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

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Date:	Tue, 5 Jan 2010 13:24:22 +0100
From:	Sascha Hauer <s.hauer@...gutronix.de>
To:	Hein_Tibosch <hein_tibosch@...oo.es>
Cc:	Pierre Ossman <pierre@...man.eu>, Ben Nizette <bn@...sdigital.com>,
	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 Tue, Jan 05, 2010 at 05:07:21AM +0800, Hein_Tibosch wrote:
> On 3-1-2010 07:38, Pierre Ossman wrote:
>>
>> <cut>
>> We all use the same pool of cards so we
>> should all be using the same init sequence. If there isn't a single
>> frequency where all cards will work, then we'll have to make something
>> more advanced where the kernel will try the init several times with
>> different clocking.
>
> On 4-1-2010 20:15, Sascha Hauer wrote:
>> The problem was that in some setups the mxcmmc driver allows frequencies
>> as low as a few Hz and the card initialization took ages. Increasing the
>> frequency to 400kHz seemed sensible since it's the value the spec
>> describes. 50kHz should be fine as well.
>>
>
> I tried out several main-stream sd-cards and with the Atmel (AP7) controller
> they all initialize with F between 138 (=f_min) and 280 Khz.
>
> Below a patch which tries mmc-initialization using several frequencies
> from an array 400, 300, 200 and 100. When it comes to 200 Khz, it works.
> The two failed attempts lasted about 10 ms together.
>
> It looks like a hell-of-a-patch but that's because the indentation changes.
>
> Note that the new mmc_host member f_init will be used later in power_restore
> and resume. As power_up always comes first, it should have a descend value
> by then.
>
> But I still wonder, why not just keep using Sascha's patch, changing 400000
> to 50000? Has anyone ever had problems with mmc-init on 50 Khz?

I already wrote it to Hein in private, once again for the rest:

The original reason to increase the minimum frequency was that the
Freescale i.MX MMC core allows minimum frequencies of just a few Hz.
With this the card initialisation took ages, so we increased the minimum
frequency. 400kHz seemed like a good value since all cards should
support this, but I'm also fine with any other value between 50kHz and
400kHz.

Sascha


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