netdev - Re: [PATCHv2 net-next 10/16] net: mvpp2: handle register mapping and access for PPv2.2

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Date:   Fri, 6 Jan 2017 14:46:48 +0000
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>
Cc:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        devicetree@...r.kernel.org, Rob Herring <robh+dt@...nel.org>,
        Ian Campbell <ijc+devicetree@...lion.org.uk>,
        Pawel Moll <pawel.moll@....com>,
        Mark Rutland <mark.rutland@....com>,
        Kumar Gala <galak@...eaurora.org>,
        Andrew Lunn <andrew@...n.ch>,
        Yehuda Yitschak <yehuday@...vell.com>,
        Jason Cooper <jason@...edaemon.net>,
        Hanna Hawa <hannah@...vell.com>,
        Nadav Haklai <nadavh@...vell.com>,
        Gregory Clement <gregory.clement@...e-electrons.com>,
        Stefan Chulski <stefanc@...vell.com>,
        Marcin Wojtas <mw@...ihalf.com>,
        linux-arm-kernel@...ts.infradead.org,
        Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>
Subject: Re: [PATCHv2 net-next 10/16] net: mvpp2: handle register mapping and
 access for PPv2.2

On Wed, Dec 28, 2016 at 05:46:26PM +0100, Thomas Petazzoni wrote:
> This commit adjusts the mvpp2 driver register mapping and access logic
> to support PPv2.2, to handle a number of differences.
> 
> Due to how the registers are laid out in memory, the Device Tree binding
> for the "reg" property is different:
> 
>  - On PPv2.1, we had a first area for the common registers, and then one
>    area per port.
> 
>  - On PPv2.2, we have a first area for the common registers, and a
>    second area for all the per-ports registers.
> 
> In addition, on PPv2.2, the area for the common registers is split into
> so-called "address spaces" of 64 KB each. They allow to access the same
> registers, but from different CPUs. Hence the introduction of cpu_base[]
> in 'struct mvpp2', and the modification of the mvpp2_write() and
> mvpp2_read() register accessors. For PPv2.1, the compatibility is
> preserved by using an "address space" size of 0.

I'm not entirely sure this is the best solution - every register access
will be wrapped with a preempt_disable() and preempt_enable().  At
every site, when preempt is enabled, we will end up with code to:

- get the thread info
- increment the preempt count
- access the register
- decrement the preempt count
- test resulting preempt count and branch to __preempt_schedule()

If tracing is enabled, it gets much worse, because the increment and
decrement happen out of line, and are even more expensive.

If a function is going to make several register accesses, it's going
to be much more efficient to do:

	void __iomem *base = priv->cpu_base[get_cpu()];

	...

	put_cpu();

which means we don't end up with multiple instances of the preempt code
consecutive accesses.

I think this is an example where having driver-private accessors for
readl()/writel() is far from a good idea.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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