linux-kernel - Re: [RFC 3/3] CMDQ: Mediatek CMDQ driver

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Date:	Fri, 29 Jan 2016 20:24:22 +0800
From:	Horng-Shyang Liao <hs.liao@...iatek.com>
To:	Daniel Kurtz <djkurtz@...omium.org>
CC:	Rob Herring <robh+dt@...nel.org>,
	Matthias Brugger <matthias.bgg@...il.com>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	"open list:OPEN FIRMWARE AND..." <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"moderated list:ARM/Mediatek SoC support" 
	<linux-mediatek@...ts.infradead.org>,
	srv_heupstream <srv_heupstream@...iatek.com>,
	Sascha Hauer <kernel@...gutronix.de>,
	Philipp Zabel <p.zabel@...gutronix.de>,
	Nicolas Boichat <drinkcat@...omium.org>,
	CK HU <ck.hu@...iatek.com>,
	cawa cheng <cawa.cheng@...iatek.com>,
	Bibby Hsieh <bibby.hsieh@...iatek.com>,
	YT Shen <yt.shen@...iatek.com>,
	Daoyuan Huang <daoyuan.huang@...iatek.com>,
	"Damon Chu" <damon.chu@...iatek.com>,
	Josh-YC Liu <josh-yc.liu@...iatek.com>,
	"Glory Hung" <glory.hung@...iatek.com>
Subject: Re: [RFC 3/3] CMDQ: Mediatek CMDQ driver

On Fri, 2016-01-29 at 16:42 +0800, Daniel Kurtz wrote:
> On Fri, Jan 29, 2016 at 3:39 PM, Horng-Shyang Liao <hs.liao@...iatek.com> wrote:
> > Hi Dan,
> >
> > Many thanks for your comments and time.
> > I reply my plan inline.
> >
> >
> > On Thu, 2016-01-28 at 12:49 +0800, Daniel Kurtz wrote:
> >> Hi HS,
> >>
> >> Sorry for the delay.  It is hard to find time to review a >3700 line
> >> driver :-o in detail....
> >>
> >> Some review comments inline, although I still do not completely
> >> understand how all that this driver does and how it works.
> >> I'll try to find time to go through this driver in detail again next
> >> time you post it for review.
> >>
> >> On Tue, Jan 19, 2016 at 9:14 PM,  <hs.liao@...iatek.com> wrote:
> >> > From: HS Liao <hs.liao@...iatek.com>
> >> >
> >> > This patch is first version of Mediatek Command Queue(CMDQ) driver. The
> >> > CMDQ is used to help read/write registers with critical time limitation,
> >> > such as updating display configuration during the vblank. It controls
> >> > Global Command Engine (GCE) hardware to achieve this requirement.
> >> > Currently, CMDQ only supports display related hardwares, but we expect
> >> > it can be extended to other hardwares for future requirements.
> >> >
> >> > Signed-off-by: HS Liao <hs.liao@...iatek.com>
> >>
> >> [snip]
> >>
> >> > diff --git a/drivers/soc/mediatek/mtk-cmdq.c b/drivers/soc/mediatek/mtk-cmdq.c
> >> > new file mode 100644
> >> > index 0000000..7570f00
> >> > --- /dev/null
> >> > +++ b/drivers/soc/mediatek/mtk-cmdq.c

[snip]

> >> > +static const struct cmdq_subsys g_subsys[] = {
> >> > +       {0x1400, 1, "MMSYS"},
> >> > +       {0x1401, 2, "DISP"},
> >> > +       {0x1402, 3, "DISP"},
> >>
> >> This isn't going to scale.  These addresses could be different on
> >> different chips.
> >> Instead of a static table like this, we probably need specify to the
> >> connection between gce and other devices via devicetree phandles, and
> >> then use the phandles to lookup the corresponding device address
> >> range.
> >
> > I will define them in device tree.
> > E.g.
> > cmdq {
> >   reg_domain = 0x14000000, 0x14010000, 0x14020000
> > }
> 
> The devicetree should only model hardware relationships, not software
> considerations.
> 
> Is the hardware constraint here for using gce with various other
> hardware blocks?  I think we already model this by only providing a
> gce phandle in the device tree nodes for those devices that can use
> gce.
> 
> Looking at the driver closer, as far as I can tell, the whole subsys
> concept is a purely software abstraction, and only used to debug the
> CMDQ_CODE_WRITE command.  In fact, AFAICT, everything would work fine
> if we just completely removed the 'subsys' concept, and just passed
> through the raw address provided by the driver.
> 
> So, I recommend just removing 'subsys' completely from the driver -
> from this array, and in the masks.
> 
> Instead, if there is an error on the write command, just print the
> address that fails.  There are other ways to deduce the subsystem from
> a physical address.
> 
> Thanks,
> 
> -Dan

Hi Dan,

Subsys is not just for debug.
Its main purpose is to transfer CPU address to GCE address.
Let me explain it by "write" op,
I list a code segment from cmdq_rec_append_command().

	case CMDQ_CODE_WRITE:
		subsys = cmdq_subsys_from_phys_addr(cqctx, arg_a);
		if (subsys < 0) {
			dev_err(dev,
				"unsupported memory base address 0x%08x\n",
				arg_a);
			return -EFAULT;
		}

		*cmd_ptr++ = arg_b;
		*cmd_ptr++ = (CMDQ_CODE_WRITE << CMDQ_OP_CODE_SHIFT) |
			     (arg_a & CMDQ_ARG_A_WRITE_MASK) |
			     ((subsys & CMDQ_SUBSYS_MASK) << CMDQ_SUBSYS_SHIFT);
		break;

Subsys is mapped from physical address via cmdq_subsys_from_phys_addr(),
and then it becomes part of GCE command via ((subsys & CMDQ_SUBSYS_MASK)
<< CMDQ_SUBSYS_SHIFT) .
Only low bits of physical address are the same as GCE address.
We can get it by (arg_a & CMDQ_ARG_A_WRITE_MASK).
MASK is used to define how many bits are valid for this op.
So, GCE address = subsys + valid low bits.

That's why we need to know the mapping between the range of physical
address and subsys.
Please guide us a better way to code such requirement.
Thanks for your help.

Thanks,
HS Liao