Received-SPF: pass (google.com: domain of linux-kernel+bounces-168093-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.80.249 as permitted sender) client-ip=147.75.80.249;
Date: Fri, 3 May 2024 14:56:33 -0400
From: Parker Newman <parker@finest.io>
To: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Ilpo =?UTF-8?B?SsOkcnZpbmVu?= <ilpo.jarvinen@linux.intel.com>, Parker
 Newman <pnewman@connecttech.com>, LKML <linux-kernel@vger.kernel.org>,
 linux-serial <linux-serial@vger.kernel.org>, Greg Kroah-Hartman
 <gregkh@linuxfoundation.org>, Jiri Slaby <jirislaby@kernel.org>
Subject: Re: [PATCH v1 11/13] serial: 8250_exar: Use BIT() in exar_ee_read()
Message-ID: <20240503145633.1ad55378@SWDEV2.connecttech.local>
In-Reply-To: <ZjUEURneUmZ4nmbC@smile.fi.intel.com>
References: <20240502144626.2716994-1-andriy.shevchenko@linux.intel.com>
	<20240502144626.2716994-12-andriy.shevchenko@linux.intel.com>
	<702a9145-5bc1-c765-a1fa-278702741637@linux.intel.com>
	<ZjPLQeTEKvt7B3mj@smile.fi.intel.com>
	<20240503102632.112a34bd@SWDEV2.connecttech.local>
	<ZjUEURneUmZ4nmbC@smile.fi.intel.com>
Organization: Connect Tech Inc.
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: quoted-printable
UI-OutboundReport: notjunk:1;M01:P0:zDp5WcS2eQM=;TSYrBciSm/jwQqbvmJA0qfCe+NU
 r3LkB5LaQkBs7ICgII1lBFUXPyymwsroAoPflGcIzdF2dksMjyFYjF9zVr812RRGGDBISC8Mf
 UcGlzImEEHT02Y0W72WVQixhWO3PbRGMZO8Zoz+ZP8oLzi71E9mJinbtkaAHTotzqNuQF9ZLQ
 MsMv5apOSxW2Utu9jjYW8+YEnYXEbeXf9z2Ksjv+S1+jFh7N+ftLkRWI2rknvwqiq4fwYbQOH
 YTnio80b8DL8xnmFWDULBaWc5yu1kOuq6XzPbOumTNIBVxBShzsZQPJldTFjZPogXRsNaSEXH
 I5Qd5I52psTm2jO/V+bPsBNTQv5kE4kMOQ6l1iYmFZcGmDcg+QvuSj/D6ZKBdBkJG5vsg9OYE
 2vIftqczONnZ9A3iZ4EBsDDsbF6uJjRnSU4XyOYNGPvgEXGqwn2VvqRx0u/u8R4JvvyvbCPKT
 VU9H6A7kiD1fXqAtR1nTDDHBcF+SGAF27d0+xOTtTM13hrlwkE7iM5fCNc/X/mEkjBz0ywgGj
 bEdz5FyBQXjZxDa0OhSQTTUcT7VOPhMpbQzGmcrOFsqzgHYkqZfOoxMkeUrOlPJSukmA+0Hhe
 VLattDaWR3yIvztXp9MZp1+R0Yo31GxXsEsz0yCiH6F7E16SJCnBCtxRZ44UvW7jIFEzOWSlV
 yE8zModPoVoCF43kAPBCIlz2+UtQmxZRgCb0cgQXNmprgKpCLuHAx2aV1DR/GgLMPJrjAUmoS
 PDuYkBDD6Dreh7przY0/nYvOnNxvRT/PYPCxNyhpw5us+jXmtEl7WM=

On Fri, 3 May 2024 18:35:45 +0300
Andy Shevchenko <andriy.shevchenko@linux.intel.com> wrote:

> On Fri, May 03, 2024 at 10:26:32AM -0400, Parker Newman wrote:
> > On Thu, 2 May 2024 20:20:01 +0300
> > Andy Shevchenko <andriy.shevchenko@linux.intel.com> wrote: =20
> > > On Thu, May 02, 2024 at 07:08:21PM +0300, Ilpo J=C3=A4rvinen wrote: =
=20
> > > > On Thu, 2 May 2024, Andy Shevchenko wrote:   =20
>=20
> ...
>=20
> > > > >  	// Send address to read from
> > > > > -	for (i =3D 1 << (UART_EXAR_REGB_EE_ADDR_SIZE - 1); i; i >>=3D 1)
> > > > > -		exar_ee_write_bit(priv, (ee_addr & i));
> > > > > +	for (i =3D UART_EXAR_REGB_EE_ADDR_SIZE - 1; i >=3D 0; i--)
> > > > > +		exar_ee_write_bit(priv, ee_addr & BIT(i));
> > > > > =20
> > > > >  	// Read data 1 bit at a time
> > > > >  	for (i =3D 0; i <=3D UART_EXAR_REGB_EE_DATA_SIZE; i++) {
> > > > > -		data <<=3D 1;
> > > > > -		data |=3D exar_ee_read_bit(priv);
> > > > > +		if (exar_ee_read_bit(priv))
> > > > > +			data |=3D BIT(i);   =20
> > > >=20
> > > > Does this end up reversing the order of bits? In the original, data=
 was left
> > > > shifted which moved the existing bits and added the lsb but the rep=
lacement
> > > > adds highest bit on each iteration?   =20
> > >=20
> > > Oh, seems a good catch!
> > >=20
> > > I was also wondering, but missed this somehow. Seems the EEPROM is in=
 BE mode,
> > > so two loops has to be aligned.
> > >  =20
> >=20
> > I just tested this and Ilpo is correct, the read loop portion is backwa=
rds as=20
> > expected. This is the corrected loop:
> >=20
> >     // Read data 1 bit at a time
> >     for (i =3D UART_EXAR_REGB_EE_DATA_SIZE; i >=3D 0; i--) {
> >         if (exar_ee_read_bit(priv))
> >             data |=3D BIT(i);
> >     }
> >=20
> > I know this looks wrong because its looping from 16->0 rather than the=
=20
> > more intuitive 15->0 for a 16bit value. This is actually correct howeve=
r=20
> > because according to the AT93C46D datasheet there is always dummy 0 bit
> > before the actual 16 bits of data.
> >=20
> > I hope that helps, =20
>=20
> Yes, it helps and means that we need that comment to be added to the code=
 Is
> it the same applicable to the write part above (for address)? Because AFA=
IU
> mine is one bit longer than yours. Maybe in the original code is a bug? H=
ave
> you tried to read high addresses?

The address portion is 6 bits, nothing extra, so what you have is correct.

The original code was legacy, I cleaned it up but didn't change those loops=
=20

Your method works out the the same number of bits as the legacy method
which sets bit 5 and has to shift right 6 times to get i =3D 0 which ends t=
he loop.

Parker