linux-kernel - Re: [RESEND PATCH v2 0/9] eeprom: at24: at24cs series serial number read

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Message-ID: <CAMpxmJWmgk+9oJXZnRFRzidLaasoMyuDuZmVWWrs6dwNM=+k6Q@mail.gmail.com>
Date:	Mon, 4 Jan 2016 15:01:54 +0100
From:	Bartosz Golaszewski <bgolaszewski@...libre.com>
To:	Wolfram Sang <wsa@...-dreams.de>
Cc:	linux-i2c <linux-i2c@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RESEND PATCH v2 0/9] eeprom: at24: at24cs series serial number read

2016-01-02 21:50 GMT+01:00 Wolfram Sang <wsa@...-dreams.de>:
> On Fri, Dec 11, 2015 at 02:55:10PM +0100, Bartosz Golaszewski wrote:
>> 2015-12-11 13:08 GMT+01:00 Wolfram Sang <wsa@...-dreams.de>:
>> > On Wed, Dec 02, 2015 at 11:25:17AM +0100, Bartosz Golaszewski wrote:
>> >> Chips from the at24cs EEPROM series have an additional read-only memory area
>> >> containing a factory pre-programmed serial number. In order to access it, a
>> >> dummy write must be executed before reading the serial number bytes.
>> >
>> > Can't you instantiate a read-only EEPROM on this second address? Or a
>> > seperate driver attaching to this address? What is the advantage of
>> > having this in at24?
>> >
>>
>> The regular memory area and serial number read-only block share the
>> internal address pointer. We must ensure that there's no race
>> conditions between normal EEPROM reads/writes and serial number reads.
>
> I don't get it. Both, regular at24 reads and the serial read, setup the
> pointer every time by using two messages, first write to set the
> pointer, then read. The per-adapter lock makes sure those two messages
> will not get interrupted.

If that's correct, then is there any need to have an additional mutex
for at24_data?

> So, it looks to me that it would be OK if a
> serial read access gets inbetween a eeprom read access. Am I wrong?
>

In that case would the preferred method be to access the regular
memory area like before - by allocating, for example, a 24c02 device -
while allocating a second device - in that case 24cs02 - on the
corresponding serial number address would give the user access to the
serial number via the eeprom sysfs attribute (which for the latter
would be read-only and 16 bytes in size)?

Best regards,
Bartosz Golaszewski
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