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Date:   Wed, 20 May 2020 16:48:26 +0300
From:   Serge Semin <Sergey.Semin@baikalelectronics.ru>
To:     Thomas Bogendoerfer <tsbogend@alpha.franken.de>
CC:     Serge Semin <fancer.lancer@gmail.com>,
        Alexey Malahov <Alexey.Malahov@baikalelectronics.ru>,
        Paul Burton <paulburton@kernel.org>,
        Ralf Baechle <ralf@linux-mips.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Rob Herring <robh+dt@kernel.org>, <linux-pm@vger.kernel.org>,
        <devicetree@vger.kernel.org>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        <linux-mips@vger.kernel.org>, <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2 18/20] mips: csrc-r4k: Decrease r4k-clocksource rating
 if CPU_FREQ enabled
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 <20200518205752.txbylbjt2zkwdwwe@mobilestation>
 <20200519155053.GB15797@alpha.franken.de>
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On Wed, May 20, 2020 at 03:38:27PM +0200, Thomas Bogendoerfer wrote:
> On Wed, May 20, 2020 at 03:12:01PM +0300, Serge Semin wrote:
> > Since you don't like the way I initially fixed it, suppose there we don't have
> > another way but to introduce something like CONFIG_MIPS_CPS_NS16550_WIDTH
> > parameter to select a proper accessors, like sw in our case, and sb by defaul).
> > Right?
> 
> to be on the safe side it's probably the best thing. But I don't know
> enough about CPS_NS16550 to judge whether shift value correlates with
> possible access width.

The base address passed to the _mips_cps_putc() leaf is UART-base address. It
has nothing to do with CPS. See:
/**
 * _mips_cps_putc() - write a character to the UART
 * @a0: ASCII character to write
 * @t9: UART base address
 */
LEAF(_mips_cps_putc)
1:      lw              t0, UART_LSR_OFS(t9)
        andi            t0, t0, UART_LSR_TEMT
        beqz            t0, 1b
        sb              a0, UART_TX_OFS(t9)
        jr              ra
        END(_mips_cps_putc)

So it's base address must be accessed with proper alignment. On our case it's
lw/sw instructions. Regarding using lw in the first line of the function. That's
must be a bug, since further in the same function they use sb to access the UART
Tx register. So reading a data from UART_LSR register should be also byte-sized
by using lb.

-Sergey

> 
> Thomas.
> 
> -- 
> Crap can work. Given enough thrust pigs will fly, but it's not necessarily a
> good idea.                                                [ RFC1925, 2.3 ]