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Re: [Qemu-devel] [PATCH v3 12/14] hw/sd.c, hw/sd.h: add receive ready qu

From: Mitsyanko Igor
Subject: Re: [Qemu-devel] [PATCH v3 12/14] hw/sd.c, hw/sd.h: add receive ready query routine to SD/MMC API
Date: Wed, 14 Dec 2011 12:29:01 +0400
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20110922 Thunderbird/3.1.15

On 12/13/2011 08:22 PM, Peter Maydell wrote:
On 13 December 2011 15:11, Peter Maydell<address@hidden>  wrote:
On 12 December 2011 06:43, Evgeny Voevodin<address@hidden>  wrote:
From: Mitsyanko Igor<address@hidden>

Data transfer direction between host controller and SD/MMC card is selected by
host controller configuration registers, but whether we actually need or need
not perform data transfer depends on type of last issued command. To avoid
memorization of which type of command host controller issued the last time, we
can use simple query procedures, to make sure that SD/MMC card is in the
right state. The only query routine currently presented in SD/MMC card
emulation is sd_data_ready(), this patch adds sd_receive_ready() routine.

I've thought about this a bit more and spent some time poking through
the standards docs. I'm kind of suspicious of both this function and
the existing sd_data_ready(). Real hardware doesn't have these signals,
both ends of the sd-controller-to-card connection just do their thing
and trust that the other end is in the state it is supposed to be.
If you (in the controller model) don't already know whether you are
OK to send/receive data then you haven't modelled some part of the
controller's state properly.

Having thought even more :-), sd_data_ready() is actually modelling a
real hardware behaviour: "have we seen the start bit on the data line?".
So it is justifiable. I still don't think sd_receive_ready() is, though.

-- PMM

I thought it's a good idea to protect user data from corruption because of inappropriate driver behaviour, even though it's obviously not a controller's concern. But after your comment about start bit it started to make sense to me that sd_data_ready() exists in current implementation, while other query functions don't :)

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