[PATCH] arm: sunxi: dram: restore previous value in DRAM

[Samuel Holland]

On 2/13/22 9:10 AM, Andrei Lalaev wrote:
> It's better to restore previous value at offset because otherwise the
> function can corrupt memory reserved by Linux (e.g. ramoops).

With this patch, does ramoops work for you? and on which hardware? If so, I am
definitely in favor of applying this patch.

I'm a bit surprised that DRAM contents are maintained (this isn't the only place
where they could be destroyed). I suppose the reset state of the PRCM enables
pad hold, which would put the DRAM in self-refresh. So we just have to be
careful during DRAM controller init.

Regards,
Samuel

> Signed-off-by: Andrei Lalaev <andrei.lalaev at emlid.com>
> ---
>  arch/arm/mach-sunxi/dram_helpers.c | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm/mach-sunxi/dram_helpers.c b/arch/arm/mach-sunxi/dram_helpers.c
> index 2c873192e6..ed6db14f3c 100644
> --- a/arch/arm/mach-sunxi/dram_helpers.c
> +++ b/arch/arm/mach-sunxi/dram_helpers.c
> @@ -32,12 +32,20 @@ void mctl_await_completion(u32 *reg, u32 mask, u32 val)
>  #ifndef CONFIG_MACH_SUNIV
>  bool mctl_mem_matches(u32 offset)
>  {
> +	ulong addr = CONFIG_SYS_SDRAM_BASE + offset;
> +	u32 prev_val = readl(addr);
> +	bool ret = false;
> +
>  	/* Try to write different values to RAM at two addresses */
>  	writel(0, CONFIG_SYS_SDRAM_BASE);
> -	writel(0xaa55aa55, (ulong)CONFIG_SYS_SDRAM_BASE + offset);
> +	writel(0xaa55aa55, addr);
>  	dsb();
>  	/* Check if the same value is actually observed when reading back */
> -	return readl(CONFIG_SYS_SDRAM_BASE) ==
> -	       readl((ulong)CONFIG_SYS_SDRAM_BASE + offset);
> +	ret = readl(CONFIG_SYS_SDRAM_BASE) == readl(addr);
> +
> +	/* Restore previous value */
> +	writel(prev_val, addr);
> +
> +	return ret;
>  }
>  #endif
>