Subject: Re: [PATCH] ARM: tegra: Fix restoration of PLLM when exiting suspend

From: Thierry Reding <[email-masked]>
Date: Tue, 10 Dec 2019 13:09:09 +0100

On Tue, Dec 10, 2019 at 10:37:08AM +0000, Jon Hunter wrote:
> The suspend entry and exit code for 32-bit Tegra devices assumes that
> the PLLM (which is used to provide the clock for external memory)
> is always enabled on entry to suspend. Hence, the current code always
> disables the PLLM on entry to suspend and re-enables the PLLM on exit
> from suspend.
> 
> Since the introduction of the Tegra124 EMC driver by commit 73a7f0a90641
> ("memory: tegra: Add EMC (external memory controller) driver"), which is
> used to scale the EMC frequency, PLLM may not be the current clock
> source for the EMC on entry to suspend and hence may not be enabled.
> Always enabling the PLLM on exit from suspend can cause the actual
> status on the PLL to be different from that reported by the common clock
> framework.
> 
> On kernels prior to v4.5, the code to set the rate of the PLLM had a
> test to verify if the PLL was enabled and if the PLL was enabled,
> setting the rate would fail. Since commit 267b62a96951
> ("clk: tegra: pll: Update PLLM handling") the test to see if PLLM is
> enabled was removed.
> 
> With these earlier kernels, if the PLLM is disabled on entering suspend
> and the EMC driver attempts to set the parent of the EMC clock to the
> PLLM on exiting suspend, then the set rate for the PLLM will fail and in
> turn cause the resume to fail.
> 
> We should not be re-enabling the PLLM on resume from suspend unless it
> was enabled on entry to suspend. Therefore, fix this by saving the state
> of PLLM on entry to suspend and only re-enable it, if it was already
> enabled.
> 
> Fixes: 73a7f0a90641 ("memory: tegra: Add EMC (external memory controller) driver")
> Cc: [email-masked]
> 
> Signed-off-by: Jon Hunter <[email-masked]>
> ---
>  arch/arm/mach-tegra/sleep-tegra30.S | 33 +++++++++++++++++++++++------
>  1 file changed, 27 insertions(+), 6 deletions(-)

Looks good to me. If I understand correctly we really only need this on
v4.4 and earlier because the issue doesn't happen on later kernels
because of that PLLM handling update change that you mentioned, right?

At the same time, this is the correct thing to do even on more recent
kernels because we currently rely on the PLLM status check being absent
for this to work.

So it seems like the safest option going forward is to apply this patch
to all versions, so that we don't rely on any assumptions.

Do you agree?

Thierry

> diff --git a/arch/arm/mach-tegra/sleep-tegra30.S b/arch/arm/mach-tegra/sleep-tegra30.S
> index 3341a12bbb9c..c2f0793a424f 100644
> --- a/arch/arm/mach-tegra/sleep-tegra30.S
> +++ b/arch/arm/mach-tegra/sleep-tegra30.S
> @@ -337,26 +337,42 @@ ENTRY(tegra30_lp1_reset)
>  	add	r1, r1, #2
>  	wait_until r1, r7, r3
>  
> -	/* enable PLLM via PMC */
> +	/* restore PLLM state */
>  	mov32	r2, TEGRA_PMC_BASE
> +	adr	r7, tegra_pllm_status
> +	ldr	r1, [r7]
> +	cmp	r2, #(1 << 12)
> +	bne	_skip_pllm
> +
>  	ldr	r1, [r2, #PMC_PLLP_WB0_OVERRIDE]
>  	orr	r1, r1, #(1 << 12)
>  	str	r1, [r2, #PMC_PLLP_WB0_OVERRIDE]
>  
>  	pll_enable r1, r0, CLK_RESET_PLLM_BASE, 0
> +	pll_locked r1, r0, CLK_RESET_PLLM_BASE
> +
> +_skip_pllm:
>  	pll_enable r1, r0, CLK_RESET_PLLC_BASE, 0
>  	pll_enable r1, r0, CLK_RESET_PLLX_BASE, 0
>  
>  	b	_pll_m_c_x_done
>  
>  _no_pll_iddq_exit:
> -	/* enable PLLM via PMC */
> +	/* restore PLLM state */
>  	mov32	r2, TEGRA_PMC_BASE
> +	adr	r7, tegra_pllm_status
> +	ldr	r1, [r7]
> +	cmp	r2, #(1 << 12)
> +	bne	_skip_pllm_no_iddq
> +
>  	ldr	r1, [r2, #PMC_PLLP_WB0_OVERRIDE]
>  	orr	r1, r1, #(1 << 12)
>  	str	r1, [r2, #PMC_PLLP_WB0_OVERRIDE]
>  
>  	pll_enable r1, r0, CLK_RESET_PLLM_BASE, CLK_RESET_PLLM_MISC
> +	pll_locked r1, r0, CLK_RESET_PLLM_BASE
> +
> +_skip_pllm_no_iddq:
>  	pll_enable r1, r0, CLK_RESET_PLLC_BASE, CLK_RESET_PLLC_MISC
>  	pll_enable r1, r0, CLK_RESET_PLLX_BASE, CLK_RESET_PLLX_MISC
>  
> @@ -364,7 +380,6 @@ _pll_m_c_x_done:
>  	pll_enable r1, r0, CLK_RESET_PLLP_BASE, CLK_RESET_PLLP_MISC
>  	pll_enable r1, r0, CLK_RESET_PLLA_BASE, CLK_RESET_PLLA_MISC
>  
> -	pll_locked r1, r0, CLK_RESET_PLLM_BASE
>  	pll_locked r1, r0, CLK_RESET_PLLP_BASE
>  	pll_locked r1, r0, CLK_RESET_PLLA_BASE
>  	pll_locked r1, r0, CLK_RESET_PLLC_BASE
> @@ -526,6 +541,8 @@ __no_dual_emc_chanl:
>  ENDPROC(tegra30_lp1_reset)
>  
>  	.align	L1_CACHE_SHIFT
> +tegra_pllm_status:
> +	.word	0
>  tegra30_sdram_pad_address:
>  	.word	TEGRA_EMC_BASE + EMC_CFG				@0x0
>  	.word	TEGRA_EMC_BASE + EMC_ZCAL_INTERVAL			@0x4
> @@ -624,10 +641,14 @@ tegra30_switch_cpu_to_clk32k:
>  	add	r1, r1, #2
>  	wait_until r1, r7, r9
>  
> -	/* disable PLLM via PMC in LP1 */
> +	/* disable PLLM, if enabled, via PMC in LP1 */
> +	adr	r1, tegra_pllm_status
>  	ldr	r0, [r4, #PMC_PLLP_WB0_OVERRIDE]
> -	bic	r0, r0, #(1 << 12)
> -	str	r0, [r4, #PMC_PLLP_WB0_OVERRIDE]
> +	and	r2, r0, #(1 << 12)
> +	str     r2, [r1]
> +	cmp	r2, #(1 << 12)
> +	biceq	r0, r0, #(1 << 12)
> +	streq	r0, [r4, #PMC_PLLP_WB0_OVERRIDE]
>  
>  	/* disable PLLP, PLLA, PLLC and PLLX */
>  	ldr	r0, [r5, #CLK_RESET_PLLP_BASE]
> -- 
> 2.17.1
> 
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