netdev - Re: [PATCH net-next 02/12] clk: sunxi-ng: r40: export a regmap to access the GMAC register

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Date:   Tue, 3 Apr 2018 17:58:05 +0800
From:   Chen-Yu Tsai <wens@...e.org>
To:     Icenowy Zheng <icenowy@...c.io>
Cc:     Maxime Ripard <maxime.ripard@...tlin.com>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Giuseppe Cavallaro <peppe.cavallaro@...com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Mark Brown <broonie@...nel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-clk <linux-clk@...r.kernel.org>,
        devicetree <devicetree@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        Corentin Labbe <clabbe.montjoie@...il.com>
Subject: Re: [PATCH net-next 02/12] clk: sunxi-ng: r40: export a regmap to
 access the GMAC register

On Tue, Apr 3, 2018 at 5:54 PM, Icenowy Zheng <icenowy@...c.io> wrote:
>
>
> 于 2018年4月3日 GMT+08:00 下午5:53:08, Chen-Yu Tsai <wens@...e.org> 写到:
>>On Tue, Apr 3, 2018 at 5:50 PM, Maxime Ripard
>><maxime.ripard@...tlin.com> wrote:
>>> On Tue, Apr 03, 2018 at 11:48:45AM +0200, Maxime Ripard wrote:
>>>> On Tue, Mar 20, 2018 at 03:15:02PM +0800, Chen-Yu Tsai wrote:
>>>> > On Mon, Mar 19, 2018 at 5:31 AM, Maxime Ripard
>>>> > <maxime.ripard@...tlin.com> wrote:
>>>> > > On Sat, Mar 17, 2018 at 05:28:47PM +0800, Chen-Yu Tsai wrote:
>>>> > >> From: Icenowy Zheng <icenowy@...c.io>
>>>> > >>
>>>> > >> There's a GMAC configuration register, which exists on
>>A64/A83T/H3/H5 in
>>>> > >> the syscon part, in the CCU of R40 SoC.
>>>> > >>
>>>> > >> Export a regmap of the CCU.
>>>> > >>
>>>> > >> Read access is not restricted to all registers, but only the
>>GMAC
>>>> > >> register is allowed to be written.
>>>> > >>
>>>> > >> Signed-off-by: Icenowy Zheng <icenowy@...c.io>
>>>> > >> Signed-off-by: Chen-Yu Tsai <wens@...e.org>
>>>> > >
>>>> > > Gah, this is crazy. I'm really starting to regret letting that
>>syscon
>>>> > > in in the first place...
>>>> >
>>>> > IMHO syscon is really a better fit. It's part of the glue layer
>>and
>>>> > most other dwmac user platforms treat it as such and use a syscon.
>>>> > Plus the controls encompass delays (phase), inverters (polarity),
>>>> > and even signal routing. It's not really just a group of clock
>>controls,
>>>> > like what we poorly modeled for A20/A31. I think that was really a
>>>> > mistake.
>>>> >
>>>> > As I mentioned in the cover letter, a slightly saner approach
>>would
>>>> > be to let drivers add custom syscon entries, which would then
>>require
>>>> > less custom plumbing.
>>>>
>>>> A syscon is convenient, sure, but it also bypasses any abstraction
>>>> layer we have everywhere else, which means that we'll have to
>>maintain
>>>> the register layout in each and every driver that uses it.
>>>>
>>>> So far, it's only be the GMAC, but it can also be others (the SRAM
>>>> controller comes to my mind), and then, if there's any difference in
>>>> the design in a future SoC, we'll have to maintain that in the GMAC
>>>> driver as well.
>>>
>>> I guess I forgot to say something, I'm fine with using a syscon we
>>> already have.
>>>
>>> I'm just questionning if merging any other driver using one is the
>>> right move.
>>
>>Right. So in this case, we are not actually going through the syscon
>>API. Rather we are exporting a regmap whose properties we actually
>>define. If it makes you more acceptable to it, we could map just
>>the GMAC register in the new regmap, and also have it named. This
>>is all plumbing within the kernel so the device tree stays the same.
>
> I think my driver has already restricted the write permission
> only to GMAC register.

Correct, but it still maps the entire region out, which means the
consumer needs to know which offset to use. Maxime is saying this
is something that is troublesome to maintain. So my proposal was
to create a regmap with a base at the GMAC register offset. That
way, the consumer doesn't need to use an offset to access it.

ChenYu