linux-kernel - Re: [PATCH 1/2] tty: pl011: Work around QDF2400 E44 stuck BUSY bit

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Date:   Wed, 8 Feb 2017 17:04:57 -0600
From:   Timur Tabi <timur@...eaurora.org>
To:     Christopher Covington <cov@...eaurora.org>,
        Jonathan Corbet <corbet@....net>,
        Marc Zyngier <marc.zyngier@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>, linux-doc@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Mark Rutland <mark.rutland@....com>,
        linux-kernel@...r.kernel.org, shankerd@...eaurora.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        Russell King <linux@...linux.org.uk>,
        linux-serial@...r.kernel.org
Cc:     Mark Langsdorf <mlangsdo@...hat.com>,
        Mark Salter <msalter@...hat.com>, Jon Masters <jcm@...hat.com>,
        Neil Leeder <nleeder@...eaurora.org>
Subject: Re: [PATCH 1/2] tty: pl011: Work around QDF2400 E44 stuck BUSY bit

On 02/08/2017 04:22 PM, Christopher Covington wrote:
>>> >> -    while (pl011_read(uap, REG_FR) & uap->vendor->fr_busy)
>>> >> +    while ((pl011_read(uap, REG_FR) ^ uap->vendor->inv_fr)
>>> >> +                        & uap->vendor->fr_busy)
>> >
>> > I really think the XOR logic needs to be documented wherever it's
>> > used. It's just too confusing.
> References such as the following for basic bit operations like
> setting, clearing, and toggling are easy enough to come by.
>
> http://stackoverflow.com/questions/47981/how-do-you-set-clear-and-toggle-a-single-bit-in-c-c

I know what XOR does.  I was thinking a more high-level comment would be 
appropriate.  With E44, we want to ignore the BUSY bit and use TXFE instead. 
So was hoping to a comment that says that:

/* Normally, we poll until BUSY=0, but E44 says we should poll until TXFE=1 
instead. So with E44, we set fr_busy to TXFE, but we have to invert it. */

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