc.txt | 7 +++++++
>>>>  1 file changed, 7 insertions(+)
>>>>
>>>> diff --git a/Documentation/devicetree/bindings/mmc/mmc.txt b/Documentation/devicetree/bindings/mmc/mmc.txt
>>>> index 8a37782..1415aa0 100644
>>>> --- a/Documentation/devicetree/bindings/mmc/mmc.txt
>>>> +++ b/Documentation/devicetree/bindings/mmc/mmc.txt
>>>
>>> The bindings in this document are common mmc DT bindings, not bindings
>>> specific to a mmc controller.
>>>
>>> So unless these bindings are applicable for another controller than
>>> sdhci, I suggest we create a new file to document these.
>>> How about Documentation/devicetree/bindings/mmc/sdhci.txt?
>>>
>>>> @@ -52,6 +52,13 @@ Optional properties:
>>>>  - no-sdio: controller is limited to send sdio cmd during initialization
>>>>  - no-sd: controller is limited to send sd cmd during initialization
>>>>  - no-mmc: controller is limited to send mmc cmd during initialization
>>>> +- sdhci-caps-mask: The sdhci capabilities registers are incorrect. This 64bit
>>>
>>> /s/registers/register
>>>
>>> This applies to some more places below as well.
>>>
>>>> +  property corresponds to the bits in the sdhci capabilty registers. If the bit
>>>> +  is on in the mask then the bit is incorrect in the registers and should be
>>>> +  turned off.
>>>> +- sdhci-caps: The sdhci capabilities registers are incorrect. This 64bit
>>>> +  property corresponds to the bits in the sdhci capability registers. If the
>>>> +  bit is on in the property then the bit should be on in the reigsters.
>>>
>>> /s/reigsters/register
>>>
>>>>
>>>>  *NOTE* on CD and WP polarity. To use common for all SD/MMC host controllers line
>>>>  polarity properties, we have to fix the meaning of the "normal" and "inverted"
>>>> --
>>>> 2.7.4
>>>>
>>>
>>> Overall, I like this idea as it gives us good flexibility. Thus it
>>> should avoid us to having to add any further new similar "sdhci broken
>>> cap" DT binding. We could also decide to start deprecate some of the
>>> existing sdhci bindings, if we think that makes sense.
>>>
>>> The downside is that we get a "magic" hex value in the dts. Although,
>>> people could address this issue by providing some comments about what
>>> the bits it means in the dts files themselves.
>>
>> I think it's not good about getting "magic" hex value.
>> In my experience, it's too difficult what bits means and calculate..
>> Because some people who i know had already used like this.(locally..)
>>
>> It needs to consider this...otherwise..it should become really complex magic code.
> 
> The bits we use are listed in sdhci.h and how we use them can be determined
> from the sdhci source code.  Also, from the hardware perspective, there is
> the SDHCI specification.  So what the bits mean is readily available.
> 
> With regard to calculating the values, won't it be obvious from testing if
> they are wrong?

You're right. But I didn't see the real use case for this properties.
If it needs to add these properties, why didn't add codes relevant to these in device-tree?
Otherwise, this code should be dead code.

Best Regards,
Jaehoon Chung

> 
>>
>> Best Regards,
>> Jaehoon Chung
>>
>>>
>>> Let's see what Rob thinks about this.
>>>
>>> Kind regards
>>> Uffe
>>> --
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>>>
>>>
>>>
>>
>>
> 
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