Subject: Re: [PATCH net-next 14/15] net: dsa: mt7530: introduce driver for MT7988 built-in switch

From: Daniel Golle <[email-masked]>
Date: Fri, 31 Mar 2023 11:16:44 +0100

On Fri, Mar 31, 2023 at 08:50:28AM +0300, Arınç ÜNAL wrote:
> On 30.03.2023 18:23, Daniel Golle wrote:
> > Add driver for the built-in Gigabit Ethernet switch which can be found
> > in the MediaTek MT7988 SoC.
> > 
> > The switch shares most of its design with MT7530 and MT7531, but has
> > it's registers mapped into the SoCs register space rather than being
> > connected externally or internally via MDIO.
> > 
> > Introduce a new platform driver to support that.
> > 
> > Signed-off-by: Daniel Golle <[email-masked]>
> > ---
> >   MAINTAINERS                   |   2 +
> >   drivers/net/dsa/Kconfig       |  12 ++++
> >   drivers/net/dsa/Makefile      |   1 +
> >   drivers/net/dsa/mt7530-mmio.c | 101 ++++++++++++++++++++++++++++++++++
> >   drivers/net/dsa/mt7530.c      |  86 ++++++++++++++++++++++++++++-
> >   drivers/net/dsa/mt7530.h      |  12 ++--
> >   6 files changed, 206 insertions(+), 8 deletions(-)
> >   create mode 100644 drivers/net/dsa/mt7530-mmio.c
> > 
> > diff --git a/MAINTAINERS b/MAINTAINERS
> > index 14924aed15ca7..674673dbdfd8b 100644
> > --- a/MAINTAINERS
> > +++ b/MAINTAINERS
> > @@ -13174,9 +13174,11 @@ MEDIATEK SWITCH DRIVER
> >   M:	Sean Wang <[email-masked]>
> >   M:	Landen Chao <[email-masked]>
> >   M:	DENG Qingfang <[email-masked]>
> > +M:	Daniel Golle <[email-masked]>
> >   L:	[email-masked]
> >   S:	Maintained
> >   F:	drivers/net/dsa/mt7530-mdio.c
> > +F:	drivers/net/dsa/mt7530-mmio.c
> >   F:	drivers/net/dsa/mt7530.*
> >   F:	net/dsa/tag_mtk.c
> > diff --git a/drivers/net/dsa/Kconfig b/drivers/net/dsa/Kconfig
> > index c2551b13324c2..de4d86e37973f 100644
> > --- a/drivers/net/dsa/Kconfig
> > +++ b/drivers/net/dsa/Kconfig
> > @@ -52,6 +52,18 @@ config NET_DSA_MT7530
> >   	  Multi-chip module MT7530 in MT7621AT, MT7621DAT, MT7621ST and
> >   	  MT7623AI SoCs is supported as well.
> > +config NET_DSA_MT7988
> > +	tristate "MediaTek MT7988 built-in Ethernet switch support"
> > +	select NET_DSA_MT7530_COMMON
> > +	depends on HAS_IOMEM
> > +	help
> > +	  This enables support for the built-in Ethernet switch found
> > +	  in the MediaTek MT7988 SoC.
> > +	  The switch is a similar design as MT7531, however, unlike
> > +	  other MT7530 and MT7531 the switch registers are directly
> > +	  mapped into the SoCs register space rather than being accessible
> > +	  via MDIO.
> > +
> >   config NET_DSA_MV88E6060
> >   	tristate "Marvell 88E6060 ethernet switch chip support"
> >   	select NET_DSA_TAG_TRAILER
> > diff --git a/drivers/net/dsa/Makefile b/drivers/net/dsa/Makefile
> > index 71250d7dd41af..103a33e20de4b 100644
> > --- a/drivers/net/dsa/Makefile
> > +++ b/drivers/net/dsa/Makefile
> > @@ -8,6 +8,7 @@ endif
> >   obj-$(CONFIG_NET_DSA_LANTIQ_GSWIP) += lantiq_gswip.o
> >   obj-$(CONFIG_NET_DSA_MT7530_COMMON) += mt7530.o
> >   obj-$(CONFIG_NET_DSA_MT7530)	+= mt7530-mdio.o
> > +obj-$(CONFIG_NET_DSA_MT7988)	+= mt7530-mmio.o
> 
> I'm not fond of this way. Wouldn't it be better if we split the mdio and
> mmio drivers to separate modules and kept switch hardware support on
> mt7530.c?

You mean this in terms of Kconfig symbols?
Because the way you describe is basically what I'm doing here:
 * mt7530.c is the shared/common switch hardware driver
 * mt7530-mdio.c contains the MDIO accessors and MDIO device drivers for
   MT7530, MT7531, MT7621, MT7623, ...
 * mt7530-mmio.c contains the platform device driver for in-SoC switches
   which are accessed via MMIO, ie. MT7988 (and yes, this could be
   extended to also support MT7620A/N).

In early drafts I also named the Kconfig symbols
CONFIG_NET_DSA_MT7530 for mt7530.c (ie. the common part)
CONFIG_NET_DSA_MT7530_MDIO for the MDIO driver
CONFIG_NET_DSA_MT7530_MMIO for the MMIO driver

However, as existing kernel configurations expect CONFIG_NET_DSA_MT7530 to
select the MDIO driver, I decided it would be better to hide the symbol of
the common part and have CONFIG_NET_DSA_MT7530 select the MDIO driver like
it was before.

Hence I decided to go with
CONFIG_NET_DSA_MT7530 selects the MDIO driver, just like before
CONFIG_NET_DSA_MT7988 selects the new MMIO driver
CONFIG_NET_DSA_MT7530_COMMON is hidden, selected by both of the above

> 
> The mmio driver could be useful in the future for the MT7530 on the MT7620
> SoCs or generally new hardware that would use MMIO to be controlled.
> 

Sure, it would be a bit confusing once we add support for MT7620A/N (if
that ever happens...), then CONFIG_NET_DSA_MT7988 would need to be
selected to support this ancient MIPS SoC...

If you are planning to work on support for MT7620A/N feel free to suggest
a better way to name the Kconfig symbols.

> Luiz did this for the Realtek switches that use MDIO and SMI to be
> controlled.
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/tree/drivers/net/dsa/realtek/Kconfig
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/tree/drivers/net/dsa/realtek/Makefile

Are you suggesting to split-off a device-specific driver which would
then select the access-method driver (MDIO vs. MMIO) and the 
common/shared driver? To me this looks like overkill for MT7530, given
that the designs of all MT7530 are pretty similar, ie. same tag format
and also otherwise very similar.

Thank you for reviewing!

Daniel