Date:	Wed, 24 Jul 2013 18:54:49 +0900
From:	Magnus Damm <magnus.damm@...il.com>
To:	Guennadi Liakhovetski <g.liakhovetski@....de>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	Simon Horman <horms@...ge.net.au>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Vinod Koul <vinod.koul@...el.com>,
	SH-Linux <linux-sh@...r.kernel.org>,
	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
Subject: Re: [PATCH v4 02/15] DMA: shdma: add r8a7740 DMAC data to the device
 ID table

Hi Guennadi,

On Wed, Jul 24, 2013 at 5:33 PM, Guennadi Liakhovetski
<g.liakhovetski@....de> wrote:
> On Wed, 24 Jul 2013, Magnus Damm wrote:
>
>> Hi Guennadi,
>>
>> On Wed, Jul 24, 2013 at 6:19 AM, Guennadi Liakhovetski
>> <g.liakhovetski@....de> wrote:
>> > On Wed, 24 Jul 2013, Magnus Damm wrote:
>> >
>> >> Hi Guennadi,
>> >>
>> >> Thanks for your efforts on this.
>> >>
>> >> On Tue, Jul 23, 2013 at 7:49 PM, Guennadi Liakhovetski
>> >> <g.liakhovetski@....de> wrote:
>> >> > This configuration data will be re-used, when DMAC DT support is added to
>> >> > r8a7740, DMAC platform data in setup-r8a7740.c will be removed.
>> >> >
>> >> > Signed-off-by: Guennadi Liakhovetski <g.liakhovetski+renesas@...il.com>
>> >> > ---
>> >> >
>> >> > v4: make struct sh_dmae_pdata r8a7740_dma_pdata "const"
>> >> >
>> >>
>> >> [snip]
>> >>
>> >> > --- /dev/null
>> >> > +++ b/drivers/dma/sh/shdma-r8a7740.c
>> >> > @@ -0,0 +1,95 @@
>> >> > +#include <linux/sh_dma.h>
>> >> > +
>> >> > +#include <mach/dma-register.h>
>> >> > +#include <mach/r8a7740.h>
>> >>
>> >> Including stuff from <mach/..> isn't really compatible with
>> >> MULTIPLATFORM,
>> >
>> > Hmm, right. I modeled this arch-specific driver code after Laurent's
>> > pinctrl driver revamp, which also includes <mach/*.h> headers. So, we'll
>> > have to think how to fix both.
>>
>> I mentioned this to Laurent when he started converting to PINCTRL, and
>> I believe the only remaining bits are the static GPIO-to-IRQ tables.
>>
>> >> so please don't write new code like this. Actually we
>> >> don't want any code under drivers/ to include stuff from the mach
>> >> directory.
>> >
>> > Sure, understood.
>>
>> Good.
>>
>> >> I suggest that you arrange your code in a way so the C version of DMAC
>> >> support has tables with slave ids as usual under
>> >> arch/arm/mach-shmobile/, but the DT bits that operate independently of
>> >> C stay in drivers/... Over time we will get rid of the C version, and
>> >> until that happens the DT and C version can coexist in parallel.
>> >
>> > That's already how it is. Data, that I took to drivers/dma/sh/ is needed
>> > for both DT and C. DMA stuff, needed only for C are only DMAC devices and
>> > resources. I think, I might be able to carry those DMA specific headers
>> > and defines over from mach/ to drivers/dma/sh. Maybe it would be easier to
>> > do this in several steps:
>> >
>> > 1. add my drivers/dma/sh/shdma-<arch>.c files *with* mach/ headers
>> > 2. switch arches over to those files
>> > (the above two steps are already done in my patch series)
>> > 3. move headers to drivers/dma/sh
>> >
>> > Ok, alternatively, I might be able to do (1) above without using mach/
>> > headers at all by directly copying them to drivers/dma/sh/ and then
>> > removing the original mach/headers in step (2)? I'll look in more detail
>> > at the code tomorrow.
>>
>> Thanks. My apologies for reviewing your code late in the cycle, but
>> I've now looked through this series and the following questions popped
>> up:
>>
>> 1) How will it look like in DT when a DMA Engine slave device will use DMA?
>
> You have already seen them by now, since you replied to that thread too,
> but just for reference, e.g. for an MMCIF controller here
> http://thread.gmane.org/gmane.linux.ports.sh.devel/25445/focus=25442
>
> +       dmas = <&dmac 0xd1
> +               &dmac 0xd2>;
> +       dma-names = "tx", "rx";

Yes, thanks for the pointer.

>> 2) Isn't it possible to leave the SHDMA_SLAVE_xx bits to only be used
>> by legacy C SoC and board code in arch/arm/mach-shmobile? I don't
>> understand why you have to move them over to drivers/... I just assume
>> these SHDMA_SLAVE bits won't be used by 1) above.
>
> That's right, those macros aren't used by (1) above, i.e. they aren't used
> in DT. But they are used pretty intensively internally by the driver. The
> C code might be "legacy," but as far as I understand, SuperH will never go
> to DT, so, as long as it has to be supported, we need to support that
> mode. Same for ARM - IIUC, board-*.c (not -reference) files are also still
> quite intensively developed and supported. So, it doesn't look like the C
> version will go any time soon, right?

In general the driver will have to keep supporting platform device
bindings for board and SoC code written in C, yes.

But for newer SoCs I imagine we will only write DT support. And the
idea is to move over the ARM mach-shmobile board and SoC code to DT.
So only SH will be left in the end unless it is migrated. So I agree
we should support SH of course, but there is no point in shuffling
around all the headers just to optimize for the legacy case.

