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

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Message-ID: <ZjiWyYlfd7NdevFq@smile.fi.intel.com>
Date: Mon, 6 May 2024 11:37:29 +0300
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Parker Newman <parker@...est.io>
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 Fri, May 03, 2024 at 02:56:33PM -0400, Parker Newman wrote:
> On Fri, 3 May 2024 18:35:45 +0300
> Andy Shevchenko <andriy.shevchenko@...ux.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@...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,  
> > 
> > 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 AFAIU
> > mine is one bit longer than yours. Maybe in the original code is a bug? Have
> > 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. 
> 
> 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 = 0 which ends the loop.

Okay, thank your for confirming the correctness of a new code!

-- 
With Best Regards,
Andy Shevchenko