linux-kernel - Re: [PATCH v1 11/13] serial: 8250_exar: Use BIT() in exar_ee

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Date: Fri, 3 May 2024 10:26:32 -0400
From: Parker Newman <parker@...est.io>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>, Parker
 Newman <pnewman@...necttech.com>, LKML <linux-kernel@...r.kernel.org>,
 linux-serial <linux-serial@...r.kernel.org>, Greg Kroah-Hartman
 <gregkh@...uxfoundation.org>, Jiri Slaby <jirislaby@...nel.org>
Subject: Re: [PATCH v1 11/13] serial: 8250_exar: Use BIT() in exar_ee_read()

On Thu, 2 May 2024 20:20:01 +0300
Andy Shevchenko <andriy.shevchenko@...ux.intel.com> wrote:

> On Thu, May 02, 2024 at 07:08:21PM +0300, Ilpo Järvinen wrote:
> > On Thu, 2 May 2024, Andy Shevchenko wrote:  
> 
> ...
> 
> > >  	// Send address to read from
> > > -	for (i = 1 << (UART_EXAR_REGB_EE_ADDR_SIZE - 1); i; i >>= 1)
> > > -		exar_ee_write_bit(priv, (ee_addr & i));
> > > +	for (i = UART_EXAR_REGB_EE_ADDR_SIZE - 1; i >= 0; i--)
> > > +		exar_ee_write_bit(priv, ee_addr & BIT(i));
> > >  
> > >  	// Read data 1 bit at a time
> > >  	for (i = 0; i <= UART_EXAR_REGB_EE_DATA_SIZE; i++) {
> > > -		data <<= 1;
> > > -		data |= exar_ee_read_bit(priv);
> > > +		if (exar_ee_read_bit(priv))
> > > +			data |= BIT(i);  
> > 
> > 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 replacement
> > adds highest bit on each iteration?  
> 
> Oh, seems a good catch!
> 
> I was also wondering, but missed this somehow. Seems the EEPROM is in BE mode,
> so two loops has to be aligned.
> 

I just tested this and Ilpo is correct, the read loop portion is backwards as 
expected. This is the corrected loop:

    // Read data 1 bit at a time
    for (i = UART_EXAR_REGB_EE_DATA_SIZE; i >= 0; i--) {
        if (exar_ee_read_bit(priv))
            data |= BIT(i);
    }

I know this looks wrong because its looping from 16->0 rather than the 
more intuitive 15->0 for a 16bit value. This is actually correct however 
because according to the AT93C46D datasheet there is always dummy 0 bit
before the actual 16 bits of data.

I hope that helps,
-Parker