linux-kernel - Re: [PATCH] hvc_dcc: Simplify assembly for v6 and v7 ARM

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Date:	Mon, 20 Dec 2010 09:51:01 -0800
From:	Daniel Walker <dwalker@...eaurora.org>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	Greg Kroah-Hartman <gregkh@...e.de>, linux-kernel@...r.kernel.org,
	linux-arm-msm@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	Tony Lindgren <tony@...mide.com>,
	Arnd Bergmann <arnd@...db.de>,
	Nicolas Pitre <nicolas.pitre@...aro.org>
Subject: Re: [PATCH] hvc_dcc: Simplify assembly for v6 and v7 ARM

On Fri, 2010-12-17 at 21:16 -0800, Stephen Boyd wrote:
> The inline assembly differences for v6 vs. v7 in the hvc_dcc
> driver are purely optimizations. On a v7 processor, an mrc with
> the pc sets the condition codes to the 28-31 bits of the register
> being read. It just so happens that the TX/RX full bits the DCC
> driver is testing for are high enough in the register to be put
> into the condition codes. On a v6 processor, this "feature" isn't
> implemented and thus we have to do the usual read, mask, test
> operations to check for TX/RX full.
> 
> Since we already test the RX/TX full bits before calling
> __dcc_getchar() and __dcc_putchar() we don't actually need to do
> anything special for v7 over v6. The only difference is in
> hvc_dcc_get_chars(). We would test RX full, poll RX full, and
> then read a character from the buffer, whereas now we will test
> RX full, read a character from the buffer, and then test RX full
> again for the second iteration of the loop. It doesn't seem
> possible for the buffer to go from full to empty between testing
> the RX full and reading a character. Therefore, replace the v7
> versions with the v6 versions and everything works the same.
> 
> While we're here, cleanup the for loops a bit and mark the inline
> assembly as volatile. Not marking it volatile causes GCC to cache
> the results of the status and RX buffer registers causing
> lockups.

I would expect to see three patches. One that adds volatile, which
appears to be a good fix. Another patch that changes the assembly lines,
and another that does the clean up. The last two are more controversial
ones.

Daniel

-- 
Sent by an consultant of the Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora
Forum.


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