linux-kernel - Re: [PATCH v15 06/16] of/fdt: add helper functions for handling properties

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Date:   Tue, 9 Oct 2018 10:47:15 -0700
From:   Frank Rowand <frowand.list@...il.com>
To:     "AKASHI, Takahiro" <takahiro.akashi@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        David Gibson <david@...son.dropbear.id.au>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        David Howells <dhowells@...hat.com>,
        Vivek Goyal <vgoyal@...hat.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        David Miller <davem@...emloft.net>, dyoung@...hat.com,
        Baoquan He <bhe@...hat.com>, Arnd Bergmann <arnd@...db.de>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        prudo@...ux.ibm.com, Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        James Morse <james.morse@....com>, bhsharma@...hat.com,
        kexec@...ts.infradead.org,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        devicetree@...r.kernel.org, Frank Rowand <frowand.list@...il.com>
Subject: Re: [PATCH v15 06/16] of/fdt: add helper functions for handling
 properties

On 10/08/18 17:37, AKASHI, Takahiro wrote:
> On Fri, Oct 05, 2018 at 08:23:57AM -0500, Rob Herring wrote:
>> On Thu, Oct 4, 2018 at 10:07 PM AKASHI, Takahiro
>> <takahiro.akashi@...aro.org> wrote:
>>>
>>> Rob,
>>>
>>> # I haven't replied to this comment yet.
>>>
>>> On Fri, Sep 28, 2018 at 08:44:42AM -0500, Rob Herring wrote:
>>>> +David Gibson
>>>>
>>>> On Fri, Sep 28, 2018 at 1:48 AM AKASHI Takahiro
>>>> <takahiro.akashi@...aro.org> wrote:
>>>>>
>>>>> These functions will be used later to handle kexec-specific properties
>>>>> in arm64's kexec_file implementation.
>>>>>
>>>>> Signed-off-by: AKASHI Takahiro <takahiro.akashi@...aro.org>
>>>>> Cc: Rob Herring <robh+dt@...nel.org>
>>>>> Cc: Frank Rowand <frowand.list@...il.com>
>>>>> Cc: devicetree@...r.kernel.org
>>>>> ---
>>>>>  drivers/of/fdt.c       | 56 ++++++++++++++++++++++++++++++++++++++++++
>>>>>  include/linux/of_fdt.h |  4 +++
>>>>>  2 files changed, 60 insertions(+)
>>>>>
>>>>> diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c
>>>>> index 800ad252cf9c..c65c31562ccb 100644
>>>>> --- a/drivers/of/fdt.c
>>>>> +++ b/drivers/of/fdt.c
>>>>> @@ -25,6 +25,7 @@
>>>>>  #include <linux/debugfs.h>
>>>>>  #include <linux/serial_core.h>
>>>>>  #include <linux/sysfs.h>
>>>>> +#include <linux/types.h>
>>>>>
>>>>>  #include <asm/setup.h>  /* for COMMAND_LINE_SIZE */
>>>>>  #include <asm/page.h>
>>>>> @@ -1323,3 +1324,58 @@ late_initcall(of_fdt_raw_init);
>>>>>  #endif
>>>>>
>>>>>  #endif /* CONFIG_OF_EARLY_FLATTREE */
>>>>> +
>>>>> +#define FDT_ALIGN(x, a)        (((x) + (a) - 1) & ~((a) - 1))
>>>>> +#define FDT_TAGALIGN(x)        (FDT_ALIGN((x), FDT_TAGSIZE))
>>>>> +
>>>>> +int fdt_prop_len(const char *prop_name, int len)
>>>>> +{
>>>>> +       return (strlen(prop_name) + 1) +
>>>>> +               sizeof(struct fdt_property) +
>>>>> +               FDT_TAGALIGN(len);
>>>>
>>>> Looks like you are using this to calculate how much space you need to
>>>> allocate in addition to the current DTB for a couple of new or
>>>> replaced properties. I'm not sure that this calculation is completely
>>>> accurate. And it is strange there doesn't seem to be any libfdt
>>>> function for this already. It would be simpler to just add some fixed
>>>> additional amount.
>>>>
>>>> Maybe David G has comments on this?
>>>>
>>>>> +}
>>>>> +
>>>>
>>>> The rest of this should go in drivers/of/fdt_address.c. Ultimately, it
>>>> should go into libfdt, but I'm fine with having it in the kernel for
>>>> now.
>>>
>>> I'd like to have this function in the kernel for now.
>>>
>>>>> +static void fill_property(void *buf, u64 val64, int cells)
>>>>> +{
>>>>> +       __be32 val32;
>>>>> +
>>>>> +       while (cells) {
>>>>> +               val32 = cpu_to_fdt32((val64 >> (32 * (--cells))) & U32_MAX);
>>>>> +               memcpy(buf, &val32, sizeof(val32));
>>>>> +               buf += sizeof(val32);
>>>>
>>>> This is kind of hard to read. I would copy u-boot's fdt_pack_reg function.
>>>
>>> Are you sure?
>>> I originally implemented this function in a similar way that fdt_pack_reg()
>>> was, but, you suggested, in your past comment[1], that we'd be better to
>>> have of_read_number()-like implementation.
>>>
>>> [1] http://lists.infradead.org/pipermail/linux-arm-kernel/2018-May/579118.html
>>
>> Yeah, you're right. Plus, I'm not sure the u-boot one would work for
>> unaligned accesses with armv5 and earlier h/w.
>>
>> My only comment then is I think you can drop the U32_MAX masking.
> 
> Okay, then I will leave this function, yet renaming it to
> cpu64_to_fdt_cells() after Frank's comment.

I have second guessed myself and do not like the name I suggested
because what the function really does is either cpu32 to be32 or
cpu64 to be64.

I agree with Rob that readability is important here.  Instead of
having a fill_property() function, how about having inline code,
something like (untested even for thinkos):

	prop = buf;

	if (addr_cells == 1) {
		*(__be32 *)prop = cpu32_to_be32(addr);
		prop += 4;
	} else {
		*(__be64 *)prop = cpu64_to_be64(addr);
		prop += 8;
	}

	if (size_cells == 1)
		*(__be32 *)prop = cpu32_to_be32(size);
	else
		*(__be64 *)prop = cpu64_to_be64(size);

You might want to also give Rob a chance to bike shed on this
suggestion.

-Frank

> 
> Thanks,
> -Takahiro Akashi
>>
>> Rob
>