linux-kernel - Re: ARC dw-mshc binding compat string

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Message-ID: <56F6ED41.5020908@mentor.com>
Date:	Sat, 26 Mar 2016 22:12:49 +0200
From:	Vladimir Zapolskiy <vladimir_zapolskiy@...tor.com>
To:	Marek Vasut <marex@...x.de>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>
CC:	<linux-kernel@...r.kernel.org>, Rob Herring <robh+dt@...nel.org>,
	Alexey Brodkin <abrodkin@...opsys.com>
Subject: Re: ARC dw-mshc binding compat string

On 26.03.2016 21:52, Marek Vasut wrote:
> On 03/26/2016 07:16 PM, Vladimir Zapolskiy wrote:
>> On 26.03.2016 20:10, Marek Vasut wrote:
>>> On 03/26/2016 06:52 PM, Vladimir Zapolskiy wrote:
>>>> Hi Marek,
>>>>
>>>> On 26.03.2016 19:30, Marek Vasut wrote:
>>>>> On 03/26/2016 06:26 PM, Vladimir Zapolskiy wrote:
>>>>>> On 26.03.2016 12:14, Marek Vasut wrote:
>>>>>>> Hi!
>>>>>>>
>>>>>>> I noticed that arch/arc/boot/dts/axs10x_mb.dtsi uses "altr," prefix in
>>>>>>> the DT compatible string:
>>>>>>>
>>>>>>> mmc@...5000 {
>>>>>>>         compatible = "altr,socfpga-dw-mshc";
>>>>>>>         reg = < 0x15000 0x400 >;
>>>>>>>         num-slots = < 1 >;
>>>>>>>         fifo-depth = < 16 >;
>>>>>>>         card-detect-delay = < 200 >;
>>>>>>>         clocks = <&apbclk>, <&mmcclk>;
>>>>>>>         clock-names = "biu", "ciu";
>>>>>>>         interrupts = < 7 >;
>>>>>>>         bus-width = < 4 >;
>>>>>>> };
>>>>>>>
>>>>>>> I don't think this is OK, since ARC is unrelated to Altera, which is
>>>>>>> what the "altr," prefix stands for. I think the socfpga-dw-mshc shim
>>>>>>> should be extended with another compatibility string, something like
>>>>>>> "snps,arc-dw-mshc" and the axs10x_mb.dtsi should be adjusted
>>>>>>> accordingly. What do you think ?
>>>>>>>
>>>>>>
>>>>>> There is "snps,dw-mshc" described in
>>>>>> Documentation/devicetree/bindings/mmc/synopsys-dw-mshc.txt and supported by
>>>>>> dw_mmc host controller driver.
>>>>>
>>>>> Thanks, that's even better.
>>>>>
>>>>> btw what do you think of using altr, prefix on non-altera system, that
>>>>> doesn't seem ok, right ?
>>>>
>>>> according to ePAPR the prefix should represent a device (IP block here
>>>> I believe) manufacturer, so it should be okay to use "altr" prefix on
>>>> non-Altera system, if Altera provides  another hardware vendor with
>>>> some own IP block.
>>>
>>> In this case, it's Synopsys who provides the SD/MMC/MS core to other
>>> chip makers (Altera etc).
>>
>> Correct.
>>
>>>> That said, I would rather prefer to see "snps,dw-mshc" prefix on description
>>>> of an MMC controller found on SoCFPGA series, "altr,socfpga-dw-mshc" seems
>>>> to be redundant.
>>>
>>> According to drivers/mmc/host/dw_mmc-pltfm.c , the Altera SoCFPGA one
>>> "altr,socfpga-dw-mshc" and also Imagination Technology Pistacio one
>>> "img,pistachio-dw-mshc" need specialty bit (SDMMC_CMD_USE_HOLD_REG),
>>> while the stock one "snps,dw-mshc" does not. I am not sure if the ARC
>>> one needs it as well, but most likely yes.
>>>
>>> I wonder if that bit is needed on some particular version of the DWMMC
>>> core. In that case, should we have "snps,dw-mshc" and "snps,dw-mshc-vN"
>>> binding ? Or should we use DT property to discern the need for this bit ?
>>>
>>
>> That's the most common way to take into account peculiarities, add
>> a property and handle it from the driver.
> 
> And by "that" you mean which of those two I listed , the
> "snps,dw-mshc-vN" or adding new DT prop ?
> 

I meant to add a new property, not a new compatible, but that's just
my experience.

Let me say it __might__ happen that a particular change you need is
specific to a particular version of the DWMMC IP (query Synopsys
by the way), but more probably it might be e.g. the same IP version with
a different reduced or extended configuration or a minor fix/improvement
to the IP block without resulting version number bump.

For example I don't remember that errata fixes in IP blocks result in
a new compatible, instead there are quite common optional "quirk"
properties for broken IPs -- e.g. check bindings/usb/dwc3.txt :)

--
With best wishes,
Vladimir