linux-kernel - Re: [PATCH v2 00/13] serial: 8250

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Message-ID: <20240911133848.2cbb1834@SWDEV2.connecttech.local>
Date: Wed, 11 Sep 2024 13:38:48 -0400
From: Parker Newman <parker@...est.io>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc: Parker Newman <pnewman@...necttech.com>, linux-kernel@...r.kernel.org,
 linux-serial@...r.kernel.org, Greg Kroah-Hartman
 <gregkh@...uxfoundation.org>, Jiri Slaby <jirislaby@...nel.org>
Subject: Re: [PATCH v2 00/13] serial: 8250_exar: Clean up the driver

On Mon, 9 Sep 2024 13:06:26 +0300
Andy Shevchenko <andriy.shevchenko@...ux.intel.com> wrote:

> On Fri, Sep 06, 2024 at 02:38:51PM -0400, Parker Newman wrote:
> > On Fri, 6 Sep 2024 17:42:26 +0300
> > Andy Shevchenko <andriy.shevchenko@...ux.intel.com> wrote:
> > > On Fri, Sep 06, 2024 at 10:33:54AM -0400, Parker Newman wrote:
> > > > On Fri, 6 Sep 2024 17:24:44 +0300
> > > > Andy Shevchenko <andriy.shevchenko@...ux.intel.com> wrote:
> > > > > On Fri, Sep 06, 2024 at 09:51:41AM -0400, Parker Newman wrote:
> > > > > > On Fri, 6 Sep 2024 15:46:51 +0300
> > > > > > Andy Shevchenko <andriy.shevchenko@...ux.intel.com> wrote:
> > > > > > > On Fri, May 03, 2024 at 02:33:03PM -0400, Parker Newman wrote:
>
> ...
>
> > > > > > > Sorry for blast from the past, but I have some instersting information
> > > > > > > for you. We now have spi-gpio and 93c46 eeprom drivers available to be
> > > > > > > used from others via software nodes, can you consider updating your code
> > > > > > > to replace custom bitbanging along with r/w ops by the instantiating the
> > > > > > > respective drivers?
> > > > > >
> > > > > > Hi Andy,
> > > > > > The Exar UARTs don't actually use MPIO/GPIO for the EEPROM.
> > > > > > They have a dedicated "EEPROM interface" which is accessed by the
> > > > > > REGB (0x8E) register. It is a very simple bit-bang interface though,
> > > > > > one bit per signal.
> > > > > >
> > > > > > I guess in theory I could either add  GPIO wrapper to toggle these bits
> > > > > > and use the spi-gpio driver but I am not sure if that really improves things?
> > > > > > Maybe using the spi-bitbang driver directly is more appropriate?
> > > > > > What do you think?
> > > > >
> > > > > Yes, spi-bitbang seems better in this case.
> > > >
> > > > I will try to make some time to implement this... Or if someone else from the
> > > > community wants to take this on in the mean time I am certainly happy to test
> > > > and help out!
> > >
> > > Sure, I shared this thought due to having lack of time to look myself,
> > > but I prepared the above mentioned drivers to make them work in this case.
> > > (If you are curios, see the Git history for the last few releases with
> > >  --author="Andy Shevchenko")
> > >
> >
> > Looking into it a bit more I think we could just use the eeprom_93cx6
> > driver without any SPI layer. Just need to add simple register_read()
> > and register_write() functions to read/write the REB register.
> >
> > That should be a pretty easy change to make, I can try to make that
> > change soon unless anyone has any objections to that method?
>
> Thank you, this is pretty wonderful news!
>

I have this mostly working however there is one issue. The eeprom_93cx6
driver doesn't seem to discard the "dummy bit" the 93C46 EEPROM outputs
between the writing of the op-code/address to the EEPROM and the reading
of the data from the EEPROM.

More info can be found on page 6 of the AT93C46 datasheet. I see similar
notes in other 93C46/93C56/93C66 datasheets.
Link: https://ww1.microchip.com/downloads/en/DeviceDoc/Atmel-5193-SEEPROM-AT93C46D-Datasheet.pdf

In summary the read operation for the AT93C46 EEPROM is:
Write to EEPROM :	110[A5-A0]	(9 bits)
Read from EEPROM: 	0[D15-D0]	(17 bits)

Where 110 is the READ OpCode, [A5-A0] is the address to read from,
0 is a "dummy bit" and then [D15-D0] is the actual data.

I am seeing the "correct" values being read from the EEPROM when using the
eeprom_93cx6 driver but they are all shifted right by one because the
dummy 0 bit is not being discarded.

The confusing part is the eeprom_93cx6 driver has behaved the same since
at least 2009 and half a dozen or so other drivers use it. I am not sure
if they just work around and/or live with this bug or if they have
different HW that handles the extra dummy bit?

I am hesitant to "fix" the eeprom_93cx6 driver and potentially break the
other users of it. I could add a flag to the eeprom_93cx6 struct to work
around this issue... Unless anyone else has some ideas or input?

Thanks,
Parker