linux-kernel - Re: [PATCH v3] OMAP: Fix for bus width which improves SD card's peformance.

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Message-ID: <4BBBD87C.1040307@ti.com>
Date:	Tue, 6 Apr 2010 19:57:32 -0500
From:	Nishanth Menon <nm@...com>
To:	"Chikkature Rajashekar, Madhusudhan" <madhu.cr@...com>
CC:	"felipe.balbi@...ia.com" <felipe.balbi@...ia.com>,
	"me@...ipebalbi.com" <me@...ipebalbi.com>,
	"'kishore kadiyala'" <kishorek.kadiyala@...il.com>,
	"'Vimal Singh'" <vimal.newwork@...il.com>,
	"tony@...mide.com" <tony@...mide.com>,
	"S, Venkatraman" <svenkatr@...com>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"'Lavinen Jarkko (Nokia-D/Helsinki)'" <jarkko.lavinen@...ia.com>
Subject: Re: [PATCH v3] OMAP: Fix for bus width which improves SD card's peformance.

Chikkature Rajashekar, Madhusudhan had written, on 04/06/2010 07:16 PM, 
the following:
> 
>> -----Original Message-----
>> From: Nishanth Menon [mailto:nm@...com]
>> Sent: Tuesday, April 06, 2010 6:39 PM
>> To: Chikkature Rajashekar, Madhusudhan
>> Cc: felipe.balbi@...ia.com; me@...ipebalbi.com; 'kishore kadiyala'; 'Vimal
>> Singh'; tony@...mide.com; S, Venkatraman; linux-omap@...r.kernel.org;
>> linux-kernel@...r.kernel.org; 'Lavinen Jarkko (Nokia-D/Helsinki)'
>> Subject: Re: [PATCH v3] OMAP: Fix for bus width which improves SD card's
>> peformance.
>>
>> Chikkature Rajashekar, Madhusudhan had written, on 04/06/2010 06:23 PM,
>> the following:
>>>> -----Original Message-----
>>>> From: Felipe Balbi [mailto:felipe.balbi@...ia.com]
>>>> Sent: Tuesday, April 06, 2010 11:57 AM
>>>> To: ext Nishanth Menon
>>>> Cc: Balbi Felipe (Nokia-D/Helsinki); Chikkature Rajashekar,
>> Madhusudhan;
>>>> me@...ipebalbi.com; 'kishore kadiyala'; 'Vimal Singh';
>> tony@...mide.com;
>>>> S, Venkatraman; linux-omap@...r.kernel.org; linux-
>> kernel@...r.kernel.org;
>>>> Lavinen Jarkko (Nokia-D/Helsinki)
>>>> Subject: Re: [PATCH v3] OMAP: Fix for bus width which improves SD
>> card's
>>>> peformance.
>>>>
>>>> On Tue, Apr 06, 2010 at 06:55:03PM +0200, ext Nishanth Menon wrote:
>>>>> some reasons why i love switch statements ;) since I dont expect other
>>>>> than precisely 4 and 8 (do we expect 5,6,7 - i might be wrong).. but
>> if
>>>>> it is so, wont the following be better?
>>>>>
>>>>> switch (mmc_slot(host).wires)
>>>>> {
>>>>> case 8:
>>>>> 	mmc->caps |= MMC_CAP_8_BIT_DATA;
>>>>> 	/* fall thru*/
>>>>> case 4:
>>>>> 	mmc->caps |= MMC_CAP_4_BIT_DATA;
>>>>> 		break;
>>>>> default:
>>>>> 	WARN("bad width");
>>>>> }
>>>> I like that, but I remember Madhu (or someone else) saying he thinks
>>>> it's less readable this way. Go figure...
>>>>
>>> Well, I did not comment on the usage of switch here. Note we only need
>> to
>>> handle 8-bit and 4-bit.The board files need not setup 8-bit or 4-bit if
>> the
>>> configuration of that board is 1-bit. The driver will still work in 1-
>> bit
>>> mode which would mean there is nothing to do in default case and should
>> not
>>> err out.
>> check the attachment out.. hope that takes care of it.. just as a
>> reference alone ofcourse..
> 
> Nope. The default case is wrong. The assumption followed by controller
> drivers is that if the board file says 4-bit or 8-bit set the capabilities
> otherwise don't do any thing. The host will continue to work in 1-bit mode
> which is a must. Your patch violates that (can not design a board without
> connecting one data line at least :))
> 
> Also you can not say 1-bit is non-optimal because the board file dictates
> the configuration based on what it is capable of. Why through a warning?
> It is subjective.
Point noted. try n++:
switch(mmc_slot(host).wires) {
case 8:
         mmc->caps |= MMC_CAP_8_BIT_DATA;
         /* Fall through */
case 4:
         mmc->caps |= MMC_CAP_4_BIT_DATA;
         break;
case 0:
         /* assuming nothing was given by board, use 1 */
case 1:
         /* nothing to crib here */
         break;
default:
         /* Completely unexpected.. try 1 bit instead  */
         dev_crit(mmc_dev(host->mmc), "Invalid width %d"
                         " used! using 1 instead\n",
                         mmc_slot(host).wires);
}

note: we should crib if the board file made a mistake here.. say it gave 
10 wire or so.. I agree that we can recover by stepping back to 1 bit 
mode.. but we gotta tell the log that something aint right..

-- 
Regards,
Nishanth Menon
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