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Subject: Re: [PATCH 1/2] tty: pl011: Work around QDF2400 E44 stuck BUSY bit
To: Christopher Covington <cov@codeaurora.org>,
        Jonathan Corbet <corbet@lwn.net>, Marc Zyngier <marc.zyngier@arm.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>, linux-doc@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org,
        Mark Rutland <mark.rutland@arm.com>, linux-kernel@vger.kernel.org,
        shankerd@codeaurora.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jiri Slaby <jslaby@suse.com>, Russell King <linux@armlinux.org.uk>,
        linux-serial@vger.kernel.org
Cc: Mark Langsdorf <mlangsdo@redhat.com>, Mark Salter <msalter@redhat.com>,
        Jon Masters <jcm@redhat.com>, Neil Leeder <nleeder@codeaurora.org>
References: <20170208005736.18724-1-cov@codeaurora.org>
 <bd4a0127-0f32-a8be-c8ee-34c1cccc6d5d@codeaurora.org>
 <6afb0a98-0973-33ec-0b76-fc6378e64309@codeaurora.org>
From: Timur Tabi <timur@codeaurora.org>
Message-ID: <5e18324f-d09d-d652-df23-d4a3f8f053fd@codeaurora.org>
Date: Wed, 8 Feb 2017 17:04:57 -0600
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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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