linux-kernel - Re: [PATCH v2] hvc_dcc : add support to armv4 and armv5 core

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Date:	Mon, 3 Sep 2012 12:57:27 +0000
From:	Arnd Bergmann <arnd@...db.de>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	Matthieu CASTET <castet.matthieu@...e.fr>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	gregkh@...uxfoundation.org, alan@...rguk.ukuu.org.uk,
	Matthieu Castet <matthieu.castet@...rot.com>
Subject: Re: [PATCH v2] hvc_dcc : add support to armv4 and armv5 core

On Friday 31 August 2012, Stephen Boyd wrote:
> > +static int hvc_dcc_put_chars_v6(uint32_t vt, const char *buf, int count)
> > +{
> > +     int i;
> > +
> > +     for (i = 0; i < count; i++) {
> > +             while (__dcc_getstatus_v6() & DCC_STATUS_TX_V6)
> > +                     cpu_relax();
> > +
> > +             __dcc_putchar_v6(buf[i]);
> > +     }
> > +
> > +     return count;
> > +}
> 
> It's unfortunate that the main logic is duplicated. I wonder if we could
> push the runtime decision slightly lower into the accessor functions
> instead and make some new functions dcc_tx_busy() and dcc_rx_busy() or
> something. Then these loops stay the same.

Agreed. Ideally, you should be able to get the code to be compiled into
the same binary as before for ARMv6+. If only the inline assembly differs,
you can do something like

static inline char __dcc_getchar(void)
{
        char __c;

        if (__LINUX_ARM_ARCH >= 6)
		asm volatile("mrc p14, 0, %0, c0, c5, 0 @ read comms data reg"
	                : "=r" (__c));
	else
		asm volatile ("mrc p14, 0, %0, c1, c0  @ read comms data reg"
			: "=r" (ret));
        isb();

        return __c;
}

	Arnd
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