linux-kernel - Re: MMCv4 support (8-bit support missing)

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Message-ID: <4628E68A.9000809@overt.org>
Date:	Fri, 20 Apr 2007 09:12:58 -0700
From:	Philip Langdale <philipl@...rt.org>
To:	Madhusudhan c <cr.madhusudhan@...il.com>
CC:	Pierre Ossman <drzeus@...eus.cx>, linux-kernel@...r.kernel.org
Subject: Re: MMCv4 support (8-bit support missing)

Madhusudhan c wrote:
> 
> Suppose a host controller is capable of suporting 8-bit and it tells
> the core that it can support 8-bit. Now the card that is plugged in
> might or might not support 8-bit based on the type of the card. There
> is no field in the ext_csd which will tell you what bus width the card
> can support.

My understanding is that 8-bit support on the card is not optional. ie:
the card is not mmc 4 compliant if it doesn't support 8-bit and this
is why there's no ext_csd field. The only case I could think of where
bus testing would be needed would be if a controller was placed into
a configuration where some pins were deliberately left unconnected
(perhaps because space constraints prevented the extra 4 pins from being
placed on a pcb) and there was no way to change the capabilities reported
by the controller.

But in any case - this is all hypothetical right now - one can't find
a controller in the wild for love or money (well - Arasan will sell you
a validation controller for $5000 but still...). As neither Pierre or
myself is in a position to do any sort of tests on an 8-bit controller,
we can't investigate or verify any of this.

It certainly seems that you have access to an 8-bit controller. If you
can help one or both of us get access to one, I'm sure we could work out
the right thing to do.

--phil
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