Re: [PATCH 0/5] Add Mediatek MT8173 subsystem clocks support

Sascha Hauer
2015-06-08T07:49:15Z

On Fri, Jun 05, 2015 at 05:59:12PM -0700, Stephen Boyd wrote:
> On 06/05, James Liao wrote:
> > Hi Stephen,
> >
> > On Thu, 2015-06-04 at 14:02 -0700, Stephen Boyd wrote:
> > > On 05/29, Sascha Hauer wrote:
> > > > Yes. I previously got the impression that the subsystem clocks are not
> > > > directly associated to the larbs, but needed to be handled by the larb
> > > > code due to some side effect. Now that I saw that the larbs are directly
> > > > in the subsystem register space it all makes sense.
> > > >
> > > > Note that the way Mediatek SoCs are designed around sub modules is bit
> > > > unusual and does not fit very well in the Linux directory structure.
> > > > Normally SoCs have a single clocks controller which controls all clocks
> > > > in the SoC. Then you often have a reset controller providing reset lines
> > > > in the SoC. In this case it's clear that the clk driver goes to
> > > > drivers/clk/, the reset controller driver to drivers/reset/. Mediatek
> > > > SoCs instead have several blocks, each with its own clock and reset
> > > > controller. Splitting each block up into parts in drivers/clk/ and
> > > > drivers/reset/ leads to quite a code fragmentation.
> > > > This is my opinion, it would be great to hear something from others.
> > > > Matthias? I'd like to avoid running into a direction that is not
> > > > acceptable in the end.
> > >
> > > We already have drivers registering clocks and resets under
> > > drivers/clk, so it's not unheard of. An alternative solution is
> > > to make child devices for the clock part and the reset part at
> > > runtime in the toplevel driver for the vencsys device (don't do
> > > any sort of DT description for this) and use regmap to mediate
> > > the register accesses and locking. That way we can put the clk
> > > driver in drivers/clk/, the reset driver in drivers/reset, etc.
> > > so that logically related code is grouped.
> >
> > I have a question about the alternative way you mentioned. Currently
> > clock providers and consumers describe what clocks they will provide /
> > consume in device tree. If we don't describe vencsys clocks in device
> > tree, how to get vencsys clocks for drivers that need to control them?
> >
>
> Perhaps an example would be best. In DT we would have:
>
> vencsys: vencsys@10000 {
> compatible = "mtk,vencsys";
> reg = <0x10000 0x1000>;
> #clock-cells = <1>;
> #reset-cells = <1>;
> };
>
> myconsumer@12000 {
> compatible = "mtk,vencsys";
> reg = <0x12000 0x100>;
> clocks = <&vencsys 10>;
> clock-names = "core";
> };
>
> (Or are the consumers only children of the subsystem?
> It's not clear to me)
>
> And then in the mtk,vencsys driver we would create a platform
> device named something like "mtk-vencsys-clk" and assign the
> of_node of the device to be the of_node that is assigned to the
> mtk,vencsys device.
>
> static int vencsys_probe(struct platform_device *pdev)
> {
> int ret;
> struct device_node *np = pdev->dev.of_node;
> struct platform_device *clk_pdev;
>
> clk_pdev = platform_device_alloc("mtk-vencsys-clk", -1);
> clk_pdev->dev.of_node = of_node;
> ret = platform_device_add(clk_pdev);
> if (ret)
> return ret;
> }
>
> Then we could put a mtk-vencsys-clk driver in drivers/clk/ that
> does the clk driver part...
>
> static int clk_vencsys_probe(struct platform_device *pdev)
> {
> int ret;
> struct device_node *np = pdev->dev.of_node;
> struct regmap *regmap;
>
> ret = of_clk_add_provider(np, of_clk_src_onecell_get, ..);
> if (ret)
> return ret;
>
> regmap = dev_get_regmap(pdev->dev.parent, NULL);
>
> }
>
> And similar things could be done for the reset driver.

The problem I see with this approach is that we scatter the code for a
otherwise simple driver over a bunch of directories. We would have

drivers/clk/mediatek/vencsys.c
drivers/reset/mediatek/vencsys.c
drivers/soc/mediatek/vencsys.c

The same must be added for vdecsys, imgsys and vencltsys. That will make
12 drivers and three maintainers for 12 registers. I think this will be
a pain to maintain, hence my suggestion to put the vencsys code into a
single file and not split this up into more subsystem specific files.

Sascha

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