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From: David Gibson <david@gibson.dropbear.id.au>
To: Peter Maydell <peter.maydell@linaro.org>
Cc: Alistair Francis <Alistair.Francis@wdc.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	QEMU Developers <qemu-devel@nongnu.org>,
	"open list:RISC-V" <qemu-riscv@nongnu.org>
Subject: Re: [PATCH v2 1/3] device_tree: Add a helper function for string arrays
Date: Sun, 10 Nov 2019 19:44:24 +0000
Message-ID: <20191110194424.GR2461@umbus.Home> (raw)
In-Reply-To: <CAFEAcA_3jgn1WrX2wpBd+sCYe=nNaqA2POvaZkj-NsSyT2N9bQ@mail.gmail.com>

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On Sat, Nov 09, 2019 at 03:59:24PM +0000, Peter Maydell wrote:
> On Fri, 8 Nov 2019 at 19:48, Palmer Dabbelt <palmer@dabbelt.com> wrote:
> >
> > The device tree format allows for arrays of strings, which are encoded
> > with '\0's inside regular strings.  These are ugly to represent in C, so
> > the helper function represents them as strings with internal '\0's that
> > are terminated with a double '\0'.  In other words, the array
> > ["string1", "string2"] is represeted as "string1\0string2\0".
> >
> > The DTB generated by this function is accepted by DTC and produces an
> > array of strings, but I can't find any explicit line in the DT
> > specification that defines how these are encoded.
> 
> > +/*
> > + * This uses a particularly odd encoding: "strings" is a list of strings that
> > + * must be terminated by two back-to-back '\0' characters.
> > + */
> > +int qemu_fdt_setprop_strings(void *fdt, const char *node_path,
> > +                             const char *property, const char *strings);
> 
> The clean API for this would be to use varargs so you could write
> 
> qemu_fdt_setprop_stringlist(fdt, node, prop, "arm,armv8-timer",
>                             "arm,armv7-timer");
> 
> and have it do the assembly into the encoding that fdt expects.
> That would require us to do a bit of allocation-and-freeing
> to assemble the string, of course, but then we only do fdt
> creation at startup.

Right, I really don't see the value in this interface.  Using
"foo\0bar" is a little ugly, but not really any uglier than
"foo\0bar\0".  The existing interface would be a drag if you had
dynamically created entries in the list (because getting the size
can't be done with sizeof() then), but I don't think that's actually a
very likely usecase.

> NB: I think that this is a good idea but not-for-4.2 material,
> so if you wanted your sifive board change to go into 4.2 you
> should probably start with the simple approach and leave the
> refactoring for the next release cycle.

I concur.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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  reply index

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-08 19:47 [PATCH v2 0/3] device_tree: Allow for and use string arrays [Was: RISC-V: virt: This is a "sifive, test1" test finisher] Palmer Dabbelt
2019-11-08 19:47 ` [PATCH v2 1/3] device_tree: Add a helper function for string arrays Palmer Dabbelt
2019-11-09 15:59   ` Peter Maydell
2019-11-10 19:44     ` David Gibson [this message]
2019-11-08 19:47 ` [PATCH v2 2/3] ARM/virt: Use fdt_setprop_strings() Palmer Dabbelt
2019-11-09 15:56   ` Peter Maydell
2019-11-10 19:47     ` David Gibson
2019-11-08 19:47 ` [PATCH v2 3/3] RISC-V: virt: This is a "sifive,test1" test finisher Palmer Dabbelt

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