Re: [PATCH v1 0/2] [PATCH] hwmon: (pmbus/max31785) Add minimum delay between bus accesses

[Wolfram Sang <wsa@kernel.org>]

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Hi Guenter,

> > > Reference to Andrew's previous proposal:
> > > https://lore.kernel.org/all/20200914122811.3295678-1-andrew@aj.id.au/
> > 
> > I do totally agree with Guenter's comment[1], though. This just affects
> > a few drivers and this patch is way too intrusive for the I2C core. The
> > later suggested prepare_device() callback[2] sounds better to me. I
> > still haven't fully understood why this all cannot be handled in the
> > driver's probe. Could someone give me a small summary about that?
> > 
> 
> Lots of PMBus devices have the same problem, we have always handled
> it in PMBus drivers by implementing local wait code, and your references
> point that out.

I am confused now. Reading your reply:

"I am not sure if an implementation in the i2c core is desirable. It
looks quite invasive to me, and it won't solve the problem for all
devices since it isn't always a simple "wait <n> microseconds between
accesses". For example, some devices may require a wait after a write
but not after a read, or a wait only after certain commands (such as
commands writing to an EEPROM)."

I get the impression you don't prefer to have a generic mechanism in the
I2C core. This I share. Your response now sounds like you do support
that idea now?

> What other summary are you looking for ?

What the actual problem is with these devices. The cover letter only
mentions "issues with small command turn-around times". More details
would be nice. Is it between transfers? Or even between messages within
one transfer? Has it been tried to lower the bus frequency? Stuff like
this.

> On a side note, if anyone plans to implement the prepare_device() callback,
> please make sure that it covers all requirements. It would be unfortunate
> if such a callback was implemented if that would still require per-driver
> code (besides the callback).

Is there a list of that somewhere? Or does it mean going through all the
drivers and see what they currently do?

Regards,

   Wolfram


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