linux-kernel - Re: [PATCH] mmc_spi: fail gracefully if host or card do not support the switch command

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Message-Id: <200906151036.05884.wolfgang.mues@auerswald.de>
Date:	Mon, 15 Jun 2009 10:36:05 +0200
From:	Wolfgang Mües <wolfgang.mues@...rswald.de>
To:	Pierre Ossman <pierre@...man.eu>
Cc:	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Matt Fleming" <matt@...sole-pimps.org>,
	"David Brownell" <dbrownell@...rs.sourceforge.net>,
	"Mike Frysinger" <vapier.adi@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mmc_spi: fail gracefully if host or card do not support the switch command

Hello Pierre,

Am Samstag, 13. Juni 2009 schrieb Pierre Ossman:
> > The switch command (typically speed switching from 25 to 50 MHz clock)
> > can be rejected by _both_ the host controller/driver and by the card.
> > Elder revisions of the SD card do not support speed switching.
> > Fail gracefully if the reject is comming from the card.

> Have you ever seen this in practice?

Yes, I have. An old 1 GByte SD card.

> For SD there are several checks 
> before the switch command is actually issued.

But there are two cases not covered by these checks:

a) the switch command is supported, but the card is not capable of switching 
to 50 MHz.

b) the card rejects the switch command in SPI mode, because it is not 
mandatory here.

So yes, the checks can be improved. But only extending the error codes was a 
simple, fast and minimal solution.

regards

i. A. Wolfgang Mües
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