linux-kernel - Re: [PATCH] i2c/busses: (mpc) Add support for SMBUS_READ_BLOCK

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Message-ID: <1321470915.7847.122.camel@oslab-l1>
Date:	Wed, 16 Nov 2011 11:15:15 -0800
From:	York Sun <yorksun@...escale.com>
To:	Jean Delvare <khali@...ux-fr.org>
CC:	<guenter.roeck@...csson.com>,
	Tabi Timur-B04825 <B04825@...escale.com>,
	<linux-i2c@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<B29983@...escale.com>
Subject: Re: [PATCH] i2c/busses: (mpc) Add support for SMBUS_READ_BLOCK_DATA

On Wed, 2011-11-16 at 20:10 +0100, Jean Delvare wrote:
> On Wed, 16 Nov 2011 10:20:38 -0800, York Sun wrote:
> > On Wed, 2011-11-16 at 19:09 +0100, Jean Delvare wrote:
> > > Your thinking is too focused on I2C block reads (or even block read of
> > > data over the network or on disk). SMBus block read is something
> > > completely different. It's not about reading 200 bytes of data and
> > > receiving it in 16-byte chunks (I2C block read works that way, on
> > > EEPROMs in particular.) There is no "data length" and "block size" to
> > > compare to each other. It's about reading the value of _one_ register
> > > and this value happens to be multi-byte. There is typically _no_
> > > register pointer increment (automatic or not) involved as can happen
> > > with EEPROMs. If an SMBus block read from register N returns 10 bytes,
> > > you're not going to read the next 10 bytes from register N+10. There
> > > are no "next 10 bytes" to read, and register N+10 is something
> > > completely unrelated.
> > > 
> > > And for this reason, it is not possible to mix SMBus block reads with
> > > byte reads, as can be done with I2C block reads.
> > > 
> > > Also note that there is a limit of 32 bytes for SMBus block transfers,
> > > per SMBus specification. All slaves and masters must comply with it.
> > > 
> > > I hope I managed to clarify the case this time...
> > 
> > You have made it much clear. If block size is fixed and block read
> > cannot mix with byte read, shall we do this
> > 
> > if length < block_size
> >    read block_size
> > else {
> >    while (length) {
> >        read block_size
> >        length -= block_size
> >    }
> 
> Which part of
> 
>   There is no "data length" and "block size" to compare to each other.
> 
> did you not understand?

For example, if the length is 40 and the block size is 32, are you going
to read 32, 72 byte or 64 byte?

York



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