> Based on that I tried to keep the difference between the C and the DT
> versions as small as possible. And that includes keeping slave IDs at
> least internally without changes.

I understand. Thanks for explaining.

> Of course, we can think about changing the driver more extensively and
> leaving those IDs only for the C case, but in fact they don't seem so
> horrifying to me. We have 2 options to keep them while eliminating the use
> of the <mach/<soc>.h> header:
> (a) redefine them in the driver
> (b) define a single list of slave IDs for all SoCs, AFAICS they don't have
> to be contiguous

Seems like a lot of churn but I can't see the benefit. Can you explain it to me?

Say that we only use DT for future devices. Then why do we want to
expose bits that no one will use in a header file? The legacy case is
already working. Reworking that code for this purpose seems backwards
to me. The legacy case is already working. Fix the DT case. =)

In the case of "struct sh_dmae_slave_config", above you mention that
the enum is used internally, but can't you simply use the index like
this?

Current style:

static const struct sh_dmae_slave_config sh73a0_dmae_slaves[] = {
    {
        .slave_id    = SHDMA_SLAVE_SCIF0_TX,
        .addr        = 0xe6c40020,
        .chcr        = CHCR_TX(XMIT_SZ_8BIT),
        .mid_rid    = 0x21,
    },

New proposed style in case enum is used:

static const struct sh_dmae_slave_config sh73a0_dmae_slaves[] = {
    [SHDMA_SLAVE_SCIF0_TX] = {
        .addr        = 0xe6c40020,
        .chcr        = CHCR_TX(XMIT_SZ_8BIT),
        .mid_rid    = 0x21,
    },

New proposed style for DT when enum is omitted:

static const struct sh_dmae_slave_config sh73a0_dmae_slaves[] = {
    {
        .addr        = 0xe6c40020,
        .chcr        = CHCR_TX(XMIT_SZ_8BIT),
        .mid_rid    = 0x21,
    },

In the driver, when parsing "struct sh_dmae_slave_config", what's
stopping you from simply using the index? Wouldn't the above allow you
to drop the enums in the DT case?

>> 3) How difficult would it be to describe the information in "struct
>> sh_dmae_slave_config" using DT?
>
> Just as difficult as anything else in DT, I presume. Technically it's not
> very difficult, but we'll have to go through all the rounds to define
> proper bindings and once defined, they'll have to be kept, as you know.

They have to be kept for that particular "compatible" string. But a
new SoC with a new compatible string can use a new format, no?

> And if in the future we need to change that information we'll have a
> problem. E.g., we could define that array as an array of tuples like
>
>         slaves = <chcr0 midrid0>,
>                 <chcr1 midrid1>,
>                 ...;

That looks quite close to my C proposal above, no? =)

> but that would mean we'll never be able to add a third element to them.

I don't think that's correct. You can add a third element in case of a
new compatible string, no?

> Whereas if kept in C as now, the full flexibility is preserved. So, I
> would really prefer to only push into DT things, for which standard
> bindings are defined, or those, which we absolutely need there.
> Information, that is SoC specific and not standard I'd rather keep in C.

Ok. I think it makes sense to use C in the case we support both DT and C.

>> 4) It seems that some patches in this series are unrelated. Can you
>> submit 4/15 and 9/15 from v4 independently somehow?
>
> 4/15 is required to avoid a compiler warning. 9/15 can be submitted
> separately, but it depends on this patch-series, so, it would be easier to
> keep them all together.

Oops, I was supposed to write 4/15 and 8/15, not 9/15. I suppose you
still want to include them in your series.

>> 5) Reducing and extending (reducing is optional of course)
>>
>> At this point you cover r8a7740, r8a73a0 and r8a73a4. I'm not sure why
>> your picked those 3 SoCs,
>
> That's simple: because I can test them.

Ok, I see. I think some of those have a USBHS-DMAC. I saw that one was
omitted. If the USB drivers bits would support DT then can you support
USBHS-DMAC with these patches?

>> but perhaps it would be a good idea to
>> select a single SoC to begin with but extend the support to also
>> provide DT code that makes use of DMA Engine in slave devices like for
>> instance MMCIF (this would cover question 1) above).
>
> I did, as you know by now.

Yes, in separate series. In next patch series version, would it be
possible to keep these things together?

>> When we have
>> agreed on the big picture then additional SoCs can easily be added
>> later.
>>
>> 6) Remove untested bits
>>
>> And while doing this conversion, perhaps this is a good opportunity to
>> only move over DMA Engine slaves that can be tested? Having tons of
>> unused DMA configuration seems overly heavy to me. If it's not tested
>> then it's broken.
>
> Tested in general or tested by me? I am sure other developers, that added
> support for various interfaces like audio would be better able to test
> them than me. And since I'm actually migrating platforms to this in-driver
> code, dropping any slaves would actually be a regression.

It's not a regression if no one is using it. Having tons of untested
code doesn't make any sense. I'm fine with you keeping your current
code as-is, but when you add DT support then please try to make it
more streamlined and only add bits that you can actually test
yourself.

> If really
> wanted, that would have to go via the standard deprecation process, right?
> E.g., I can well imagine that noone is actually using DMA on SCIF* serial
> interfaces on sh73a0, but removing them should probably be done as a
> separate patch-set.

So you are thinking that you will move over the old C legacy bits into
drivers/. I'm not so keen on that since it will result in a lot of
churn.

I am thinking that you can keep the legacy C bits as-is (perhaps
rework slightly) at the same location, but for the DT reference
support you can add a subset of the code to drivers/. At this time I
want you to only add code that will be used. Then over time we will
get rid of the legacy C bits either by moving to DT or phasing out
software support.

What do you think?

Thanks,

/ magnus
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