linux-kernel - Re: [PATCH v3] mmc: Make ID freq configurable

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Date:	Sat, 02 Jan 2010 17:04:08 -0600
From:	Robert Hancock <hancockrwd@...il.com>
To:	Ben Nizette <bn@...sdigital.com>
CC:	Pierre Ossman <pierre@...man.eu>,
	Hein_Tibosch <hein_tibosch@...oo.es>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Adrian Hunter <adrian.hunter@...ia.com>,
	linux-mmc@...r.kernel.org, linux-kernel@...r.kernel.org,
	Matt Fleming <matt@...sole-pimps.org>
Subject: Re: [PATCH v3] mmc: Make ID freq configurable

On 01/02/2010 04:23 PM, Ben Nizette wrote:
>
> On 02/01/2010, at 11:08 PM, Pierre Ossman wrote:
>
>> On Sat, 2 Jan 2010 20:07:01 +1100
>> Ben Nizette<bn@...sdigital.com>  wrote:
>>
>>>
>>> Pierre, thoughts?  All my cards were borderline spec but Hein's got a mainstream, common card which seems to completely ignore the spec.  Can we just take the few-hundred-uS hit and allow initialization down at the bottom end, say 100kHz?
>>>
>>
>> Broken cards seem to be all over the spectrum, so I wouldn't be
>> suprised if you find ones that break if you go too low as well.
>
> Yea good point, though given there might not even be a One Freq to Rule Them All, how about:
>
> ---8<---
>
> From: Ben Nizette<bn@...sdigital.com>
> Subject: [PATCH v3] mmc: Make ID freq configurable
>
> While the SD spec specifies a range of frequencies for the ID phase from 100-400kHz, not all cards play by the rules.  This patch adds a Kconfig option to allow the user to tweak this to their card/host config, though the default of 250kHz should be fine for all spec-abiding cards and most others besides.
>
> Signed-off-by: Ben Nizette<bn@...sdigital.com>
>
> ---
> diff --git a/drivers/mmc/core/Kconfig b/drivers/mmc/core/Kconfig
> index ab37a6d..a8e5003 100644
> --- a/drivers/mmc/core/Kconfig
> +++ b/drivers/mmc/core/Kconfig
> @@ -14,3 +14,15 @@ config MMC_UNSAFE_RESUME
>            This option is usually just for embedded systems which use
>            a MMC/SD card for rootfs. Most people should say N here.
>
> +config MMC_ID_FREQ
> +       int "SD/MMC Card initialisation frequency (Hz)"
> +       default 250000
> +       help
> +         The SD card specification allows for initialisation
> +         frequencies from 100-400kHz however some broken cards are
> +         known not to work across this whole range.
> +
> +         If you're having problems with a particular card not being
> +         detected, you may try adjusting this frequency up or down
> +         but for widest compatibility just leave it default.

Don't know that a kconfig option is best for this, it'd suck to have to 
recompile the kernel to change it. Module option might be better..
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