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Date: Sat, 27 Oct 2012 06:12:00 +0200
From: "Edgar E. Iglesias" <edgar.iglesias@gmail.com>
Message-ID: <20121027041159.GA20153@zapo>
References: <CAEgOgz4q1oA=W2yktQ0SgZ_3WTjT9DZaDqB3r-0nfod0m1QX8Q@mail.gmail.com>
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Subject: Re: [Qemu-devel] [RFC] Memory API and fine grained Memory Regions
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To: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Cc: Peter Maydell <peter.maydell@linaro.org>, John Williams <john.williams@xilinx.com>, "qemu-devel@nongnu.org Developers" <qemu-devel@nongnu.org>, Avi Kivity <avi@redhat.com>

On Fri, Oct 26, 2012 at 05:03:04PM +1000, Peter Crosthwaite wrote:
> In my recent USB series Avi mentioned he wanted to do some work with
> the memory API and encourage devices to use the memory API to do
> fine-grained register decoding, i.e. each register is its own
> MemoryRegion. This has the advantage of getting rid of the symmetric
> switch statements in the read and write handlers. The big drawback I
> am seeing is however is indexing into the register file. For example
> in a device i'm working on ATM I have this read handler for my device
> registers:
> 
> static uint64_t devcfg_read (void *opaque, hwaddr addr,
>         unsigned size)
> {
>     struct XilinxDevcfg *s = opaque;
>     uint32_t ret;
> 
>     addr >>= 2;
>     switch (addr)
>     {
>         //TODO: implement any read side effects
>     }
>     ret = s->regs[addr];
>     DB_PRINT("addr=" TARGET_FMT_plx " = %x\n", addr * 4, ret);
>     return ret;
> }


Hi,

If well designed, most hw has a well designed symtery between regs
that makes things simpler if you can just opencode the decding.
IMO, an mr per reg would just add a massive overhead for no win.
And also, hw implemented decoders look very much like
switch cases in qemu which make things easy to map.

What is the win?

Cheers,
Edgar