linux-kernel - Re: [RFT PATCH v2 2/3] hwmon: (spd5118) Use spd5118 specific read/write operations

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Date: Wed, 19 Jun 2024 11:13:41 +0200
From: Thomas Weißschuh <linux@...ssschuh.net>
To: Guenter Roeck <linux@...ck-us.net>
Cc: Paul Menzel <pmenzel@...gen.mpg.de>, Armin Wolf <W_Armin@....de>, 
	Wolfram Sang <wsa+renesas@...g-engineering.com>, linux-kernel@...r.kernel.org, 
	René Rebe <rene@...ctcode.de>, Stephen Horvath <s.horvath@...look.com.au>, 
	Sasha Kozachuk <skozachuk@...gle.com>, John Hamrick <johnham@...gle.com>, 
	Chris Sarra <chrissarra@...gle.com>, linux-hwmon@...r.kernel.org, Jean Delvare <jdelvare@...e.com>, 
	Heiner Kallweit <hkallweit1@...il.com>
Subject: Re: [RFT PATCH v2 2/3] hwmon: (spd5118) Use spd5118 specific
 read/write operations

On 2024-06-18 18:02:51+0000, Guenter Roeck wrote:
> On 6/18/24 17:50, Thomas Weißschuh wrote:
> > On 2024-06-18 17:23:44+0000, Guenter Roeck wrote:
> > > On 6/18/24 16:39, Paul Menzel wrote:
> > > > [Cc: +Heiner]
> > > > 
> > > > 
> > > > Dear Armin,
> > > > 
> > > > 
> > > > Am 19.06.24 um 01:28 schrieb Armin Wolf:
> > > > > Am 19.06.24 um 00:28 schrieb Wolfram Sang:
> > > > > 
> > > > > > > to 86 degrees C. If that doesn't work, we'll be really out of luck
> > > > > > > with that controller (or at least I don't have an idea what else to try).
> > > > > > 
> > > > > > Try CCing Heiner Kallweit for ideas about the i801 controller.
> > > > 
> > > > > i am not Heiner Kallweit, but i found something interesting in
> > > > > commit ba9ad2af7019 ("i2c: i801: Fix I2C Block Read on 8-Series/C220 and later").
> > > > > 
> > > > > Basically, it seems that the i802 i2c controller indeed features a SPD write disable bit which blocks all writes for slave addresses 0x50-0x57.
> > > > > 
> > > > > Does the i801 i2c controller driver print something like "SPD Write Disable is set" during boot?
> > > > 
> > > > Nice find. Yes, it does:
> > > > 
> > > 
> > > Yes, definitely. I didn't have any recent datasheets, so I missed that flag.
> > > Oh well :-(.
> > > 
> > > >       [    5.462605] i801_smbus 0000:00:1f.4: SPD Write Disable is set
> > > >       [    5.468399] i801_smbus 0000:00:1f.4: SMBus using PCI interrupt
> > > > 
> > > 
> > > Bummer. That explains the problem. It means that the BIOS effectively
> > > blocks reading the eeprom on your system (because that would require writing
> > > the page register), as well as changing temperature limits. That is really
> > > annoying, but there is nothing we can do about it. Maybe the BIOS has a
> > > configuration flag to enable or disable write protect, but I doubt it.
> > 
> > What about using 16bit addressing mode?
> > 
> >      Alternatively, at initial power on, the host can set the Table 112, “MR11” [3] = ‘1’ to address the entire 1024 bytes of
> >      non-volatile memory with 2 bytes of address and hence not required to go through page selection to address entire
> >      non-volatile memory.
> > 
> > regmap-i2c allows 16bit addresses when I2C_FUNC_SMBUS_I2C_BLOCK is supported,
> > which to me looks like it should be the case on i801 for ICH5.
> > 
> 
> Good idea, but it doesn't work. I can get write operations with
> 16-bit register addresses to work even on piix4, but read operations
> require writing a 16-bit register address followed by byte reads (see
> regmap_i2c_smbus_i2c_read_reg16). Unfortunately, spd5118 devices
> don't auto-increment the address on byte read operations, meaning
> each byte read returns data from address 0x00 (i.e., it returns
> 0x51). Try "i2cdump -y -f 0 0x50 c" and you'll see what I mean.
> Maybe there is a way around it, but I have not found it.

Thanks for the pointer to regmap_i2c_smbus_i2c_read_reg16().
I'm not really familiar with I2C/SMBUS ...

Did you look into "2.6.8.3 Default Read Address Pointer Mode"?

I am failing to understand how that address pointer mode would ever make
sense without address auto-increment.

> On top of that, configuring 16-bit mode requires a write operation
> into the page register, and that is blocked.

... this one on the other hand is really obvious.


Thanks,
Thomas