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

[Nishanth Menon]

Chikkature Rajashekar, Madhusudhan had written, on 04/06/2010 07:16 PM, 
the following:
> > -----Original Message-----
> > From: Nishanth Menon [mailto:nm@xxxxxx]
> > Sent: Tuesday, April 06, 2010 6:39 PM
> > To: Chikkature Rajashekar, Madhusudhan
> > Cc: felipe.balbi@xxxxxxxxx; me@xxxxxxxxxxxxxxx; 'kishore kadiyala'; 'Vimal
> > Singh'; tony@xxxxxxxxxxx; S, Venkatraman; linux-omap@xxxxxxxxxxxxxxx;
> > linux-kernel@xxxxxxxxxxxxxxx; '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@xxxxxxxxx]
> > > > Sent: Tuesday, April 06, 2010 11:57 AM
> > > > To: ext Nishanth Menon
> > > > Cc: Balbi Felipe (Nokia-D/Helsinki); Chikkature Rajashekar,
> > Madhusudhan;
> > > > me@xxxxxxxxxxxxxxx; 'kishore kadiyala'; 'Vimal Singh';
> > tony@xxxxxxxxxxx;
> > > > S, Venkatraman; linux-omap@xxxxxxxxxxxxxxx; linux-
> > kernel@xxxxxxxxxxxxxxx;
> > > > 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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