[U-Boot] [PATCH V2 1/4] arm: mx5: Fix memory slowness on MX53QSB

[Stefano Babic]

Hi Marek,

On 28/03/2014 08:30, Marek Vasut wrote:
> Fix memory access slowness on i.MX53 MX53QSB board. Let us inspect the
> issue: First of all, the i.MX53 CPU has two memory banks mapped at
> 0x7000_0000 and 0xb000_0000 and each of those can hold up to 1GiB of
> DRAM memory. Notice that the memory area is not continuous. On MX53QSB,
> each of the banks contain 512MiB of DRAM, which makes a total of 1GiB
> of memory available to the system.
> 
> The problem is how the relocation of U-Boot is treated on i.MX53 . The
> U-Boot is placed at the ((start of first DRAM partition) + (gd->ram_size)) .
> This in turn poses a problem, since in our case, the gd->ram_size is 1GiB,
> the first DRAM bank starts at 0x7000_0000 and contains 512MiB of memory.
> Thus, with this algorithm, U-Boot is placed at offset:
> 
>     0x7000_0000 + 1GiB - sizeof(u-boot and some small margin)
> 
> This is past the DRAM available in the first bank on MX53QSB, but is still
> within the address range of the first DRAM bank. Because of the memory
> wrap-around, the data can still be read and written to this area, but the
> access is much slower.
> 
> There were two ideas how to solve this problem, first was to map both of
> the available DRAM chunks next to one another by using MMU, second was to
> define CONFIG_VERY_BIG_RAM and CONFIG_MAX_MEM_MAPPED to size of the memory
> in the first DRAM bank. We choose the later because it turns out the former
> is not applicable afterall. The former cannot be used in case Linux kernel
> was loaded into the second DRAM bank area, which would be remapped and one
> would try booting the kernel, since at some point before the kernel is started,
> the MMU would be turned off, which would destroy the mapping and hang the
> system.
> 
> Signed-off-by: Marek Vasut <marex at denx.de>
> Cc: Fabio Estevam <fabio.estevam at freescale.com>
> Cc: Stefano Babic <sbabic at denx.de>
> Cc: Wolfgang Denk <wd at denx.de>
> ---
>  include/configs/mx53loco.h | 2 ++
>  1 file changed, 2 insertions(+)
> 
> V2: Reword the commit message
> 
> diff --git a/include/configs/mx53loco.h b/include/configs/mx53loco.h
> index 1415584..82e0249 100644
> --- a/include/configs/mx53loco.h
> +++ b/include/configs/mx53loco.h
> @@ -199,6 +199,8 @@
>  #define PHYS_SDRAM_2		CSD1_BASE_ADDR
>  #define PHYS_SDRAM_2_SIZE	(512 * 1024 * 1024)
>  #define PHYS_SDRAM_SIZE         (PHYS_SDRAM_1_SIZE + PHYS_SDRAM_2_SIZE)
> +#define CONFIG_VERY_BIG_RAM
> +#define CONFIG_MAX_MEM_MAPPED	PHYS_SDRAM_1_SIZE
>  
>  #define CONFIG_SYS_SDRAM_BASE		(PHYS_SDRAM_1)
>  #define CONFIG_SYS_INIT_RAM_ADDR	(IRAM_BASE_ADDR)
> 

Nice works, thanks ! I will push it soon.

Best regards,
Stefano Babic

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