linux-kernel - Re: [PATCH v2] i2c: stub: Add support for SMBus block commands

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Date:	Tue, 08 Jul 2014 13:05:41 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Jean Delvare <jdelvare@...e.de>
CC:	Wolfram Sang <wsa@...-dreams.de>,
	Randy Dunlap <rdunlap@...radead.org>,
	linux-i2c@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] i2c: stub: Add support for SMBus block commands

Hi Jean,

On 07/08/2014 12:54 PM, Jean Delvare wrote:
> Hi Guenter,
>
> On Mon,  7 Jul 2014 07:23:03 -0700, Guenter Roeck wrote:
>> SMBus block commands are different to I2C block commands since
>> the returned data is not normally accessible with byte or word
>> commands on other command offsets. Add linked list of 'block'
>> commands to support those commands.
>>
>> Access mechanism is quite simple: Block commands must be written
>> before they can be read. The first write selects the block length.
>> Subsequent writes can be partial. Block read commands always return
>> the number of bytes selected with the first write.
>>
>> Signed-off-by: Guenter Roeck <linux@...ck-us.net>
>> ---
>> v2: Make new functionality only available on request via functionality
>>      module parameter
>>      Add more details about SMBus block mode support to documentation
>>      Use correct sizeof() variable in devm_kzalloc
>>      Use stub_find_block() only in SMBus block command itself.
>>      Store first word of block data in chip->words[].
>>      When writing block data and the written data is longer than
>>      the first write, bail out with debug message indicating the reason
>>      for the error.
>
> Looks good, thanks for the quick update.
>
> Reviewed-by: Jean Delvare <jdelvare@...e.de>
>
> Just one thing I have been thinking about while reviewing the updated
> code... You decided to make the first SMBus block write select the
> maximum block length, and you always use that for SMBus block reads.
> However you accept partial writes. The fact that the order in which
> writes are performed has an effect on which writes are accepted is
> somewhat unexpected.
>
> Wouldn't it make more sense to accept all SMBus block writes,
> regardless of the size (as long as it is within the limits of the SMBus
> standard, of course)? Then the only thing left to decide is whether
> SMBus block reads use the maximum size or the size of the most recent
> SMBus block write.
>
> I suspect this would mimic the behavior of real chips better. What do
> you think?
>

Not really sure what the expected behavior is. My original code
accepted all writes and returned the most recent write, including
the most recent write length. I thought this was untypical, and that
it would be more typical for the chip to return a fixed length.
But ultimately I don't really know, and I am fine either way.

Guenter

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