Message-ID: <20200518205752.txbylbjt2zkwdwwe@mobilestation>
Date:   Mon, 18 May 2020 23:57:52 +0300
From:   Serge Semin <Sergey.Semin@...kalelectronics.ru>
To:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>
CC:     Serge Semin <fancer.lancer@...il.com>,
        Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>,
        Paul Burton <paulburton@...nel.org>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        Rob Herring <robh+dt@...nel.org>, <linux-pm@...r.kernel.org>,
        <devicetree@...r.kernel.org>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        <linux-mips@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 18/20] mips: csrc-r4k: Decrease r4k-clocksource rating
 if CPU_FREQ enabled

On Mon, May 18, 2020 at 06:32:06PM +0200, Thomas Bogendoerfer wrote:
> On Mon, May 18, 2020 at 04:48:20PM +0300, Serge Semin wrote:
> > On Fri, May 15, 2020 at 11:06:47PM +0200, Thomas Bogendoerfer wrote:
> > > On Fri, May 15, 2020 at 10:48:27AM +0300, Serge Semin wrote:
> > > > Thomas,
> > > > Could you take a look at my comment below so I could proceed with the
> > > > patchset v3 development?
> > > 
> > > I can't help, but using r4k clocksource with changing frequency is
> > > probaly only usefull as a random generator. So IMHO the only two
> > > options are disabling it or implement what arch/x86/kernel/tsc.c does.
> > > 
> > > Thomas.
> > 
> > Thomas, could you proceed with the rest of the patches review?
> > ├─>[PATCH v2 16/20] bus: cdmm: Add MIPS R5 arch support
> > ├─>[PATCH v2 15/20] mips: cdmm: Add mti,mips-cdmm dtb node support
> 
> both are not my call, but look ok to me.

Can I add your Reviewed-by tag there then?

> 
> > ├─>[PATCH v2 13/20] mips: early_printk_8250: Use offset-sized IO-mem accessors
> 
> that's broken. A reg shift of 2 doesn't mean we could use 32bit access
> to the registers on other platforms. As I don't think adding some ifdefery
> makes things nicer, just implement the your prom_putchar in board code.

I thought about that initially, but then I decided to alter the generic
early_printk_8250 code instead. My version of prom_putchar() would be almost
the same as one implemented in the early_printk_8250 module except minor
modification of replacing readb/writeb methods with readl/writel. So I didn't
want to duplicate the code, but wanted to provide a general way to fix the
problem potentially also for another platforms.

Since you don't like this fix alternatively I'd suggest to add the reg_width
parameter passed to the setup_8250_early_printk_port() method like this:
-setup_8250_early_printk_port(unsigned long base, unsigned int reg_shift,
-                             unsigned int timeout)
+setup_8250_early_printk_port(unsigned long base, unsigned int reg_shift,
+                             unsigned int reg_width, unsigned int timeout)

By reg_width parameter we could determine the actual width of the register:
 static inline u8 serial_in(int offset)
 {
-       return readb(serial8250_base + (offset << serial8250_reg_shift));
+       u8 ret = 0xFF;
+
+       offset <<= serial8250_reg_shift;
+       switch (serial8250_reg_width) {
+       case 1:
+               ret = readb(serial8250_base + offset);
+               break;
+       case 2:
+               ret = readw(serial8250_base + offset);
+               break;
+       case 4:
+               ret = readl(serial8250_base + offset);
+               break;
+       default:
+               break;
+       }
+
+       return ret;
 }

The similar modification will be implemented for serial_out(). I'll also modify
the currently available setup_8250_early_printk_port() calls so they would pass
the reg_with as 1 to activate the normal readb/writeb IO methods.

What do you think about this?

> 
> > ├─>[PATCH v2 12/20] mips: MAAR: Add XPA mode support
> 
> looks ok so far.

Can I add your Reviewed-by tag there then?

> 
> > ├─>[PATCH v2 10/20] mips: Add CONFIG/CONFIG6/Cause reg fields macro
> 
> that is fine

Can I add your Reviewed-by tag there then?

> 
> > └─>[PATCH v2 09/20] mips: Add CP0 Write Merge config support
> 
> this is IMHO a dangerous change. Enabling write merging for any
> CPU supporting it might triggers bugs. Do it in your board bringup
> code and at the moment I don't see a reason for the rest of that
> patch.

Let's at least leave the mm_config() implementation but without the write-merge
enabling by default. Providing features availability macro
cpu_has_mm_sysad/cpu_has_mm_full and c0 config fields
MIPS_CPU_MM_SYSAD/MIPS_CPU_MM_FULL defined will be useful in the platform-specific
Write-Merge enable/disable procedure. For instance, in the my prom_init() method
I could use them to implement a code pattern like:

+	if (cpu_has_mm_full) {
+		unsigned int config0 = read_c0_config();
+               config0 = (config0 & ~MIPS_CONF_MM) | MIPS_CONF_MM_FULL;
+               write_c0_config(config0);
+	}

By doing so I can manually enable/disable the MM feature in the
cpu-feature-overrides.h. Without that I'd have to locally define these macro,
which isn't good seeing they are in fact generic and can be useful for other
platforms with SYSAD and FULL MM feature available. What do you think?

-Sergey

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

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