linux-kernel - Re: [PATCH 0/6] soc: Add Baikal-T1 SoC APB/AXI EHB and L2-cache drivers

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Date:   Wed, 1 Apr 2020 21:10:20 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Sergey Semin <Sergey.Semin@...kalelectronics.ru>
Cc:     Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>,
        Maxim Kaurkin <Maxim.Kaurkin@...kalelectronics.ru>,
        Pavel Parkhomenko <Pavel.Parkhomenko@...kalelectronics.ru>,
        Ramil Zaripov <Ramil.Zaripov@...kalelectronics.ru>,
        Ekaterina Skachko <Ekaterina.Skachko@...kalelectronics.ru>,
        Vadim Vlasov <V.Vlasov@...kalelectronics.ru>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Paul Burton <paulburton@...nel.org>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Olof Johansson <olof@...om.net>, SoC Team <soc@...nel.org>,
        DTML <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/6] soc: Add Baikal-T1 SoC APB/AXI EHB and L2-cache drivers

On Wed, Apr 1, 2020 at 5:06 PM Sergey Semin
<Sergey.Semin@...kalelectronics.ru> wrote:
> On Fri, Mar 06, 2020 at 04:19:47PM +0100, Arnd Bergmann wrote:
> > On Fri, Mar 6, 2020 at 2:07 PM <Sergey.Semin@...kalelectronics.ru> wrote:
> > >
> > > From: Serge Semin <fancer.lancer@...il.com>
> > >
> > > Aside from PCIe/SATA/DDR/I2C/CPU-reboot specific settings the Baikal-T1
> > > system controller provides three vendor-specific blocks. In particular
> > > there are two Errors Handler Blocks to detect and report an info regarding
> > > any problems discovered on the AXI and APB buses. These are the main buses
> > > utilized by the SoC devices to interact with each other. In addition there
> > > is a way to tune the MIPS P5600 CM2 L2-cache up by setting the Tag/Data/WS
> > > L2-to-RAM latencies. All of this functionality is implemented in the
> > > APB/AXI EHB and L2-cache control block drivers to be a part of the kernel soc
> > > subsystem (as being specific to the Baikal-T1 SoC) and introduced in the
> > > framework of this patchset.
> > >
> > > This patchset is rebased and tested on the mainline Linux kernel 5.6-rc4:
> > > commit 98d54f81e36b ("Linux 5.6-rc4").
> >
> > I have no objection to the drivers, but I wonder if these should be
> > in drivers/bus and drivers/memory instead of drivers/soc, which have
> > similar drivers already. The driver for the L2 cache is not really a
> > memory controller driver, but it may be close enough, and we
> > already have a couple of different things in there.
> >

> Regarding l2 driver. Do you really think that L2 cache should be in
> drivers/memory? First there is no any cache-related drivers in that
> subsystem (at least I couldn't find any). Second the Baikal-T1
> L2-cache-RAM config block has just indirect connection with RAM.
> The block just tunes the L2-cache<->RAM stall clock cycles up on
> WS/Tag/Data RAM IO-operations. This config seems more SoC-specific,
> than memory-like. Do you think that the driver should still be in
> drivers/memory?

Either way could work, and both locations are a bit of a dumping
ground for different kinds of drivers. My preference would be
drivers/memory, but if anyone has a strong opinion the other way,
drivers/soc would be acceptable as well.

> On the other hand the block is part of the System Controller. I could
> just embed the l2-cache driver functionality into the System Controller
> MFD driver. Though honestly IMHO the functionality should live in
> a dedicated driver and drivers/soc is a better place for it. I also have
> doubts this part will be well accepted by Lee (drivers/mfd maintainer).
>
> So what do you think?

If you make it a combined driver with the system controller,
drivers/soc would be the most logical place. drivers/mfd should
only be used for a multiplexer with child drivers doing the
actual functionality.

        Arnd