Re: [PATCH] I2C: i.MX: early: Use internal udelay

[Sascha Hauer <sha@pengutronix.de>]

On Mon, Jan 30, 2023 at 09:42:53PM +1100, John Watts wrote:
> On Mon, Jan 30, 2023 at 11:27:27AM +0100, Sascha Hauer wrote:
> > This takes around 5 times too long on a i.MX8MM and around 50 times too
> > long on a i.MX6Q. This was measured under a regular barebox on the
> > shell. In an early environment with MMU disabled it takes 730 times too
> > long.
> 
> Yikes! I'm glad I broke this in to its own patch then. :)
> 
> I did copy this code from these boards:
> 
> arch/arm/boards/cm-fx6/lowlevel.c
> arch/arm/boards/skov-imx6/lowlevel.c
> arch/arm/boards/technexion-wandboard/lowlevel.c
> arch/arm/mach-imx/xload-gpmi-nand.c
> arch/arm/mach-imx/imx6-mmdc.c (uses 1000 loops)
> 
> Maybe it's time for an i.MX6-wide early udelay?

i.MX6 is not enough, the code could run on other i.MX SoCs as well.

> Though this code might break if its timing is already wrong.
> 
> > Maybe we could do this:
> > 
> > static void __udelay(void *base, int us)
> > {
> > 	int i;
> > 
> > 	for (i = 0; i < us * 4; i++)
> > 		readb(base);
> > }
> > 
> > The time spent for a register read depends on the bus clock which
> > doesn't change that much between the different SoCs.
> 
> This seems like a better solution if you've tested it, I'm not too sure
> how to check this. Would I just then specify the controller as the base?

Yes, the I2C controller base should be passed here.

I tested this with a call to this udelay function with a sufficiently
great number of microseconds surrounded by a printf.

> 
> Though the actual goal of the code is to wait for the controller to bestable.
> Maybe there's a better way than a delay?

I briefly looked into this, but the commit history of both barebox or
Linux do not give a real conclusion what we are waiting here for.

Sascha

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