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SubjectRe: Disabled LocalPlus Controller (LPC) clock on MPC512x
On 26/11/2014 12:49, Alexander Popov wrote:
> Hello.
>
Hi.

> My Freescale TWR-MPC5125 board instantly reboots if I touch
> any physical address on the LocalPlus Bus (LPB) for the first time
> when Linux has already booted.
>
> This effect is reproduced by using /dev/mem or loading a kernel module
> which works with any peripherals on LPB.
>
> It took me some time to find out that such crash is caused by
> clk_disable_unused() in drivers/clk/clk.c, which disables
> LocalPlus Controller (LPC) clock if I don't touch LPB addresses in the
> previous initcalls. So starting Linux with clk_ignore_unused bootparam
> or inserting dummy LPB reading to some initcall is a temporary fix.
>
> Is it correct to gate LPC clock?
For me yes, because it's physically present on SoC and could not be muxed.

> If yes, how to avoid the mentioned
> crashes properly?
>
> There's a piece of code in arch/powerpc/platforms/512x/clock-commonclk.c
> which is doubtful for me:
>
> /*
> * pre-enable those "internal" clock items which never get
> * claimed by any peripheral driver, to not have the clock
> * subsystem disable them late at startup
> */
> clk_prepare_enable(clks[MPC512x_CLK_DUMMY]);
> clk_prepare_enable(clks[MPC512x_CLK_E300]); /* PowerPC CPU */
> clk_prepare_enable(clks[MPC512x_CLK_DDR]); /* DRAM */
> clk_prepare_enable(clks[MPC512x_CLK_MEM]); /* SRAM */
> clk_prepare_enable(clks[MPC512x_CLK_IPS]); /* SoC periph */
> clk_prepare_enable(clks[MPC512x_CLK_LPC]); /* boot media */
>
> Does it mean that these clocks should be registered with
> CLK_IGNORE_UNUSED flag?
>
Yes, in my opinion this groups of clocks may be considered as "always on",
but I don't think that MPC512x_CLK_LPC is an "internal" clock and then
It could be enable if really used only.
In detail:
- may be good to enable MPC512x_CLK_LPC only when localbus is enabled
by the dts
- if enabled, MPC512x_CLK_LPC have to setup with CLK_IGNORE_UNUSED
flag because never get claimed by any driver.

I put in CC "Gerhard Sittig" also beacuse it might be interesting to
know his point of view as the author of mpc512x common clock driver.

Regards,
Matteo

> Thanks a lot.
> Best regards,
> Alexander
> _______________________________________________
> Linuxppc-dev mailing list
> Linuxppc-dev@lists.ozlabs.org
> https://lists.ozlabs.org/listinfo/linuxppc-dev



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