[U-Boot] [PATCH] sunxi: Fix buggy sun6i/sun8i DRAM size detection logic

[Siarhei Siamashka <siarhei.siamashka@gmail.com>]

On Wed, 24 Dec 2014 18:58:17 +0200
Siarhei Siamashka <siarhei.siamashka@gmail.com> wrote:

> After reboot, reset or even short power off, DRAM typically retains
> the old stale data for some period of time (for this type of memory,
> the bits of data are stored in slowly discharging capacitors).
> 
> The current sun6i/sun8i DRAM size detection logic, which is
> inherited from the Allwinner code, relies on using a large magic
> signature with the hope that it is unique enough and unlikely to
> ever accidentally match this leftover garbage data in RAM. But
> this approach is inherently unsafe, as can be demonstrated using
> the following test program:
> 
> /***** A testcase for reproducing the problem ******/
> #include <stdlib.h>
> #include <stdio.h>
> #include <stdint.h>
> 
> void main(int argc, char *argv[])
> {
>     size_t size, i;
>     uint32_t *buf;
>     /* Allocate the buffer */
>     if (argc < 2 || !(size = (size_t)atoi(argv[1]) * 1048576) ||
>                     !(buf = malloc(size))) {
>         printf("Need buffer size in MiB as a cmdline argument\n");
>         exit(1);
>     }
>     /* Fill it with the Allwinner DRAM "magic" values */
>     for (i = 0; i < size / 4; i++)
>         buf[i] = 0xaa55aa55 + ((uintptr_t)&buf[i] / 4) % 64;
>     /* Try to reboot */
>     system("reboot");
>     /* And wait */
>     for (;;) {}
> }
> /***************************************************/
> 
> If this test program is run on the device (giving it a large
> chunk of memory), then the DRAM size detection logic in u-boot
> gets confused after reboot and fails to initialize DRAM properly.
> 
> A better approach is not to rely on luck and abstain from making
> any assumptions about the properties of the leftover garbage
> data in RAM. Instead just use a more reliable code for testing
> whether two different addresses refer to the same memory location.
> 
> Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>
> ---
> 
> Done only very limited testing on MSI Primo81 tablet (Allwinner A31s),
> which is currently a rather impractical device for doing any sun6i code
> development due to having no access to the serial console, USB or other
> convenient ways to interact with the device.
> 
> It might be a good idea to backup/restore the data in RAM when doing
> this check in the code.
> 
> Using the standard u-boot 'get_ram_size' function could be also an
> option to replace the loops and simplify the sun6i/sun8i dram code
> in the future. The only inconvenience is that 'get_ram_size' returns
> 'size' instead of 'log2(size)'. This could be probably resolved by
> introducing a new 'get_ram_size_log2' common function.

Just noticed that there is actually '__ilog2' function in u-boot. This
makes it easier to switch the sun6i/sun8i dram code to using the
standard 'get_ram_size' function.

-- 
Best regards,
Siarhei Siamashka