linux-kernel - Re: [PATCH v3 10/13] mmc: mmci: add Qcom specifics of clk and datactrl registers.

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Date:	Wed, 28 May 2014 12:03:15 +0200
From:	Ulf Hansson <ulf.hansson@...aro.org>
To:	Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
Cc:	Russell King <linux@....linux.org.uk>,
	linux-mmc <linux-mmc@...r.kernel.org>,
	Chris Ball <chris@...ntf.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-arm-msm@...r.kernel.org,
	Linus Walleij <linus.walleij@...aro.org>
Subject: Re: [PATCH v3 10/13] mmc: mmci: add Qcom specifics of clk and
 datactrl registers.

On 28 May 2014 11:41, Srinivas Kandagatla
<srinivas.kandagatla@...aro.org> wrote:
> Hi Ulf,
>
>
> On 26/05/14 22:38, Srinivas Kandagatla wrote:
>>>>
>>>>   2 files changed, 28 insertions(+)
>>>>
>>>> diff --git a/drivers/mmc/host/mmci.c b/drivers/mmc/host/mmci.c
>>>> index 17e7f6a..6434f5b1 100644
>>>> --- a/drivers/mmc/host/mmci.c
>>>> +++ b/drivers/mmc/host/mmci.c
>>>> @@ -185,6 +185,10 @@ static struct variant_data variant_qcom = {
>>>>          .fifosize               = 16 * 4,
>>>>          .fifohalfsize           = 8 * 4,
>>>>          .clkreg                 = MCI_CLK_ENABLE,
>>>> +       .clkreg_enable          = MCI_QCOM_CLK_FLOWENA |
>>>> +                                 MCI_QCOM_CLK_FEEDBACK_CLK,
>>>
>>>
>>> Obviously I don't have the in-depth knowledge about the Qcom variant,
>>> but comparing the ST variant here made me think.
>>>
>>> Using the feeback clock internal logic in the ST variant, requires the
>>> corresponding feedback clock pin signal on the board, to be
>>> routed/connected. Typically we used this for SD cards, which involved
>>> using an external level shifter circuit.
>>>
>>> Is it correct to enable this bit for all cases, including eMMC?
>>>
>> You are correct, FBCLK should specific to the board, and I will try to
>> do something on the same lines as ST variant in next version.
>
> I get lot of I/O errors when I remove this flag for test.

Running eMMC I suppose?

>
> I rechecked schematics and datasheet, the feedback clk that we refer here is
> the the feedback clk from CLK pad, there is no separate input pad for fbclk.
> So I think this is internally feedbacked clk.
>
> This selection is configuring bits to latch data and command coming in using
> feedback clock from CLK pad.

Seems like it's strange to have this bit configurable then. I guess it
would  be hard to tell under what circumstances you don't want this
bit set.

Anyway, it's not a big deal to me - let's keep it as is for now.

Kind regards
Ulf Hansson

>
> I will make sure that the macro is named more appropriately to reflect the
> same.
>
> thanks,
> srini
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