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

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Date:	Tue, 6 Apr 2010 19:16:49 -0500
From:	"Madhusudhan" <madhu.cr@...com>
To:	"'Nishanth Menon'" <nm@...com>
Cc:	<felipe.balbi@...ia.com>, <me@...ipebalbi.com>,
	"'kishore kadiyala'" <kishorek.kadiyala@...il.com>,
	"'Vimal Singh'" <vimal.newwork@...il.com>, <tony@...mide.com>,
	"'S, Venkatraman'" <svenkatr@...com>, <linux-omap@...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.



> -----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.

Regards,
Madhu

> --
> Regards,
> Nishanth Menon

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